Hayabusa-2, Japan's second Asteroid Sample Return Mission
Hayabusa-2 is JAXA's (Japan Aerospace Exploration Agency) follow-on mission to the Hayabusa mission, the country's first round-trip asteroid mission that sent the Hayabusa (MUSES-C) spacecraft to retrieve samples of asteroid Itokawa. The initial Hayabusa mission launched in May 2003 and reached Itokawa in 2005; it returned samples of Itokawa — the first asteroid samples ever collected in space — in June 2010. Hayabusa means 'falcon' in Japanese. 1) 2) 3) 4)
The objective of the Hayabusa-2 sample return mission is to visit and explore the C-type asteroid 1999 JU3, a space body of about 920 m in length and of particular interest to researchers, because it consists of 4.5 billion-year-old material that has been altered very little. Measurements taken from Earth suggest that the asteroid’s rock may have come into contact with water. The carbonaceous or C-type asteroid is expected to contain organic and hydrated minerals, making it different from Itokawa, which was a rocky S-type (stony composition) asteroid.
Table 1: Some background on the programmatic exploration of primitive bodies
Table 2: Objectives : Hayabusa vs Hayabusa-2 5)
Figure 1: Overall schedule of the Hayabusa-2 mission (image credit: JAXA)
Detailed information of asteroid 1999JU3 has been obtained by observations of ground-based telescopes. According to observation data of 2008, the diameter of 1999JU3 is estimated to be about 900 m, larger than that Itokawa, and the rotation period is around 7.6 hours. Observation of the reflected sunlight spectrum showed, that it has features of a C-type asteroid. It is rather difficult to determine the spin axis of asteroid 1999JU3 because of its rather spherical shape.
Figure 2 shows the orbit of asteroid 1999 JU3. The orbit is similar to that of Itokawa, and it is orbiting from just inside the orbit of the Earth to just outside the orbit of Mars. The inclination of the orbit is small like the one of Itokawa. Such an orbit is suitable for a small spacecraft like Hayabusa-2 to reach and return to Earth.
Legend to Figure 2: The blue circled lines in the figure illustrate the orbits of Mercury, Venus, Earth and Mars from inside, respectively, Itokawa's orbit is green, while the yellow orbit is that of 1999JU3.
The asteroid was discovered in 1999 by the LINEAR (Lincoln Near-Earth Asteroid Research) project, and given the provisional designation 1999JU3 (it hasn't been named so far). LINEAR is a collaboration of the United States Air Force, NASA, and MIT/LL (Massachusetts Institute of Technology /Lincoln Laboratory) for the systematic discovery and tracking of near-Earth asteroids.
Figure 3: Schematic of the science of Hayabusa and Hayabusa-2 missions (image credit: JAXA)
Project short history:
The Hayabusa-2 mission was proposed in 2006 at first. In this first proposal, the spacecraft was almost same as that of Hayabusa, because the project team wanted to start it as soon as possible. Of course, the team realized that parts had to be modified where trouble occurred in Hayabusa, but there were no major changes. The launch windows to go for launch to asteroid 1999 JU3 were in 2010 and 2011. However, JAXA could not start Hayabusa-2 mission immediately, because no budget was available. Hence, the launch opportunity was missed. The next launch window came up in 2014. Thus, the project postponed the launch date, and continued proposing Hayabusa-2. Since the launch was delayed, the project had time to change the spacecraft a little. New instruments were added, such as a Ka-band antenna and what is called “impactor.” The project even calls Hayabusa-2 a new spacecraft. 6)
In May 2011, the status of Hayabusa-2 project shifted to Phase-B, starting with the design of the spacecraft. In March 2012, the CDR (Critical Design Review) was done, and the team started manufacturing the flight model. The initial integration test started at the beginning of 2013, and the final integration test started at the end of 2013. At present (September 2014), the team has almost finished the final integration test, and the spacecraft will be shipped to the launch site soon.
International collaborations: The Hayabusa-2 mission involves international collaborations with Germany, the United States, and Australia. DLR (German Aerospace Center) and CNES (French Space Agency) are providing the small lander MASCOT. NASA was already a partner in the Hayabusa mission, a similar collaboration is under consideration for Hayabusa-2. The third collaboration is with Australia for capsule reentry as in the case of the Hayabusa mission.
Japan's Hayabusa-2 spacecraft is designed to study asteroid 1999 JU3 from multiple angles, using remote-sensing instruments, a lander and a rover. It will collect surface- and possibly also subsurface materials from the asteroid and return the samples to Earth in a capsule for analysis. The mission also aims to enhance the reliability of asteroid exploration technologies. 7) 8) 9)
In the current plan, the launch window for Hayabusa-2 is in late 2014. With this schedule, Hayabusa-2 will reach the asteroid in the middle of 2018, and return to the Earth at the end of 2020.
Figure 4: Artist's rendition of the Hayabusa-2 spacecraft (image credit: JAXA, Akihiro Ikeshita)
The Hayabusa-2 mission will utilize new technology while further confirming the deep space round-trip exploration technology by inheriting and improving the already verified knowhow established by Hayabusa to construct the basis for future deep-space exploration.
The configuration of Hayabusa-2 is basically the same as that of Hayabusa, with modifications of some parts by introducing novel technologies that evolved after the Hayabusa era. For example:
• The HGA (High Gain Antenna) for Hayabusa featured a parabolic shape, while Hayabusa-2 uses two planar HGAs with a considerably lower mass but with the same performance characteristics. The reason why Hayabusa-2 has two HGAs is that spacecraft has two communication links, Ka-band as well as the X-band links. In daily operations support, the team uses the X-band for data transmission, but for the download of the asteroid observation data, the Ka-band is used to take advantage of the higher data rate of 32 kbit/s, provided by the Ka-band link. The DDOR (Delta-Differential One-way Ranging) technique is used for very accurate plane-of-sky measurements of spacecraft position which complement existing line-of-sight ranging and Doppler measurements.
• The AOCS (Attitude and Orbit Control Subsystem) of Hayabusa-2 was improved, now featuring 4 reaction wheels for a more reliable service in case of need.
- During the cruise phase, Hayabusa-2 controls its attitude with only one reaction wheel to bias the momentum around the Z-axis of the body. This is to save the operating life of reaction wheels for other axes, because the project experienced that two reaction wheels of three equipped on Hayabusa were broken after the touchdown mission. 10)
- In this one wheel control mode, the angular momentum direction is slowly moved in the inertial space (generally called precession) due to the solar radiation torque. This attitude motion caused by the balance of the total angular momentum and solar radiation pressure is known to trace the Sun direction automatically with ellipsoidal and spiral motion around Sun direction. Based on this knowledge of the past, the attitude dynamics model for the Hayabusa-2 mission had been developed before the launch. According to the newly developed attitude dynamics model of Hayabusa-2, the precession trajectory is almost the ellipsoid around the attitude equilibrium point, and this equilibrium point is determined mainly by the phase angle around Z-axis of the body.
- In the actual operation of Hayabusa-2, the spacecraft experienced already the one wheel control mode, and the attitude motion in this mode is nearly corresponding to the expected motion based on the dynamics model developed before the launch. The precession trajectory is ellipsoid around the equilibrium point, and the attitude dynamics model is verified by the actual flight data. In this one wheel operation, the Sun aspect angle is restricted within a certain limit angle in terms of the thermal condition of the spacecraft. Because the precession radius is determined by the initial attitude and the equilibrium point, the Sun aspect angle almost exceeds the limit angle due to the precession without change of the equilibrium point. At this operation, the project executes the attitude maneuver around the Z-axis to change the equilibrium point, in order to reduce the Sun aspect angle - and succeeded. After that, the project executed the maneuver again to change the equilibrium point to a close point in order to make the small precession trajectory (Ref. 10).
• IES (Ion Engine System) has been modified to account for the aging effect during extended support periods. The thrust level of IES was increased by 25%, using the same Xe microwave discharge ion engine system.
IES will be used for orbit maneuvers during the cruising of the Hayabusa-2’s onward journey to the asteroid and return trip to Earth. The engine enables to make the round trip with only one tenth of the power consumption compared to that of chemical propellant.
Major improvements from the Hayabusa mission are:
- Countermeasures to plasma ignition malfunction of one ion source of an ion engine. Carefully coordinating each part of the ion engine to improve both ion source propulsion generation efficiency and ignition stability.
- Countermeasures to degradation and malfunction of three neutralizers that occurred after 10,000 to 15,000 hours of operation. To make the neutralizer’s lifespan longer, the walls of the electric discharge chamber are protected from plasma and the magnetic field has been strengthened to decrease the voltage necessary for electron emission.
- The maximum power was successfully increased to 10 mN per ion engine from the conventional 8 mN.
Figure 5: Photo of the IES assembly (image credit: JAXA)
Figure 7: Bottom view of Hayabusa-2 spacecraft illustrating the various elements of the spacecraft (image credit: JAXA)
The Hayabusa-2 spacecraft has a stowed size of 1.6 m x 1 m x 1.25 m (height). With the solar panels deployed, the 600 kg satellite as a width of 6 m.
Table 3: Spacecraft system parameters
Figure 8: Photo of the Hayabusa-2 flight model, taken in Aug. 31, 2014, before shipping to the launch site, TNSC (Tanegashima Space Center). The SRC (Sample Return Capsule) is mounted to the bottom front side center of the spacecraft (image credit: JAXA)
Launch: The Hayabusa-2 spacecraft was launched on December 03, 2014 (04:22:04 UTC) on a H-IIA vehicle (No. 26) from TNSC (Tanegashima Space Center), Japan. The launch service provider was MHI (Mitsubishi Heavy Industries, Ltd). The launch was nominally and about 1 hr 47 minutes and 21 seconds after liftoff, the separation of the Hayabusa-2 spacecraft into an Earth-escape trajectory was confirmed. 11) 12)
• Shin'en-2, a nanosatellite technology demonstration mission (17 kg) of Kyushu Institute of Technology and Kagoshima University, Japan. The objective is to establish communication technologies with a long range as far as moon. Shin'en-2 carries into deep space an F1D digital store-and-forward transponder which offers an opportunity for earthbound radio amateurs to test the limits of their communication capabilities.
• ArtSat-2 (Art Satellite-2)/DESPATCH (Deep Space Amateur Troubadour’s Challenge), a joint project of of Tama Art University and Tokyo University. DESPATCH is a microsatellite of ~30 kg. The microsatellite carries a “deep space sculpture” developed using a 3D printer, as well as an amateur radio payload and a CW beacon at 437.325 MHz.
• PROCYON (PRoximate Object Close flYby with Optical Navigation) is a microsatellite (67 kg) developed by the ISSL (Intelligent Space Systems Laboratory) of the University of Tokyo and JAXA. The objective is to demonstrate microsatellite bus technology for deep space exploration and proximity flyby to asteroids performing optical measurements. 13)
Orbit: The trajectory of Hayabusa-2 for the whole mission is shown in the sun-earth fixed coordinate in Figure 9. The total cruising time is about 4.5 years, and the asteroid proximity period is about 1.5 years. So the total flight time is about 6 years. The departure C3 is 21 km2/s2, the total impulse of the ion engine is 2 km/s, and the reentry speed of the capsule is 11.6 km/s.
Legend to Figure 9: Hayabusa-2 is equipped with a high-specific impulse ion engine system to enable the round-trip mission. First one year after launch is an interplanetary cruise phase called EDVEGA (Electric Delta-V Earth Gravity Assist).
• November 27, 2019: Analysis of the impact craters on Ryugu using the spacecraft Hayabusa 2's remote sensing image data has illuminated the geological history of the Near-Earth asteroid. 14)
- A research group led by Assistant Professor Naoyuki Hirata of the Department of Planetology at Kobe University's Graduate School of Science revealed 77 craters on Ryugu. Through analyzing the location patterns and characteristics of the craters, they determined that the asteroid's eastern and western hemispheres were formed at different periods of time.
- It is hoped that the collected data can be used as a basis for future asteroid research and analysis. These results were first published in the American Scientific Journal 'Icarus' on November 5 2019.
- JAXA's Hayabusa-2 has been used to carry out various missions to increase our understanding of the spinning top-shaped, Near-Earth asteroid Ryugu. Since arriving in June 2018, the unmanned spacecraft has taken samples and a great number of images of the asteroid. It is hoped that these can reveal more about Ryugu’s formation and history.
- This research group focused on using the image data to determine the number and location of impact craters on the asteroid. Impact craters are formed when a smaller asteroid or a comet hits the surface of the asteroid. Analyzing the spatial distribution and the number of impact craters can reveal the frequency of collisions and aid researchers in determining the age of different surface areas.
- First of all, the image data from Hayabusa-2 was analyzed. Hayabusa 2 has many different types of camera including Optical Navigation Cameras (ONC). The ONC team has been able to take around 5000 images of Ryugu, which have revealed many surface features- including impact craters. For this study, image data obtained from the ‘ONC-T’ camera between July 2018 and February 2019 was utilized. The research group had to determine which of these images showed craters. 340 images were used for crater counting, with stereopair images making it easier to identify the craters. A global image mosaic map was constructed from the ONC images and rendered onto the computer model of Ryugu’s shape. Small Body Mapping Tool software was then used to measure the size, latitude and longitude of the craters. A LiDAR (Light Detection and Ranging pulsed laser) was also utilized to determine the overall size of Ryugu.
- The depressions identified on Ryugu were divided into four categories- depending on how evident their circular appearance was. Category I to III depressions were classified as distinct craters. Category IV depressions only had quasi-circular features, therefore it was hard to determine whether they were craters or not. Many craters were filled with boulders or lacked a distinct shape. Depressions that were too vague to determine were left out of the results.
- The research team were able to identify all impact craters over 10 to 20m in diameter on Ryugu’s entire surface- a total of 77 craters. Furthermore, a pattern was discovered in their distribution. The section of the eastern hemisphere near the meridian was found to have the most craters. This is the area near the large crater named Cendrillon - which is one of Ryugu’s biggest. In contrast, there are hardly any craters in the western hemisphere- suggesting that this part of the asteroid was formed later. The analysis also revealed that there are more craters at lower latitudes than at higher latitudes on Ryugu. In other words, there are very few craters in Ryugu’s polar regions.
- The equatorial ridge in the eastern hemisphere was determined to be a fossil structure. When asteroids like Ryugu rotate at high speeds, this can alter their shape. It is thought that this ridge formed in the distant past during a period when it only took Ryugu 3 hours to rotate. As the eastern hemisphere and western hemisphere were formed at different periods of the asteroid’s history- this suggests that there have been at least two instances where Ryugu’s rotational speed has increased.
Figure 10: Size and location of craters on Ryugu (Figure from the Journal paper): The craters are numbered in order of size (image credit: Kobe University) 15)
Figure 11: Individual images of each identified crater on Ryugu (image credit: Kobe University)
• November 13, 2019: JAXA confirmed Hayabusa-2, JAXA's asteroid explorer, left the target asteroid Ryugu. 16)
- On November 13, 2019, JAXA operated the Hayabusa-2 chemical propulsion thrusters for the spacecraft's orbit control. The confirmation of the Hayabusa-2 departure made at 10:05 a.m. [JST (Japan Standard Time), 01:05 GMT] was based on the following data analyses:
a) The thruster operation of Hayabusa-2 occurred nominally
b) The velocity leaving from Ryugu is approximately 9.2 cm/s
c) The status of Hayabusa2 is nominal.
- The project is planning to conduct performance tests of onboard instruments, including the electric propulsion system, for the return to Earth. According to JAXA, the probe is expected to drop off its precious samples some time in late 2020.
• October 2019: Hayabusa-2 arrived at the C-type asteroid Ryugu in June 2018. During one and a half year of the Ryugu-proximity operation, we succeeded in two rovers landing, one lander landing, two spacecraft touchdown/sample collection, one kinetic impact operation and two tiny reflective balls and one rover orbiting. Among the two successful touchdowns, the second one succeeded in collecting subsurface material exposed by the kinetic impact operation. This paper describes the asteroid proximity operation activity of the Hayabusa-2 mission, and gives an overview of the achievements done so far. Some important engineering and scientific activities, which have been done in synchronous operations with the spacecraft to tackle the unexpected Ryugu environment, are also described. 17)
Asteroid Proximity Operation
- Summary of Asteroid-Proximity Activity: Hayabusa2 arrived at HP (Home Position) on 27 June 2018, when the asteroid proximity phase began. The HP is defined as the position 20 km from the asteroid center toward asteroid-Earth (sub-Earth) line. The HP is always located on the day side of the asteroid, since the Sun-asteroid-Earth angle varies between 0 to 39º during the 1.5 years of the asteroid proximity phase. The Sun-asteroid distance during the asteroid proximity phase varies between 0.96-1.4AU, and the Earth-asteroid distance varies between 2.0-2.4 AU, which corresponds to the round trip light time of 33-40 minutes.
- All the descent operations to lower altitude is defined as “critical operation” in the project, which were managed by a large operation team participated by 20-40 specifically-trained operators. Table 4 shows the list of the descent operations and the other important operations performed by Hayabusa-2. Overall, the asteroid-proximity activity proceeded as planned despite the fact that the environment of Ryugu was found to be unexpectedly severe, and some operations were aborted (i.e. the spacecraft detected a failure and ascent back to HP while descending).
Figure 12: Top left: Landing site candidates of Hayabusa-2 designated by L01...L13 and M01...M04. The size of each box is 100 m x100 m. Top right: Landing site candidates of MASCOT, designated by MA-1...MA-10, superimposed with some landing candidates of Hayabusa-2 to check the interference. Bottom Left: Landing site candidates of MINERVA-II-1, designated by N1...N7, superimposed with some landing candidates of Hayabusa-2. Bottom right: Finally selected landing candidates for Hayabusa-2 [Primary: L08, Backup: L07, M04), MINERVA-II-1 (N6) and MASCOT (MA-6)]. (image credit: JAXA, U. of Tokyo, DLR, CNES and collaborators)
- Initial Ryugu Observation: On establishing the hovering state at HP, Hayabusa2 started initial observations of Ryugu using the remote science instruments, such as ONC-T (Optical Navigation Camera Telescopic); multi-band imager, ONC-W1 (Optical Navigation Camera-Wide), TIR (Thermal Infrared Imager), NIRS3 (Near-Infrared Spectrometer) and LIDAR (Laser Altimeter).
- The first two months of the proximity phase were dedicated to the LSS (Landing Site Selection) observation campaign. The objective of this campaign was to measure and acquire basic properties of Ryugu and to derive a set of landing sites for two MINERVA-II-1 rovers, one MASCOT lander, and a one spacecraft touchdown. The other sites (for the kinetic impact, the second touchdown and the MINERVA-II2 rover) were to be decided in the next LSS activity in 2019.
- Through this operation, Ryugu has been revealed to have an oblate body with an equatorial radius of 502 m and polar to equatorial axis ratio of 0.872. The asteroid spin state is upright and retrograde, having the obliquity of 171.64º with the period of 7.63262 hours. The spectral data obtained by ONC-T and NIRS3 indicate Ryugu is a Cb-type (carbon rich) with a very low geometric albedo of 4.5%, and is later proved to contain hydroxyl(OH)-bearing minerals all over the globe. One of the extraordinary features of Ryugu is that the number density of boulders is uniformly high across the surface, twice as high as that of Itokawa for boulders having diameter larger than 20 m, which led to the primary difficulty that the Hayabusa2 mission faced for deriving feasible landing sites. The gravity of Ryugu was measured to be GM=30.0 m3/s2 by a free-fall operation conducted on August 5-7, 2018. This GM corresponds to the bulk density of 1.19 g/cm3, showing that Ryugu is a rubble-pile.
- Two critical operations were conducted during the LSS observation camping phase. On July 31-August 2, “Mid-altitude descent observation” was conducted, in which the spacecraft has descent down to 5 km and stayed there for 8 hours to obtain high resolution observations for one rotational period. On August 5-7, “Gravity measurement descent” was conducted, in which the spacecraft descent down to 851 m above surface. The free-fall descent and free-ascent were conducted at the altitude below 5 km to measure the gravity. These two descent operations also enabled to create a landmark database necessary for the subsequent lower altitude descents, and contributed as an in-situ operation practice of the terrain-relative precision guidance.
- LSS (Landing Site Selection) Activity: Selecting landing sites required a variety of high-level products of the asteroid property. Many level-2 products, such as the shape model, surface composition map, gravity potential model, temperature map and boulder distributions map were created by integrating and cross-evaluating the raw observation data (level-1 products) obtained through the LSS observation campaign.
- Based on these level-2 products, level-3 maps which define candidate sites of the rover/lander/spacecraft landing were created. This level-3 maps were derived by integrating engineering feasibility, operation safety and scientific values.
- Figure 12 top-left shows 15 candidate sites for the spacecraft touchdown derived from engineering constraints. The spacecraft is to land to the surface along the sub-Earth line, and therefore the geometry with relative to the sub-Earth line is important. The sun angle, surface slope and terrain roughness with respect to the sub-Earth line have been evaluated based on the shape model of Ryugu. The pre-launch spacecraft design required 50 m (2.5 σ) landing accuracy. Hence the 15 boxes shown in the top left of Figure12 all have the size of 100 m x100 m.
- Specifically for Ryugu, the major drivers for deriving the spacecraft touchdown site candidates were engineering constraints (especially safety), reflecting the fact that the surface property of Ryugu is homogenous. Table 4 shows the scores evaluated by the science team for some of the candidate sites (the evaluation itself was done for all the candidate sites), showing that the scientific values of all the candidate sites are almost equal. 18)
- The landing site candidates for MINERVA-II-1 (Figure 12 bottom-left) and MASCOT (Figure 12 top-right) were derived by the Hayabusa-2 spacecraft operation team and the DLR/CNES MASCOT team, respectively from the viewpoint of engineering constraints (c.f. surface temperature, surface roughness, day-night ratio, landing dispersion) and scientific values.
- The “LSS Decision Meeting” was held on August 17, 2018 to review all the level-3 products and to derive a consistent set of landing sites for MINERVA-II-1, MASCOT and the spacecraft. The meeting was participated by 109 members including spacecraft engineers, science PIs (Principal Investigators) and international scientists. Thanks to extensive training and dry-run having done before arrival, including “Simulated LSS decision meeting” held on September 5, 2017, the meeting has reached the conclusion smoothly in one day with dense and constructive discussions. A set of landing site (MINERVA-II-1: N6, MASCOT: MA-9, Spacecraft: L08) with two backup spacecraft touchdown sites (L07 and M04) was selected such that these sites should not interfere with each other while maximizing scientific value and success probability of landings (Figure 12 bottom-right).
- The conclusion includes an important collateral condition for the spacecraft touchdown site; i.e. since none of the candidate sites perfectly fulfil the touchdown safety due to the high boulder density, as seen in Figure 13, the project is to search for a safer region within selected candidate sites by additional observations, and at the same time, to improve the landing accuracy to cope with narrower landing sites.
Figure 13: Left: An ONC-T image of the Hayabusa-2 landing candidates L08, L07 and M04, captured at the altitude of 5 km. Right: Corresponding boulder size distributions (image credit: Hayabusa-2 Team and the DLR/CNES MASCOT Team)
- Post-LSS Operations: Based on the LSS decision described above, several critical operations have been conducted.
- On September 11-12, the first touchdown rehearsal (TD1-R1) was conducted, targeting the center of the L08 site. This was the first attempt to descend to lower than 30 m to demonstrate the surface accessibility performance of Hayabusa-2 and to acquire the characteristics of LRF (Laser Range Finder) which is a critical component for the autonomous sequence of Hayabusa-2 below 30 m. The operation was aborted at the altitude of 600 m and spacecraft automatically ascent back to HP. The cause of it was an immatur LIDAR setting due to very low reflectance surface of the asteroid, and the spacecraft failed to measure the altitude.
- On September 20-21, the MINERVA-II-1 rovers release operation (MNV1) was conducted, followed by the MASCOT lander release operation (MSC) on October 2-5, both of which were successful.
- At the time MSC operation completed, the project decided a new strategy toward realizing the spacecraft touchdown. Based on three descent operations done in September-October, 2018, our terrain relative guidance accuracy performance was evaluated to be around 15 m, which was far better than the original specification of 50 m. On the other hand. due to the aborted TD1-R1 operation, the LRF performance had not been obtained. Thus the project decided to do additional two touchdown rehearsals (TD1-R1A and TD1-R3) before the actual touchdown operation. TD1-R1A is basically a retrial of TD1-R1 to acquire the LRF performance. TD1-R3 is to deploy the target marker (TM) on ground to precisely evaluate the our terrain relative guidance capability and the autonomous TM-tracking control performance of Hayabusa2. The TM is a 10 cm diameter ball covered with retro-reflective sheet. Hayabuas-2 is equipped with a flash lamp (FLA). The combination of TM and FLA enables an autonomous terrain relative control by a bright-spot tracking which is tolerant against highly uncertain surface condition. The target site of these two operations was set at L08-B1 whose diameter is 20 m and was selected because it is the flattest region in the L08 area.
- We succeeded in TD1-R1A on October 14-15 and TD1-R3 on October 24-25. After TD1-R3 the TM was identified settled on ground, 15.4 m off from the targeted center (Figure 14). The success of TD1-R3 marked a critical milestone for the project for two reasons, (i) the dropped TM location indicates that our terrain-relative guidance is as good as 15 m, (ii) the autonomous TM tracking capability at very low altitude (~12 m) is confirmed on Ryugu environment with ±1 m TM-relative positioning accuracy. These results opened a possibility toward a TM-relative pin-point touchdown which had not been planned originally.
Critical Operation Achievements
- MINERVA-II-1 Rover Deployment Operation: The two MINERVA-II-1 rovers (Rover-1A named “HIBOU” and Rover-1B named “OWL”, Figure15 top-left) were deployed from the spacecraft at altitude of 55 m at September 21, 4:06 UT. A horizontal ΔV of 0.2 m/s was executed right before the separation to compensate for fast ejection velocity of the rovers, ensuring that the rovers should not exceed the escape velocity of Ryugu. The terrain-relative guidance accuracy at the release was 3.3 m.
- Two rovers were confirmed active immediately after the deployment, and some images captured by the rover onboard cameras were received by Hayabusa-2. On Sol 4 (4th asteroid day), Rover-1A was confirmed doing some autonomous hops, and Rover-1B as well on Sol 7. The final decodable telemetry was received on Sol 114 for Rover-1A and Sol 10 for Rover-1B. The carrier signal of Rover-1B was successfully received again on August 2, 2019 for about 10 minutes at a rather distant location from the original landing point, which indicates that the rover has acted on the Ryugu surface for more than 10 months.
Figure 15: Top left: MINERVA-II-1 rovers, left: Rover-A “HIBOU”, right: Rover-B “OWL”. Top right: MASCOT lander. Bottom left: DCAM3 deployable camera. Bottom right: SCI (Small Carry-on Impactor), image credit: Hayabusa-2 Team and the DLR/CNES MASCOT Team
- Over the whole MINERVA-II-1 operation, a few hundreds of images (including movies and stereo images) as well as the surface temperatures were obtained (Figure 16). The two rovers have become the world’s first mobile landers to successfully act on a small body. The landing site N6 was nicknamed “Tritonis” by the MINERVA-II-1 team.
Figure 16: Top left: Image of Hayabusa-2 taken by Rover-1A on 21 September at 4:08UT right after the separation. Top right: Close-up image of the Ryugu surface taken by Rover-1B on September 23 at 0:46 UT. Bottom: A series of images captured by Rover-1B on the Ryugu surface between 1:14-02:48 UT on 23 September 2018 (image credit: Hayabusa-2 Team and the DLR/CNES MASCOT Team)
- MASCOT Lander Deployment Operation: The MASCOT lander (Figure 15 top-right), developed by DLR and CNES was successfully deployed from the spacecraft at the altitude of 51m at October 3,1:57UT. The separation was later confirmed by the Hayabusa-2 onboard cameras ONC-W2 and ONC-W1 (Figure 17). No horizontal compensation was executed for this release, as the MASCOT separation mechanism deploys the lander very slowly (~5 cm/s). The terrain-relative guidance accuracy at the release was 30 m.
- Different from MINERVA-II rovers, MASCOT is driven by primary battery and can be active for 17 hours from the separation (i.e. 3-4 Sols). The project team prepared the biggest operation organization for this operation. The operation center for the MASCOT was placed in Cologne, Germany, and was connected online to SSOC (Sagamihara Space Operation Center), the operation center for Hayabusa-2 in Japan. A 24 hours continuous coverage was prepared to cover the whole life time of MASCOT using ground stations of JAXA Usuda Deep Space Center, NASA Deep Space Network, and ESA (ESTRACK) Malargüe station. The MASCOT telemetry and command operations were performed through the rover communication component (OME-E) which realizes a simultaneous two-way communication with MASCOT and all the MINERVA-II rovers.
- The MASCOT was confirmed active immediately after release, and landed to MA-6 region a few minutes later after some bounces. On Sol 1, the lander was found to sit on the ground with inappropriate orientation. Hence the MASCOT team decided to command a “hop” on Sol 2, which was successful. Thereafter, extensive scientific observations were performed on Sol 2-3. On Sol 3 a small relocation hop was commanded by the MASCOT team to do some additional observations with different attitude, which was also successful. The last telemetry was received in the dawn of Sol 3, and the battery was estimated to deplete after 17 hours of the successful surface activity. The landing site MA-6 was nicknamed “Alice’s Wonderland” by the MASCOT team.
Figure 17: Top: Images taken by ONC-W2, attached on the side of the spacecraft, at the moment of MASCOT separation on 3October 2018 at 1:57:54-1:58:14 UT. Bottom: Falling MASCOT taken by ONC-W1 on 3 October 2018 at 1:59:40 UT. MASCOT (arrow a) and its shadow (arrow b) are seen in the image (image credit: Hayabusa-2 Team and the DLR/CNES MASCOT Team)
- First Touchdown and Sampling Operation: The original touchdown sequence used the Target Marker (TM) to eliminate the terrain-relative velocity at the final phase of landing. From the result of TD1-R3, however, the project team acquired a great confidence on the TM-tracking capability of Hayabusa-2, and decided to re-configure the onboard guidance program such that the final landing sequence should use the TM for terrain-relative position control in addition to the velocity compensation.
- This technique had been prepared for the crater landing and is called “pin-point touchdown”, which would have been planned after the kinetic impact operation. With this pin-point touchdown as a basis, some modifications were applied to be suited to the L08-E1 landing. For example, four “check points” (Figure 18) were implemented onboard for the autonomous sequence below 45 m such that, if the pre-programmed status conditions were not fulfilled, the spacecraft is to automatically abort and the ascent-back ΔV is to be triggered. The second example is a “tail-up” attitude (Figure 18) adopted for the final touchdown attitude so as to minimize the risk for any part of the spacecraft body other than the sampler horn hitting terrain. By tilting up the attitude by 10 degree from the locally horizontal attitude, additional 10 cm margin has been gained on the minimum distance between the local terrain and the spacecraft body.
Figure 18: Low altitude autonomous sequence of touchdown operation TD1-L08E1. CP indicates onboard autonomous “Check Points”; CP1: TM lock and start tracking, CP2: LIDAR to LRF takeover, CP3: 8.5 m arrival convergence, CP4: Final pre-free fall position and attitude convergence (image credit: Hayabusa-2 Team)
- In terms of our terrain knowledge, high resolution images obtained in a series of low altitude operations (MNV, MSC, TD1-R1A and TD1-R3) enabled us to create a precise boulder distribution map (Figure 19) and a digital elevation map (Figure 20 right) with a few centimeter accuracy, which is good enough for a sub-meter level safety evaluation. By using these maps, two new landing candidate sites were found within the L08 area, L08-B1 and L08-E1 (Figure 20 left). L08-B1 is located 15 m distant from the dropped TM, and spans about 12 m. L08-E1 is located 4.2 m distant from the TM, and spans about 6 m.
- Combining these two progresses (i.e. improvements in the touchdown accuracy and the terrain knowledge), the project team decided to choose L08-E1 for the first touchdown target. L08-E1 is a narrower region but closer to the dropped TM than L08-B1, and was found to be only feasible region that Hayabusa-2 can target with the updated landing accuracy (Figure 20 right).
- The touchdown operation 1 (TD1-L08E1) was conducted on February 19-22, 2019. Due to a miss-configuration in the pre-descent activity, the start of descent was delayed by 5 hours, which was recovered by adopting a faster descent rate trajectory to catch up with the original descent path at around the altitude of 6.5 km. The spacecraft was commanded “Go” for autonomous landing at February 21, 21:10 UT at the altitude of about 500 m, and reached the altitude of 45 m at 22:07 UT. Then the spacecraft successfully captured and locked the dropped TM, and started TM-tracking descent to 8.5 m. At 8.5 m, a horizontal maneuver was triggered, and the spacecraft moved about 4 m toward northeast. On converging the horizontal motion, the final free-fall descent was triggered autonomously, and the spacecraft touched the ground at 22:29:10 UT. One of four touch down detection sources (attitude rate limit) was triggered, when a projectile was ejected and an ascent-back ΔV was executed 0.1 seconds later as planned. Four 20 N thrusters on the bottom of the spacecraft were activated for 4.7 seconds to attain the total ascend-back ΔV of 0.64 m/s. The autonomous activity below 45 m was done with non-Earth pointing attitude, limiting the communication link to the Earth only with a Low Gain Antenna. Therefore, the low altitude onboard activity was monitored on ground solely by the carrier signal receive power and the Doppler signals (Figure 21).
- The successful touchdown was confirmed soon through the play-back telemetry of rising temperature of the fired projector at the touchdown, and also of the sequence progressing normally as pre-programmed. There are three container rooms in the Hayabusa-2 sampling system. As we confirmed the touchdown, the sampler room A was commanded “Close” at February 22, 2:20 UT, and the sample collected in the TD1-L08E1 operation has been secured.
Figure 20: Left: Two final candidates for touchdown site L08-B1 and L08-E1. Right: A Monte-Carlo simulation result based on finalized touchdown sequence of Hayabusa-2 for TD1-L08E1 operation (image credit: Hayabusa-2 Team)
Figure 21: The Doppler signal observed on ground at the moment of touchdown in TD1-L08E1 operation. The vertical axis is “2-way O-C”, which is twice the value of the line-of-sight velocity of the spacecraft, relative to the asteroid (image credit: Hayabusa-2 Team)
- The moment of the touchdown was recorded by ONC-W1 (Figure 22) and CAM-H (a sampler-horn monitor camera, Figure 23). As observed in these images, many fragments were seen to be ejected as the reaction of the projectile shooting and the ascend-back ΔV. Processing images by these cameras revealed that the resulting landing accuracy was 1 m, which was within the L08-E1 requirement accuracy of 3 m. The sampling location has been also resolved precisely, which was 20 cm off from the center of L08-E1 (Figure 24).
Figure 22: ONC-W1 image captured after touchdown on 21 February 2019 at 22:30 UT at the altitude of approximately 25 m. Boulder distributions around the dropped TM. The circle indicates the 6 m circle of the landing target L08-E1, and a bright dot at the arrow tip is the Target Marker (image credit: Hayabusa-2 Team)
- The landing site L08-E1 was then nicknamed “Tamatebako” (meaning “Treasure Box”) by the Hayabusa-2 project team.
- At this moment, as we had spent 4 months longer than the original plan to realize the first touchdown, a decision was made to cancel the second touchdown, which had been performed after the first touchdown, and to directly move on to the kinetic impact operation.
- Kinetic Impact Operation: The kinetic impact is to be done by SCI (Small Carry-on Impactor), Figure 15 bottom-right. It has a mass of 15 kg and is to be separated from the bottom of the spacecraft. SCI is to be ignited by an SCI-onboard timer, and the 2 kg copper bullet is to be ejected at the speed of 2 km/s. For the operation planning purpose, the targeting accuracy from the nominal stand-off distance of 500 m was set to be as conservative as 200 m (3σ), which included the accuracy of SCI acceleration itself as well as the spacecraft attitude and guidance errors. Because of this relatively large dispersion, SCI was planned to be released exactly on the Sub-Earth line, leading to a constraint that the center of the impact target should be on the equatorial ridge of Ryugu.
- The target point of the kinetic impact was called “S01” region, which is located on the equatorial ridge of Ryugu, apart longitude-wise from the L08 landing site (Figure 25). There are two requirements on the kinetic impact target site; (i) maximizing the possibility to create a large crater detectable by the post-impact observations by the onboard instruments, and (ii) maximizing the landability in terms of terrain condition (for safety). S01 was found through the global survey activity which had been done in parallel with the first touchdown activity. Due to the highly homogeneous surface property of Ryugu, the main driver for selecting the impact site was (ii) rather than (i). S01 was found to maximize (but not to fulfil due to the very large impact dispersion) the landability condition.
- Prior to the SCI operation, two critical operations were performed. One is the S01 low altitude descent observation (DO-S01, March 6-8, 2019), another is a pre-impact crater search operation (CRA1, March 19-22, 2019). These two operations provided the original (pre-impact) terrain information around the SCI target. After the SCI operation, a post-impact crater search operation (CRA2) was conducted, which enabled us to compare images before and after the impact.
- The SCI operation was conducted on April 14-16. The spacecraft reached the bottom altitude of approximately 500 m and released the SCI on 5 April 5 at 1:56 UT. The spacecraft autonomy worked perfectly as planned and followed the pre-programmed path to the backside of the asteroid from the impact point to avoid the ejecta and debris of the SCI ignition and impact. On its way of escaping, the Hayabusa-2 released a deployable camera called “DCAM3” (Figure 15 bottom-left) at 2:14 UT, which successfully observed the impact event (Figure 26). The impact occurred on 5 April 5 at 2:37 UT.
- As the result of the CRA2 operation conducted on April 13-16, the artificial crater generated by SCI was found (Figure 27). The diameter of the crater was measured to be 13 m, and was located approximately 25 m off from the aim point, which is far better targeting accuracy than anticipated. The SCI crater was then nicknamed “Omusubi-Kororin (meaning “rolling rice ball” from a Japanese fairly tail).
Figure 26: Impact event captured by two cameras equipped on DCAM3. Left: DCAM3-A (640 x 480 pixels analog camera), 2 seconds after impact. Right: DCAM3-D (2000 x 2000 pixels digital camera), 3 seconds after impact (image credit: Hayabusa-2 Team)
Figure 27: Terrain change by kinetic impact operation. Left: impact point observed before the SCI impact in CRA1 operation. Right: after the SCI impact observed in CRA2 operation. The white dashed line shows the crater diameter of 13 m (image credit: Hayabusa-2 Team)
- Second Touchdown and Sampling Operation: The objective of the second touchdown (codenamed as PPTD) is to sample subsurface material exposed by the kinetic impact. By evaluating images from the CRA2 operation, and by applying the same landability condition as the TD1-L08E1 operation, as many as 11 touchdown candidate sites with wider than 6 m diameter flat region were found within 30 m from the crater. On the other hand, a scientific evaluation revealed that the ejecta deposit distribution is thick in the North of the crater and very faint in the South (Figure 28). Hence the project chose C01-Cb site which is closest among the sites located North to the crater. C01-Cb spans 7 m in diameter and is located 20 m distant from the center of the SCI crater.
- Prior to PPTD, three rehearsal descents were conducted for the purpose of landing site survey and operation practice. In the second rehearsal descent (designated as PPTD-TM1A), a TM was dropped in the C01-Cb site. The operation was successful, and the TM was placed within the C01-Cb region, just 2.6 m off from the center (Figure 29). The project team decided to use this TM for PPTD, and to aim at the center of C01-Cb for landing.
- The spacecraft suffered degradation in two optical instruments ONC-W1 and LRF in the TD1-L08E1 operation. These two instruments play critical role in the low altitude autonomous touchdown sequence. Due to a dynamic reaction from the surface soil in the TD1-L08E1 operation (as seen in Figure 23), optical lenses have been covered by dust and the sensitivity was reduced by approximately 70%. To compensate for this optical degradation, the altitude for entering the full-autonomous mode was lowered to 30 m from the original 45 m. This required better guidance accuracy above 30 m to appropriately “lock” the TM by the onboard camera with narrower FOV coverage, and also finer altitude control precision below 30 m to guarantee collision-safe maneuver against closer terrain.
Figure 28: Map of ONC-T v-band reflectance factor difference between before and after the SCI impact around the SCI crater. Dark region corresponds to thick ejecta deposit. Blue ellipsoids indicate identified touchdown candidate sites. C01-Cb is the decided touchdown target for PPTD (image credit: Hayabusa-2 Team)
- The spacecraft left HP at the descent rate of 0.4m/s on July 10, 2019. The descent rate was reduced to 0.1 m/s at the altitude of 5 km a half day later. The project gave “Go” for the full-autonomous mode on 11 July 11 at 00:12 UT. The spacecraft reached 30 m at 00:41 UT and successfully locked the TM with ONC-W1. The all autonomous sequence proceeded perfectly as planned, and the touchdown to the ground occurred on 11 July at 01:06:18 UT. The sample container room C was used in PPTD, which was closed after we confirmed the successful touchdown at 5:10 UT on 11 July 2019.
- The post-operation analysis revealed that the landing accuracy for PPTD was 0.6 m (Figure 30). The landing site C01-Cb was then nicknamed “Uchideno-kozuchi” (meaning “Treasure Hummer”) by the Hayabusa-2 project team.
Figure 29: Left: Landing dispersion analysis. Green dots indicate the landing dispersion obtained by a Monte-Carlo simulation. Blue region indicates safe for landing, and red region indicates unsafe. Right: High resolution image taken in PPTD-TM1B operation. Red ellipsoids indicate boulders marked for special safety caution with their estimated height. “TM” indicates the location of the target maker dropped in PPTD-TM1A operation, and “C01-Cb” indicates the landing target of the PPTD operation (image credit: Hayabusa-2 Team)
- Target Markers/MINERVA-II-2 Orbiting Operation: Two more semi-critical operations were conducted after PPTD. One is the Target Markers (TMs) orbiting operation (TM-ORB), in which the two TMs were released from the spacecraft at the terrain altitude of 1 km with fine-tuned velocity. The TM-ORB operation was planned to provide an additional contribution to the gravity science, and was also prepared as a rehearsal for the following MINERVA-II-2 orbiting operation. The first TM was inserted onto an equatorial orbit at 16:17:40 UT on 16 September 16 2019, and the second TM was inserted onto a polar orbit at 16:24:20 UT on the same day. The TMs were then observed from the spacecraft holding at 20 km sub-solar point using ONC-T camera from the separation until 23 September successfully.
- The last semi-critical operation was the MINERVA-II-2 orbiting operation (MNRV-ORB), in which the MINERVA-II-2 rover, developed by Tohoku University and the Japanese University Consortium, was released from the spacecraft at the terrain altitude of 1 km onto an equatorial orbit. The main computer of the MINERVA-II-2 rover had been found to be nonfunctional, and therefore the objective of the mission was changed to more focus on the behavior of the rover from separation until landing rather than post-landing activity. The separation occurred at 15:57:20 UT on 2 October 2019 and the orbiting MINERVA-II-2 rover was observed from the spacecraft’s ONC-T/ONC-W1 camera at the altitude of 8-10 km from the separation until 8 October. The radio link between Hayabusa-2 and MINERVA-II-2 was also confirmed. These measurements are to be also used for the gravity science.
- In summary, Hayabusa-2 arrived at an unexplored C-type asteroid Ryugu, and conducted all the planned missions successfully. Fundamental scientific questions about Ryugu have been answered through successful scientific activities, and more fundamental questions have been raised. From the engineering point of view, during one and a half year of the Ryugu-proximity phase, Hayabusa-2 has achieved seven “world’s firsts”, that are (1) mobile activity of rovers on small body, (2) multiple rover deployment on a small body, (3) 60 cm-accuracy landing and sampling, (4) artificial crater forming and observation of impact process, (5) multiple landing on extra-terrestrial planet, (6) subsurface material sampling, (7) Smallest-object constellation around extra-terrestrial planet. Hayabusa-2 will leave Ryugu in the middle of November 2019 and is scheduled to return to Earth in November-December, 2020.
• July 15, 2019: Ryugu and other asteroids of the common ‘C-class’ consist of more porous material than was previously thought. Small fragments of their material are therefore too fragile to survive entry into the atmosphere in the event of a collision with Earth. This has revealed the long-suspected cause of the deficit of this meteorite type in finds on Earth. Researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have come to this conclusion in a scientific paper published in the journal Nature Astronomy. The results are based on high-resolution measurements of the surface temperature with the DLR radiometer MARA on board the German-French Mobile Asteroid Surface Scout (MASCOT) lander. On 3 October 2018, as part of the Japanese Hayabusa-2 mission, MASCOT descended onto the almost 1 km diameter asteroid Ryugu and sent spectacular images and physical measurements from the surface back to Earth. 19) 20)
- “Ryugu surprised us,” says Matthias Grott, Principal Investigator for the MARA radiometer experiment at the DLR Institute of Planetary Research in Berlin and lead author of the study. “On the asteroid, we observed only larger fragments that are highly porous and probably very fragile.” Earlier telescopic infrared light curves of such carbon-rich asteroids acquired from Earth had been interpreted by researchers studying their thermal properties as bodies covered in sand- to pebble-sized particles. In total, 21 DLR scientists from institutes in Berlin, Bremen and Cologne participated in the study, together with international partners. “MASCOT has brought together DLR’s broad range of expertise in space research – from design, development and testing to experience in the scientific exploration of the Solar System,” says Hansjörg Dittus, DLR Executive Board Member for Space Research and Technology. “The first published results are impressive proof of this.”
Deficit in meteorite finds reveals the protection provided by Earth's atmosphere
- Until now, only a few chondritic meteorites found on Earth have been identified as fragments of C-type asteroids, which are very common in the Solar System ('C' is the chemical symbol for the element carbon). Chondrules are small, millimeter-sized rock globules that formed in the solar nebula 4.5 billion years ago and are considered to be the basic building blocks of planet formation. "We can now confirm that fragments of these asteroids are very likely to break up further when they enter Earth’s atmosphere, and then usually burn up completely. This means that only the largest fragments reach the Earth’s surface,” explains Grott. “That is why meteorites from this type of asteroid are so rarely found on Earth.”
- The good news is that, because of this, Earth’s atmosphere offers increased protection from C-type asteroids, which account for 75 percent of all asteroids. Ryugu is a C-class asteroid, a carbon-rich representative of the oldest bodies in the 4.5 billion-year-old Solar System, and thus a building block of planet formation. It is one of the oldest of the 17,000 asteroids with orbits known to intersect that of Earth. However, further research is necessary to determine the maximum asteroid size for which this atmospheric protection is effective.
- The international research team led by Matthias Grott determined the increase and decrease of the surface temperature over the course of the asteroid’s roughly seven-and-a-half-hour diurnal cycle. This was accomplished by measuring the infrared radiation emitted by the surface during the day and at night, using the MARA radiometer. The MARA measurements made it possible to infer the thermal properties and density of the material. The data from MASCOT were transmitted to the Japanese Hayabusa-2 spacecraft. The craft was located at an observing position 3 km above the asteroid’s surface. From there, MASCOT sent all measurement and operational data to Earth.
- MASCOT landed on asteroid Ryugu on 3 October 2018 by free falling at walking pace. Six minutes after separating from Hayabusa-2 at an altitude of 42 m, the lander touched down on the asteroid’s surface having followed a ballistic trajectory. MASCOT bounced back up several meters, before the 10 kg experiment package finally came to rest. A rotating swing arm allowed MASCOT to turn to the 'correct' side and ‘hop’ across the surface. In total, MASCOT was active on Ryugu for 17 hours, one hour longer than anticipated.
- The gravitational attraction of Ryugu is 66,500 times weaker than that of Earth, so the small amount of momentum produced by the arm was sufficient. This technical innovation for an unconventional form of mobility on an asteroid surface was used for the first time in the history of space exploration as part of the Hayabusa-2 mission. The Hayabusa-2 mission on Ryugu will continue until the end of 2019, with the goal of returning samples of the asteroid material to Earth by 2020. On 11 July, Hayabusa-2 successfully completed the second touchdown operation on the asteroid.
About the Hayabusa-2 mission and MASCOT
- Hayabusa-2 is a Japanese space agency (Japan Aerospace Exploration Agency; JAXA) mission to the near-Earth asteroid Ryugu. The German-French lander MASCOT carried on board Hayabusa-2 was developed by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and built in close cooperation with the French space agency CNES (Centre National d’Etudes Spatiales). The scientific experiments on board MASCOT were devised by DLR, the Institut d’Astrophysique Spatiale and the Technical University of Braunschweig. The MASCOT lander and its experiments are operated and controlled by DLR with support from CNES and in constant interaction with the Hayabusa-2 team.
- The DLR Institute of Space Systems in Bremen was responsible for developing and testing the lander together with CNES. The DLR Institute of Composite Structures and Adaptive Systems in Braunschweig was responsible for the stable structure of the lander. The DLR Robotics and Mechatronics Center in Oberpfaffenhofen developed the swing arm that allowed MASCOT to hop on the asteroid. The DLR Institute of Planetary Research in Berlin contributed the MASCAM camera and the MARA radiometer. The asteroid lander was monitored and operated from the MASCOT Control Center in the Microgravity User Support Center (MUSC) at the DLR site in Cologne.
• July 11, 2019: Japan's Hayabusa-2 probe made a "perfect" touchdown Thursday on a distant asteroid, collecting samples from beneath the surface in an unprecedented mission that could shed light on the origins of the solar system. "We've collected a part of the solar system's history," project manager Yuichi Tsuda said at a jubilant press conference hours after the successful landing was confirmed. "We have never gathered sub-surface material from a celestial body further away than the Moon," he added. "We did it and we succeeded in a world first." 21)
- The fridge-sized probe made its second landing on the asteroid around 10:30 am (01:30 GMT), with officials from JAXA (Japan Aerospace Exploration Agency) breaking into applause and cheers as initial data suggested the touchdown had been a success.
- Confirmation of the landing came only after Hayabusa2 lifted back up from the asteroid and resumed communications with the control room.
- Research director Takashi Kubota told reporters that the touchdown operation was "more than perfect." - And Tsuda, with a grin, said he rated it "1000 points out of 100."
- "The probe moved perfectly and the team's preparation work was perfect," he said.
Figure 31: Officials from Japan's space agency celebrated news of Hayabusa-2's successful second touchdown on the asteroid Ryugu (image credit: JAXA)
- The brief landing Thursday is the second time Hayabusa2 has touched down on the desolate asteroid Ryugu, some 300 million km from Earth.
- Ryugu, which means "Dragon Palace" in Japanese, refers to a castle at the bottom of the ocean in an ancient Japanese tale.
- The complex multi-year Hayabusa-2 mission has also involved sending rovers and robots down to the surface.
- Thursday's touchdown was intended to collect pristine materials from beneath the surface of the asteroid that could provide insights into what the solar system was like at its birth, some 4.6 billion years ago.
- To get at those crucial materials, in April an "impactor" was fired from Hayabusa-2 towards Ryugu in a risky process that created a crater on the asteroid's surface and stirred up material that had not previously been exposed to the atmosphere.
- Hayabusa-2's first touchdown was in February, when it landed briefly on Ryugu and fired a bullet into the surface to puff up dust for collection, before blasting back to its holding position.
- The second touchdown required special preparations because any problems could mean the probe would lose the precious materials already gathered during its first landing.
Figure 32: Overview of the Hayabusa-2 mission to asteroid Ryugu (image credit: CNES, JAXA, NASA)
• On 11 July 2019, the Hayabusa-2 spacecraft performed a 2nd touchdown on the surface of asteroid Ryugu. The touchdown occurred at 10:06 JST (Japan Standard Time) at the onboard time and was successful. Samples of asteroid Ryugu were collected. Below we show images taken before and after the touchdown. As this is a quick bulletin, more detailed information will be given in the future. 22)
- Images were acquired with ONC-W1 (Optical Navigation Camera – Wide angle). Immediately after touchdown, we captured images with the ONC-W1. Here are two bulletin images from this camera.
Figure 33: This image was taken on 11 July 2019 at 10:06:32 JST (onboard time, 01:06:32 UTC) with the ONC-W1 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST)
Figure 34: This image was taken on 11 July 2019 at 10:08:53 JST (onboard time, 01:08:53 UTC) with the ONC-W1 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST)
The following images were taken with CAM-H (Small Monitor Camera-Head). CAM-H operated before and after touchdown, capturing images 4 seconds before touchdown, the moment of touchdown and 4 seconds after touchdown. (CAM-H is the camera that was developed and installed on Hayabusa-2 through public donations. The field of view is downwards beside the sampler horn.)
Figure 35: This image was taken 4 seconds before touchdown with CAM-H (image credit: JAXA)
Figure 36: This image was acquired at the moment of touchdown with CAM-H (image credit: JAXA)
Figure 37: This image was acquired 4 seconds after touchdown with CAM-H (image credit: JAXA)
• April 25, 2019: JAXA separated the SCI (Small Carry-on Impactor), which had been onboard the asteroid explorer Hayabusa-2, on April 5, 2019, for deployment to Ryugu, and then put the SCI into operation. 23) — Hayabusa-2 is operating nominally.
- As a result of checking the images captured by the Optical Navigation Camera - Telescopic (ONC-T) onboard the asteroid explorer Hayabusa-2, we have concluded that a crater was created by the SCI.
Figure 38: ONC-T images. The left image was acquired on 22 March 2019. The right image was acquired on 25 April 2019. By comparing the two images, we have confirmed that an artificial crater was created in the area surrounded by dotted lines. The size and depth of the crater are now under analysis (image credit: JAXA, The University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, The University of Aizu, AIST)
- After the start of the operation, the camera (DCAM3) separated from Hayabusa-2 captured an image that shows ejection from Ryugu’s surface, which implies that the SCI had functioned as planned. The SCI was released from the spacecraft, carrying a plastic explosive charge that shot a 2.5 kg copper projectile at the surface of the 900 m diameter Ryugu asteroid at a velocity of around 2 km/s. The objective is to uncover subsurface material to be brought back to Earth for detailed analysis.
Figure 39: Plume from impact. This image, captured by the camera separated from Hayabusa2 (DCAM3), shows ejection from Ryugu’s surface, which was caused by the collision of the SCI against Ryugu. The image was taken at 11:36 a.m. JST on April 5, 2019 (image credit: JAXA/The University of Tokyo/Kochi University/Rikkyo University/Nagoya University/Chiba Institute of Technology/Meiji University/The University of Aizu/AIS)
• March 19,2019: The first data received from the Hayabusa-2 spacecraft in orbit of asteroid Ryugu helps space scientists explore conditions in the early solar system. The space probe gathered vast amounts of images and other data which gives researchers clues about Ryugu's history, such as how it may have formed from a larger parent body. These details in turn allow researchers to better estimate quantities and types of materials essential for life that were present as Earth formed. 26) 27)
- "The ground shook. My heart pounded. The clock counted. 3... 2... 1... Liftoff!" regaled Professor Seiji Sugita of the University of Tokyo's Department of Earth and Planetary Science. "I've never felt so excited and nervous at the same time, that wasn't just another science experiment on top of that rocket. That was the culmination of my life's work and the hopes and dreams of my entire team."
- On Wednesday, 3rd December 2014, an orange and white rocket over 50m tall weighing almost 300 tons launched from Tanegashima Space Center in South West Japan and successfully sent the Hayabusa2 spacecraft hurtling into space. Its carefully calculated trajectory swung Hayabusa2 round the Earth to pick up speed so it could reach its destination in the asteroid belt between Mars and Jupiter. The target was the asteroid Ryugu and Hayabusa2 arrived on schedule on Wednesday 27 June 2018.
- Since then the spacecraft has used a wide range of cameras and instruments to collect images and data about Ryugu which it continually sends to researchers back on Earth. It has even made a brief soft landing in preparation for a second where it will collect loose surface material — regolith — to return to Earth. We'll have to wait until 2020 before that sample returns, but researchers are far from idle in the meantime.
- "Just a few months after we received the first data we have already made some tantalizing discoveries," said Sugita. "The primary one being the amount of water, or lack of it, Ryugu seems to possess. It's far dryer than we expected, and given Ryugu is quite young (by asteroid standards) at around 100 million years old, this suggests its parent body was much largely devoid of water too."
- According to colleagues of Sugita writing in a companion paper, various instruments on Hayabusa2 including a visible-light camera and a near-infrared spectrometer confirm the lack of water. This fact is important as it's thought all of Earth's water, including that comprising 70% of you, came from local asteroids, distant comets and the nebula or dust cloud that became our sun. The presence of dry asteroids in the asteroid belt would change models used to describe the chemical composition of the early solar system. But why does this matter?
- "Life," explained Sugita. "This has implications for finding life. There are uncountably many solar systems out there and the search for life beyond ours needs direction. Our findings can refine models that could help limit which kinds of solar systems the search for life should target."
- But there's more to this than water; other compounds crucial to life exist in asteroids and Ryugu has some surprises here too. To understand why, it's important to know that Hayabusa-2 is not the only terrestrial robot out there exploring asteroids right now. In 2016 NASA launched OSIRIS-REx which arrived at its target asteroid Bennu on 3 December 2018, four years to the day from the launch of Hayabusa-2.
- The two projects are not in competition but actively share information and data which could help one another. Researchers compare their asteroids to learn even more than would be possible if they could only probe one. Although alike in most ways, Bennu and Ryugu differ significantly in some areas. They are both extremely dark, have spinning-top-like shapes and are covered in large boulders, but Ryugu contains far less water. This discrepancy has researchers scratching their heads.
- "I hoped the surface of Ryugu had more variety as previous ground-based observations had suggested. But every surface feature and boulder on Ryugu seems to be like every other, showing the same scarcity of water," said Sugita. "However, what felt limiting is now enlightening; Ryugu's homogeneity demonstrates the capacity of our instruments to capture nuanced data. It also serves as a necessary constant to compare subsequent data against. So much of science is about controlling variables and Ryugu does this for us."
- As Hayabusa-2 continues to explore our little rocky neighbor researchers gradually piece together its history, which is entwined with our own. Sugita and his colleagues believe Ryugu comes from a parent asteroid several tens of kilometers wide, most likely in the asteroid families Polana or Eulalia.
Figure 40: An oblique view of Ryugu showing the circum-equatorial ridge (yellow arrows), trough (blue arrows) extending from the equatorial region through the south polar region to the other side of Ryugu, and the large and bright Otohime Saxum (red arrow) near the south pole. The location of the poles and the spin direction are indicated with white arrows (image credit: 2019 Seiji Sugita et al., Science)
- "Thanks to the parallel missions of Hayabusa-2 and OSIRIS-REx, we can finally address the question of how these two asteroids came to be," concludes Sugita. "That Bennu and Ryugu may be siblings yet exhibit some strikingly different traits implies there must be many exciting and mysterious astronomical processes we have yet to explore."
Figure 41: Hayabusa2 touchdown on Asteroid Ryugu. JAXA’s Asteroid Explorer “Hayabusa2” collected a sample from asteroid Ryugu on 22 February 2019. The touchdown was captured using the onboard small monitor camera (CAM-H). The image of the site immediately after touchdown was taken with the Optical Navigation Camera – Wide angle (ONC-W1) on 22 February 2019 (video credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST, Published on 5 March 2019)
• February 22, 2019: JAXA executed the asteroid explorer Hayabusa-2 operation to touch down the surface of the target asteroid Ryugu for sample retrieval. Data analysis from Hayabusa-2 confirms that the sequence of operation proceeded, including shooting a projectile into the asteroid to collect its sample material. The Hayabusa-2 spacecraft is in nominal state. This marks the Hayabusa-2 successful touchdown on Ryugu. 28)
Figure 42: Schematic of the TD1 (Touchdown 1)-L08E1 operation (image credit: JAXA)
Figure 43: TD1-L08E1 low altitude sequence (image credit: JAXA)
- The descent to Ryugu began on 21 February at 4:45 UTC, a delay of about 5 hours later than initially planned. The reason for the delay wasn't clear, but mission controllers made up for lost time by sending Hayabusa-2 towards Ryugu at a speed of 90 cm/s instead of 40 cm/s. Around the same time, images from the spacecraft's optical navigation cameras started coming in, and continued to do so until the spacecraft crossed beneath 200 meters shortly after 22:02 UTC. 29)
- At the 45-meter hold point, Hayabusa-2 oriented itself for landing and turned its high-gain antenna away from Earth, shutting off the flow of telemetry in the process. From there, mission controllers could only watch for Doppler shifts in the signal from Hayabusa-2's low-gain antenna, indicating the spacecraft had pushed its sample horn into Ryugu and was starting to ascend.
- That shift occurred around 22:49 UTC.
Figure 44: Hayabusa-2 photo of Ryugu during sampling descent. Hayabusa-2 took this photo with its optical navigation camera at an altitude of about 180 meters, before it entered its final descent to grab a sample from asteroid Ryugu, on 21 February 2019 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST)
- A cheer erupted at JAXA's mission control center.
- Hayabusa-2 began to ascend, and just minutes later was able to swing its high-gain antenna back toward Earth. Mission controllers confirmed that the spacecraft was healthy and the command to fire the tantalum bullet executed as expected.
- The next step will be for JAXA to download imagery, particularly from the camera on the sample horn, and further confirm the touchdown sequence went as planned.
Figure 45: Celebration in Hayabusa-2 mission control room after successful touchdown on Ryugu. A packed control room celebrates a subtle shift in Hayabusa-2's radio signal, marking the moment of its touchdown and sample grab on Ryugu on 21 February 2019 at 22:49 UTC (image credit: JAXA)
• February 22, 2019: Up until now, the Hayabusa-2 mission has progressed smoothly. One particular success was the landing of the small rovers on the surface of Ryugu, which could not be achieved during the first Hayabusa mission. Now on February 22, 2019, we plan to touchdown on the asteroid surface; another challenge that did not go as expected for Hayabusa. 30)
- The original schedule was planned for touchdown in late October of last year (2018). However, Ryugu was revealed as a boulder strewn landscape that extended across the entire surface, with no flat or wide-open regions. Before arriving at Ryugu, it was assumed there would be flat areas around 100 meters in size. But far than finding this, we have not even seen flat planes 30 meters across!
- During the scheduled time for touchdown in late October, we did not touchdown but descended and dropped a target marker near the intended landing site. We were able to drop the target marker in almost the planned spot and afterwards we examined the vicinity of the target marker landing site in detail. Finally, the area denoted L08-E1 was selected as the place for touchdown. L08-E1 will be described later (Figure 48), but the final area where the touchdown is planned is a region of radius 3 m within L08-E1 as shown in Figure 46.
Figure 46: Location where Hayabusa-2 will touchdown. The touchdown aims for inside the purple circle (about 6 m diameter). The cross indicates the location of the target marker. The illustration of the spacecraft in the lower left is the same scale as the picture (image credit: JAXA)
- During touchdown, the spacecraft will descend towards the center of the circle shown in Figure 46, which is located 4 or 5 m away from the target marker location. As the guidance error of the spacecraft is a maximum of 2.7 m, the spacecraft can land in a circle of radius 3 m. Although this size of the site is just barely sufficient, we will try to touchdown here.
Figure 47: Figure 2 shows an animation of the area in Figure 46 in three dimensions (3D) using a DEM (Digital Elevation Map). You can see the elevation of the area surrounding the touchdown location (video credit: JAXA)
- Prior to Hayabusa's arrival at Ryugu, the project planned to touchdown in a flat area about 100 m wide, but ultimately, we selected a region with a diameter of about 6 m (radius 3m). We were able to improve the necessary landing accuracy using a technique called “pinpoint touchdown”. Pinpoint touchdown was originally planned for touching down around the artificial crater generated with the onboard small carry-on impactor (SCI), but the environment we discovered on the asteroid surface has made this method necessary from the start.
• February 18, 2019 updated: On December 28 —the last day of work in 2018— the sampler team conducted an important experiment. As a final test before touchdown (TD), the team fired an identical bullet to that onboard Hayabusa-2 into a simulated soil of the surface of Ryugu to test how much sample would be ejected. 31) 32)
- Hayabusa-2 uses a projector to inject metal bullets into the asteroid surface and release material, before passively collecting these samples through the sampler horn. This projector, including the pyrotechnic products, were manufactured with multiple flight spares (equivalent products manufactured at the same time as the flight model, Figure 49).
- The original purpose of this experiment was to confirm that one month before the TD operation, the flight spare projector was operating normally after it had been stored for a long period of four years.
- As we now know, the expected topography of a powdery fine regolith was not found on the surface of Ryugu. But cm-sized or larger gravel was observed by the MASCOT and MINERVA-II1 rovers that landed on the asteroid surface. This is quite different from the prediction before launch, so it took time to investigate the safety of the spacecraft during TD. Additionally, it was necessary to review whether sample material would still be released from the asteroid surface as originally assumed.
- Therefore, we decided to use the flight spare projector to perform an operation confirmation test, whereby we examined what happens if a bullet identical to that onboard Hayabusa2 is fired into a target that simulates the observed surface of Ryugu.
Figure 49: The projector (barrel) and the projectile (bullet) used in the experiment. As this is a flight spare, the shape and the material are all the same as those of onboard Hayabusa-2 (image credit: JAXA)
• January 21, 2019: Place names for locations on the surface of Ryugu were discussed by Division F (Planetary Systems and Bioastronomy) of the International Astronomical Union (IAU) Working Group for Planetary System Nomenclature (hereafter IAU WG) and approved in December 2018. We will introduce the place names in this article and the background to their selection. 33)
- As the appearance of Ryugu gradually became clear during the approach phase in June 2018, we used nicknames amongst the Hayabusa-2 Project team to distinguish regions of the terrain. (For example, the crater now named “Urashima” was referred to as the Death Star crater in Star Wars!) However, in order to introduce Ryugu to the world, it is necessary to have names that are intentionally recognized rather than nicknames, which can be referred to in scientific papers and other articles. Therefore, the discussion regarding naming the Ryugu surface topology began within the team.
- To name a place on a celestial body in the Solar System, you must first decide on a theme. For example, the theme for places on Venus is the “names of goddesses”. During discussions between the domestic and overseas project members, suggestions such as “names of castles around the world”, “word for ‘dragon’ in different languages” and the “names of deep-sea creatures” were proposed for the place name theme on Ryugu. After an intense debate, the theme was selected to be “names that appear in stories for children” and a theme proposal was put to the IAU WG. The proposal was accepted on September 25, after which the discussion moved to selecting the topographical features to be named and the choice of name.
- Names cannot be attributed to any location. Instead, there are restrictions on the places that can be assigned an official name involving considerations such as scientific importance or size on the celestial body. With this in mind, volunteers from the project members as well as planetary geology experts (hereinafter referred to as the Place Name Core Members) discussed the place selection and completed the application forms for naming based on the exploration data. On October 12, we proposed 13 place names to the IAU WG. After additional discussion with the WG, 9 were accepted as proposed by the team and the remaining 4 names were approved after an amendment suggested by the IAU.
- The surface of celestial bodies has a range of different topologies. We applied to give names to four different topology types on the Ryugu surface. The first type is “dorsum” which originates from the Latin for peak or ridge. The second type is “crater” which are familiar structures on the Moon and asteroids. Then “fossa” meaning grooves or trenches and finally the Latin word “saxum” for the rocks and boulders that are a main characteristic of the Ryugu terrain. Saxum is actually a new classification of terrain type that we applied to introduce due to the nature of Ryugu.
- Numerous boulders are distributed on the surface of Ryugu. Regardless of where you look, there are rocks, rocks and more rocks. This is a major characteristic of Ryugu and continues to make plans for the touchdown operation of the spacecraft difficult. Additionally, spectroscopic observations revealed that the giant boulder (Otohime saxum) at the south pole has not only a substantial size, but also a distinct visible light spectrum that reveals materials and surface conditions that are different from the surrounding areas. Since this boulder is the most important topographical feature for understanding the formation history of Ryugu, the Project strongly hoped to name it. However, there was no precedent for boulder nomenclature and even the name type did not exist (during the exploration of the first Hayabusa mission, naming the huge boulder protruding from asteroid Itokawa was not allowed). We therefore proposed the type name for boulders at the same time as applying for the place names. Since terrain type names are usually Latin, we proposed “saxum” (meaning rocks and stones in Latin) as the type name for boulders. The IAU accepted this nomenclature for boulders with a few conditions (such as the boulder must be 1% or more of the diameter of the celestial body) and the type name that we suggested was adopted (!). This is how the new terrain type “saxum” was born.
- Figure 50 shows a map of Ryugu with the place names labelled. Additionally, Figure51 shows the location of the places on images of Ryugu taken from four different directions. In these figures, the north pole of Ryugu is at the image top. Please keep in mind that the north pole of Ryugu is in the same direction as the south pole on Earth, as Ryugu rotates in the opposite direction. Table 5 shows a list of the place names.
Figure 50: Map of Ryugu showing the place names. Trinitas and Alice’s Wonderland are nicknames of the MINERVA-II1 and MASCOT landing sites, respectively, and not place names recognized by the IAU (image credit: JAXA naming team)
Figure 51: The location of place names on Ryugu. Trinitas and Alice’s Wonderland are nicknames of the MINERVA-II1 and MASCOT landing sites, respectively, and not place names recognized by the IAU (image credit: JAXA naming team)
- As it is difficult to get a feel for how the place names were chosen from just a list, we will introduce the story behind the main choices below.
- The asteroid name “Ryugu” comes from the Japanese fairy tale of Taro Urashima. In the story, Urashima is a fisherman who rescues a sea turtle from the cruelty of a group of children. The turtle takes Urashima to the underwater palace of Ryugo-jo (Dragon Palace), where he meets the princess, Otohime. After 3 years, Urashima wishes to return home and is given a treasure box (tamatebako) by Otohime with instructions never to open it. But when Urashima returns to the surface, he discovers everything he knew has changed as 300 years has actually past. In confusion, Urashima opens the treasure box and is engulfed in white fog. When it clears, he has become an old man, as the box contained his age.
- With the name of the asteroid being Ryugu, there was a strong desire from the Project to use other names that appear in Urashima’s story for major asteroid topography. However, place names cannot be common nouns so words such as “sea bream”, “flounder” and “turtle” do not work and we were limited to names such as Taro Urashima, Otohime etc.
- Therefore, “Urashima” was chosen for the biggest crater on Ryugu and “Otohime” for the largest boulder near the south pole. Both of these are very important features for deciphering the formation history of Ryugu. However, Otohime had already been used! Venus (whose place theme uses the names of goddesses) had already a location named Otohime Tholus. Otohime was therefore initially refused by the IAU when it was proposed. But Otohime is an extremely important person in the story of Taro Urashima and how can we collect the tamatebako if Otohime is not on Ryugu?! (That was a joke, but we did want to use such a relevant name.) Since the name was important to the Project, the place name core members refined the proposal to the IAU, explaining why Otohime should be one of the main topological features on Ryugu and this was accepted.
- A defining feature of Ryugu is that the shape is similar to a spinning top or abacus bead. This shape is the combination of two cones which appear almost circular when seen from the north pole. The ridge where they join was named “Ryujin”, after the ruler of the Dragon Palace who is the father of princess Otohime. This name came from the Place Name Core Members who felt the ridge resembled a dragon coiling around the asteroid or an ouroboros (the image of the serpent or dragon that swallows its own tail).
- On either side of Otohime saxum there are large grooves extending in the equatorial direction. In the story of Taro Urashima, Otohime lives in this mysterious place at the bottom of the ocean which is sometimes depicted as a different world in the various retellings of the tale. This world is often called “Horai”, “Tokoyo” or “Niraikanai”. The grooves adjacent to Otohime saxum were therefore named Horai fossa and Tokoyo fossa.
- There is a reasonably big boulder to the southeast of the Urashima crater. According to one version of the tale, the place where Taro Urashima helped the turtle and left to travel to Ryugu-jo is the place “Ejima”, which gave the boulder its name Ejima saxum.
- There are also large craters on both sides of Urashima crater. In particular, there are two craters stuck together along the north-south direction to the west. This state reminded us of the kibidango (Japanese dumplings) in another Japanese fairy tale called Momotaro. The northern crater of the pair was therefore named “Momotaro crater” and the southern crater became “Kibidango crater”. To the east of the Urashima crater, there is a crater with big black boulder inside. This reminded us of the Japanese tale of Kintaro, a boy with super strength who carried a broad-axe, and so was named “Kintaro crater”.
- Ryugu also has topological names derived from children’s stories from outside Japan. For example, while you might not immediately recognize the name of the Cendrillion crater, the name is from the original French name for the familiar fairy tale, “Cinderella”. The name of the Brabo crater is derived from the name of the hero of a Netherlands tale, which was proposed by the overseas project members. The Kolobok crater and Catafo saxum were both names proposed by the IAU WG. They are taken from Russian and Cajun (famous for Cajun cuisine in the USA) folktales.
- These are the place names formally recognized by the IAU WG. In addition, there are two nicknames shown in Figures 50 and 51; Trinitas (the MINERVA-II1 landing site and named for the goddess Minerva’s birth place) and Alice’s Wonderland (the MASCOT landing site). These were places named by the project to identify the points where MINERVA-II1 and MASCOT landed, but are not official names recognized by the IAU.
• December 28, 2018: This week is the second half of the solar conjunction operation. Signals from the beacon operation are becoming cleaner every day and it is possible to differentiate between “0” and “1” without overlapping the signal multiple times. Telemetry can now also be clearly received, transmitting a detailed account about the state of the spacecraft. Data collected while communication with Hayabusa-2 was not possible, was saved to onboard memory. This week, we accessed the recorded data for the first time in a long while and we are relieved to confirm that Hayabusa-2 has been functioning normally during the period when telemetry could not be received. Although the communication environment is returning to normal operation, the return to the home position (at 20 km from the asteroid) is still a little way away. 34)
• December 25, 2018: From late November 2018 until the end of December, the solar conjunction operation is underway for Hayabusa-2. Solar conjunction refers to the situation where the direction to the spacecraft almost overlaps with that to the Sun when viewed from the Earth. This is the same “conjunction” as in astronomy, whereby planets and stars appear to line up on the sky. During this time, communication with Hayabusa-2 is disrupted due to radio waves emitted from the Sun and from its surrounding plasma. We therefore do not perform operations such as descending towards Ryugu during this period. 35)
- In order not to risk a collision with Ryugu while communication is disrupted, we place the spacecraft slightly further away from the asteroid in a “conjunction transition orbit”. Figure 52 shows an animation of the trajectory of the spacecraft in the Hill coordinate system. In this coordinate system, the Sun is always to the left and outside the figure. The black dot on the right is Ryugu. From late November to the end of December, the spacecraft will travel along the blue line. The red dots are points where a trajectory control maneuver will be performed.
• December 21, 2018: Until now, “astrodynamics” has been one of the less frequently reported operations for Hayabusa2. In space engineering, the movement, attitude, trajectory and overall handling of the flight mechanics of the spacecraft is referred to as “astrodynamics”. For example, astrodynamics played an active role in the gravity measurement descent operation in August 2018. While this was a short time ago, let’s look at a few of the details. 36)
- From August 6 - 7, 2018, the “Gravity Measurement Descent Operation” was performed to estimate the strength of asteroid Ryugu’s gravity. Hayabusa-2 initially descended from the home position at an altitude of 20 km to an altitude of 6100 m. Orbital control was then temporarily stopped to allow the spacecraft to “free-fall” towards Ryugu, moving due to the gravitational pull of the asteroid alone. When the altitude decreased to about 850 m, the thrusters were instantaneously fired to give the spacecraft an upward velocity, whereupon Hayabusa-2 performed a “free-rise” to an altitude of about 6100 m (the spacecraft’s movement here is similar to throwing a ball vertically upwards).
- From the spacecraft’s motion during the free-fall and free-rise, the strength of Ryugu’s gravity could be measured and the mass of the asteroid obtained. As a result of this measurement, the mass of Ryugu was calculated to be about 450 million tons.
- The shape and volume of Ryugu are known thanks to the construction of the three-dimensional shape model (article on July 11). Using this volume and the measured mass of Ryugu from the gravity measurement descent operation, the average density of the asteroid can be calculated. The average density and shape of Ryugu could then be used to find the gravitational strength (gravitational acceleration) on the surface of Ryugu, which was found to have the following distribution:
- The gravitational acceleration on the surface of Ryugu is approximately 0.11~0.15 mm/s2, which is about eighty thousandths (~ 1/80000th) the strength of the Earth’s gravity and a few times stronger than that of Itokawa. We can additionally see that the gravity near the poles of Ryugu is stronger than near the asteroid’s equator. This is due to the equatorial ridge protruding from the surface.
- The information on the asteroid’s gravitational acceleration obtained through this method has been used for operations that approach close to the surface of Ryugu. Of course, it will also be used during touchdown. The gravity measurement descent operation described here is one application of astrodynamics. The astrodynamics team for Hayabusa-2 uses a variety of similar methods to estimate the trajectory of the spacecraft and Ryugu, and to evaluate the dynamic environment for operating around Ryugu.
Figure 53: Distribution of the gravitational acceleration on the surface of asteroid Ryugu (image credit: JAXA)
• October 26, 2018: The second touchdown rehearsal (TD1-R1-A) was performed from October 14 to 16. On October 15, just before 22:44 JST when the spacecraft reached a new low altitude of 22.3m, we successfully photographed the surface of Ryugu using the Optical Navigation Camera – Telescopic (ONC-T). This is the highest resolution image to date (Figure 54). 37)
- The image resolution is about 4.6 mm/pixel. This is the highest resolution image that Hayabusa-2 has taken so far and even small rocks with a diameter of 2 – 3 cm are clearly visible. The maximum resolution of AMICA –the camera at the time of the first Hayabusa mission— was 6 mm/pixel, so even its resolution has now been exceeded. As the image captured of the asteroid surface from the spacecraft, it will be one of the highest resolution to be taken of Ryugu (MINERVA-II1 and MASCOT which landed on the surface, have captured even higher resolution images).
- A feature from the image is the lack of regolith (sandy substance). This was suspected to be true from the images obtained so far, but it is more clearly seen in this high resolution photograph. There is also a collection of pebbles with different colors, which may be evidence that the surface material of Ryugu is mixed.
- It is a landmark for the mission that such high resolution images were captured by the spacecraft before landing. Such a detailed image that can be used to visually recognize anything above about 1cm in size is extremely useful in analyzing the surface photographs returned from the MINERVA-II1 rovers and MASCOT lander and also for understanding the microanalysis from the sample once it is returned to Earth.
Figure 54: The surface of Ryugu photographed on October 15 at 22:40 JST using the Optical Navigation Camera – Telescopic (ONC-T). The altitude here is about 42m. (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST)
Figure 55: Surface of Ryugu photographed by the ONC-W1 at an altitude of about 49m. The image was captured on October 15, 2018 at 22:39 JST. The yellow square indicates the image area in Figure 54 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST)
• October 26, 2018: Onboard Hayabusa-2 is a Small Monitor Camera (CAM-H, also called the Small Monitor Camera Head), which was funded by contributions from the public (Figure 1). 38)
Figure 56: The Small Monitor Camera (CAM-H), image credit: JAXA
- The Hayabusa-2 project used this camera to capture a picture of the sampler horn on 14 August. This is shown in Figure 57. From this photograph, we can perform a visual inspection of the sampler horn and confirm it is sound after Hayabusa-2 has arrived at Ryugu.
- In connection with the sampler horn, the project also tested a laser device called the LRF-S2 in April this year. LRF-S2 is designed to measure the distance to the tip of the sampler horn. During touchdown, the sampler horn will compress as it touches the surface of Ryugu. When the distance measurement from LRF-S2 changes or the laser from LRF-S2 deviates from its target at the base of the sampler horn, bullets will be fired to stir up the surface material for collection. This makes the LRF-S2 an important device for successful sample collection. The photograph captured during this test is shown in Figure 58.
Figure 58: Test photograph of the sampler horn taken by the Small Monitor Camera (CAM-H) on April 16, 2018. The shiny part in the red circle is the target plate of the sampler horn which is integrated with the LRF-S2 laser. If the laser misses this plate, touchdown is in process and bullets will fire to stir up surface material (image credit: JAXA)
• October 26, 2018: Approximately one month has passed since the laser altimeter (LIDAR) onboard Hayabusa-2 measured the distance to asteroid Ryugu for the first time. Figure 59 shows the analysis of the data acquired during this period, with each point showing where the laser reflected from the surface of Ryugu. Normally, the attitude of the spacecraft is controlled so that that the laser altimeter faces the equator. But during this operation, the attitude was changed to move along the asteroid’s axis of rotation; this is an “attitude scan” that has been performed twice so far. In the vicinity of the poles, there are currently fewer data points, but you can see the global shape of Ryugu. 39)
- In order to construct this shape, the orbit of the spacecraft must be accurately predicted. The accuracy is currently (in early August) several hundred meters. That the position of a spacecraft 300 million km away from Earth can be determined to within hundreds of meters is impressive, but this error is still too big to describe the shape of the 900 m diameter Ryugu. The project is therefore improving the trajectory of the spacecraft using the distance measurement data from the laser altimeter. The improved orbital data of the spacecraft also aids the other equipment teams on Hayabusa-2 and is being used to select the best landing site.
Figure 59: To create the figure from the LIDAR data, the project used the shape model of Ryugu to guide the three-dimensional structure (image credit: National Astronomical Observatory of Japan (NAOJ), JAXA, Chiba Institute of Technology, University of Aizu, Nihon University, Osaka University)
• October 25, 2018: DPS is the Division for Planetary Sciences of the American Astronomical Society and is one of the largest academic societies for the planetary field in the world. This year will be the 50th meeting for the DPS ( https://aas.org/meetings/dps50 ), held in Knoxville, Tennessee in the USA from October 21 to 26. A special session dedicated to Hayabusa-2 will be held during this meeting, where one session in the conference will be devoted to announcements only from this mission. This is a first for Hayabusa2. Hayabusa2 will also be the subject of a press briefing held during the conference. 40)
- The Hayabusa-2 special session involves thirteen studies presented in the poster session on October 25 and nine oral presentations held on October 26. The titles and abstracts of these research presentations are listed as sessions 411 (poster presentation) and 501 (oral presentation) in the table of contents of the conference abstract book available on the DPS website (https://aas.org/files/final_abstract_program.pdf ). In addition to these special sessions, there is one additional presentation related to Hayabusa2 in session 309 (309.03 in the abstract book). This brings the total number of Hayabusa2 presentations up to 23.
- There will also be a press conference held on October 25 at 12:00 (October 26, 01:00 JST) with the title “Hayabusa-2 Explores Asteroid Ryugu”. The information for this press conference is at: https://aas.org/meetings/dps50/2nd-media-advisory
• October 14, 2018: The second touchdown rehearsal of Hayabusa-2 will be performed from October 14 – 16. The purpose of this rehearsal is to confirm the operation characteristics of the LRF (Laser Range Finder) which performs the altitude measurement at short distances. To test this, the spacecraft will descend to an altitude of about 25 m. This will be the lowest altitude reached to date. 41)
Figure 60: Schematic of the TD1-R1-A operation (image credit:JAXA)
- The first touchdown rehearsal (TD1-R1) was conducted between September 10 – 12. During this operation, problems occurred with the distance measurement taken with the LIDAR (laser altimeter) once the spacecraft had descended to about 600 m. This caused the spacecraft to rise autonomously. The issue was addressed by adjusting the settings on the laser altimeter and it was confirmed that there were no further issues during the subsequent separation operations of MINERVA-II1 and MASCOT. As the spacecraft was only able to descend to an altitude of 600 m during TD-R1, the LRF characteristics could not be verified. Therefore, we are performing this check with this rehearsal.
- Although this is the second time a rehearsal operation has been performed, the name is TD1-R1-A as it is re-starting the challenge of TD1-R1.
• October 12, 2018: Six minutes of free fall, a gentle impact on the asteroid and then 11 minutes of rebounding until coming to rest. That is how, in the early hours of 3 October 2018, the journey of the MASCOT asteroid lander began on Asteroid Ryugu – a land full of wonder, mystery and challenges. Some 17 hours of scientific exploration followed this first 'stroll' on the almost 900 m diameter asteroid. The lander was commanded and controlled from the MASCOT Control Center e at the DLR (German Aerospace Center) site in Cologne in the presence of scientific teams from Germany, France and Japan. MASCOT surpassed all expectations and performed its four experiments at several locations on the asteroid. Never before in the history of spaceflight has a Solar System body been explored in this way. It has now been possible to precisely trace MASCOT’s path on Ryugu’s surface on the basis of image data from the Japanese Hayabusa-2 space probe and the lander’s images and data. 42)
- "This success was possible thanks to state-of-the-art robotic technology, long-term planning and intensive international cooperation between the scientists and engineers of the three space nations Japan, France and Germany," says Hansjörg Dittus, DLR Executive Board Member for Space Research and Technology about this milestone in Solar System exploration. "We are proud of how MASCOT was able to master its way across the asteroid Ryugu over boulders and rocks and send so much data about its composition back to Earth," says DLR Chair Pascale Ehrenfreund.
- MASCOT had no propulsion system and landed in free fall. Six minutes after separating from Hayabusa-2, and following the end of a ballistic trajectory, the landing module made its first contact with asteroid Ryugu. On the surface, MASCOT moved through the activation of a tungsten swing arm accelerated and decelerated by a motor. This made it possible for MASCOT to be repositioned to the 'correct' side or even perform hops across the asteroid's surface. The gravitational attraction on Ryugu is just one 66,500th of the Earth's, so the little momentum provided was enough: a technological innovation for an unusual form of mobility on an asteroid surface used for the first time in the history of space travel as part of the Hayabusa-2 mission.
Through a rock garden full of rough boulders and no flat surfaces
- To reconstruct MASCOT's path across the surface of Ryugu, the cameras aboard the Hayabusa-2 mother probe were aimed at the asteroid. The Optical Navigation Cameras (ONC) captured the lander's free fall in several images, detected its shadow on the ground during the flight phase, and finally identified MASCOT directly on the surface in several images. The pattern of the countless boulders distributed on the surface could also be seen in the direction of the respective horizon in oblique photographs of the lander's DLR MASCAM camera. The combination of this information unlocked the unique path traced by the lander.
- After the first impact, MASCOT smoothly bounced off a large block, touched the ground about eight times, and then found itself in a resting position unfavorable for the measurements. After commanding and executing a specially prepared correction maneuver, MASCOT came to a second halt. The exact location of this second position is still being determined. There, the lander completed detailed measurements during one asteroid day and night. This was followed by a small ‘mini-move’ to provide the MicrOmega spectrometer with even better conditions for measuring the composition of the asteroid material.
- Finally, MASCOT was set in motion one last time for a bigger jump. At the last location it carried out some more measurements before the third night on the asteroid began, and contact with Hayabusa-2 was lost as the spaceship had moved out of line of sight. The last signal from MASCOT reached the mother probe at 21:04 CEST. The mission was over. "We were expecting less than 16 hours of battery life because of the cold night, says MASCOT project manager Tra-Mi Ho from the DLR Institute of Space Systems. "After all, we were able to operate MASCOT for more than one extra hour, even until the radio shadow began, which was a great success." During the mission, the team named MASCOT's landing site (MA-9) 'Alice's Wonderland', after the eponymous book by Lewis Carroll (1832-1898).
Figure 61: MASCOT's approach to Ryugu and its path across the surface (image credit: JAXA/U. Tokyo/Kochi U/Rikkyo U/Nagoya U/Chiba Inst Tech/Meiji U/U Aizu/AIST)
A true wonderland
- Having reconstructed the events that took place on asteroid Ryugu, the scientists are now busy analyzing the first results from the acquired data and images. "What we saw from a distance already gave us an idea of what it might look like on the surface," reports Ralf Jaumann from the DLR Institute of Planetary Research and scientific director of the MASCOT mission. "In fact, it is even crazier on the surface than expected. Everything is covered in rough blocks and strewn with boulders. How compact these blocks are and what they are composed of, we still do not know. But what was most surprising was that large accumulations of fine material are nowhere to be found – and we did not expect that. We have to investigate this in the next few weeks, because the cosmic weathering would actually have had to produce fine material," continues Jaumann.
- "MASCOT has delivered exactly what we expected: an 'extension' of the space probe on the surface of Ryugu and direct measurements on site," says Tra-Mi Ho. Now there are measurements across the entire spectrum, from telescope light curves from Earth to remote sensing with Hayabusa2 through to the microscopic findings of MASCOT. "This will be of enormous importance for the characterization of this class of asteroids," emphasizes Jaumann.
- Ryugu is a C-type asteroid – a carbon-rich representative of the oldest bodies of the four-and-a-half-billion year-old Solar System. It is a 'primordial' building block of planet formation, and one of 17,000 known Near-Earth asteroids.
- On Earth, there are meteorites with a composition that could be similar to Ryugu's, which are found in the Murchison Range, Australia. However, Matthias Grott from the DLR Institute of Planetary Research and responsible for the radiometer experiment MARA is skeptical as to whether these meteorites are actually representative of Ryugu in terms of their physical properties: "Meteorites such as those found in Murchison are rather massive. However, our MARA data suggests the material on Ryugu is slightly more porous. The investigations are just beginning, but it is plausible to assume that small fragments of Ryugu would not survive the entry into the Earth's atmosphere intact."
Table 6: MASCOT's 17 hours and 7 minutes on Ryugu
• October 11, 2018: JAXA told reporters the Hayabusa-2 probe is now expected to touch down on the Ryugu asteroid in "late January" at the earliest, rather than at the end of this month as initially expected. JAXA project manager Yuichi Tsuda said they needed more time to prepare the landing as the latest data showed the asteroid surface was more rugged than expected. 43)
- "The mission... is to land without hitting rocks," Tsuda said, adding this was a "most difficult" operation. "We had expected the surface would be smooth... but it seems there's no flat area."
- Scientists are already receiving data from other subsatellites deployed on the surface of the asteroid. Last week, JAXA successfully landed a new 10 kg observation robot known as MASCOT (Mobile Asteroid Surface Scout). Ten days earlier, a pair of MINERVA-II micro-rovers were dropped onto the asteroid - marking the first time that moving, robotic observation devices have been successfully deployed.
- These rovers are taking advantage of Ryugu's low gravity to jump around on the surface - travelling as far as 15 m and staying above the surface for as long as 15 minutes - to survey the asteroid's physical features with cameras and sensors.
• October 05,2018: It was a day full of exciting moments and a happy team of scientists and engineers: late in the afternoon of 3 October 2018, the German-French lander MASCOT completed its historic exploration of the surface of the asteroid Ryugu at 21:04 CEST, as its battery ran out. On-asteroid operations were originally scheduled to last 16 hours after separation from the Japanese mother craft Hayabusa-2. But in the end, the battery lasted more than 17 hours. Upon landing in the early morning and subsequently relocating using the built-in swing arm, all instruments collected detailed data on the composition and nature of the asteroid. The on-board camera provided pictures of the landing, hopping maneuvers and various locations on the surface. 44) 45)
a) As planned, MASCOT was able to acquire data about the composition and texture of the asteroid at several locations.
b) Before the battery depleted, the lander sent all scientific data to the Hayabusa-2 mothercraft.
c) New images from MASCOT's landing on asteroid Ryugu were presented by DLR, JAXA and CNES today at the International Astronautical Congress (IAC).
e) Focus: Space, exploration.
- For MASCOT, the Sun set three times on Ryugu. The lander was commanded and controlled from the MASCOT Control Center at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) in Cologne, in the presence of teams of scientists from Japan, France and Germany. All scientific data was transferred to the mother probe according to plan.
- "With MASCOT, it has been possible to, for the first time, explore the surface of an asteroid directly on site so extensively," says Hansjörg Dittus, DLR Executive Board Member for Space Research and Technology. "A mission like this can only be done working in close cooperation with international partners – bringing together all their expertise and commitment." With MASCOT, DLR has been working closely with the Japanese space agency JAXA and the French space agency CNES.
Figure 62: Hayabusa2 acquires images of MASCOT on its approach to Ryugu. Three consecutive images acquired on 3 October 2018 between 03:57:54 and 03:58:14 CEST with the wide-angle optical navigation camera (ONC-W2). MASCOT can be seen at the top (image credit: JAXA, Tokyo University, Kochi Univ., Rikkyo Univ., Nagoya Univ., Chiba Institute of Technology, Meiji Univ., Aizu Univ., AIST)
- Jumps and a mini-move: MASCOT landed safely on Ryugu in the early morning of 3 October 2018. "After a first automated reorientation hop, it ended up in an unfavorable position. With another manually commanded hopping maneuver, we were able to place MASCOT in another favorable position thanks to the very precisely controlled swing arm," says MASCOT operations manager Christian Krause from DLR. From that position, MASCOT completed a complete measurement sequence with all instruments over one asteroid day and an asteroid night. "Later, we were able to continue the activities on Ryugu with a special maneuver," adds Ralf Jaumann, DLR planetary scientist and scientific director of MASCOT. "With a 'mini-move' we recorded image sequences that will be used to generate stereo images of the surface once they have been analyzed."
- During the first maneuvers, MASCOT moved several meters to the next measuring point. Finally, and seeing that the lander still had battery power left, the researchers dared to make a bigger jump. All in all, MASCOT explored Ryugu for three asteroid days and two asteroid nights. A day-night cycle on Ryugu lasts about 7 hours and 36 minutes. At 21:04 CEST, communications with Hayabusa-2 were interrupted, because of the radio shadow entering with each asteroid rotation. Hayabusa-2 is now returning to its home position, at an altitude of 20 km above the asteroid’s surface.
- In addition to the images acquired by the DLR camera MASCAM, a DLR radiometer, a magnetometer from TU Braunschweig and a spectrometer from the Institut d'Astrophysique Spatiale provided a variety of measurements on the temperature, magnetic properties and the composition of the near-Earth asteroid Ryugu.
- Waiting for the scientific data: MASCOT is now a silent inhabitant of Ryugu. "The evaluation of the valuable data has just begun," says MASCOT project manager Tra-Mi Ho from the DLR Institute of Space Systems. "We will learn a lot about the past of the Solar System and the importance of near-Earth asteroids like Ryugu. Today, I look forward to the scientific publications that will result from MASCOT and the remarkable Hayabusa-2 mission of our Japanese partners. "Hayabusa-2 played a crucial role in the success of MASCOT. The Japanese probe brought the lander to the asteroid. Thanks to precise planning and control, the communication links to the lander could be optimally used for data transmission, so that the first pictures were received on the very day of landing. The remaining scientific data, which was transmitted to Hayabusa-2, will be sent to Earth in the coming days.
• October 03, 2018: The near-Earth asteroid Ryugu, located approximately 300 million km from Earth, has a new inhabitant: On 3 October 2018, the Mobile Asteroid Surface Scout (MASCOT) landed on the asteroid and began to work. The lander successfully separated from the Japanese Hayabusa-2 space probe at 03:58 CEST (Central European Summer Time, corresponding to 01:58 UTC). The 16 hours in which the lander will conduct measurements on the asteroid’s surface have begun for the international team of engineers and scientists. The day before, JAXA's Hayabusa-2 began its descent towards Ryugu. MASCOT was ejected at an altitude of 51 meters and descended in free fall – slower than an earthly pedestrian – to its destination, the asteroid. The relief about the successful separation and subsequent confirmation of the landing was clearly noticeable in the MASCOT Control Center at DLR in Cologne as well as in the adjoining room: "It could not have gone better," explained MASCOT project manager Tra-Mi Ho from the DLR Institute of Space Systems. "From the lander's telemetry, we were able to see that it separated from the mothercraft, and made contact with the asteroid surface approximately 20 minutes later." The team is now in contact with the lander. 46) 47)
- The moment of separation was one of the risks of the mission: If MASCOT had not successfully separated from Hayabusa-2 as planned and often tested, the lander’s team would hardly have had the opportunity to solve this problem. But everything went smoothly: Already during the descent on the asteroid, MasCam (MASCOT Camera) took 20 pictures, which are now stored on board the Japanese space probe. "The camera worked perfectly," says Ralf Jaumann, DLR planetary scientist and scientific director of the camera instrument. "The team's first images of the camera are therefore safe." The magnetometer team was also able to recognize in the data sent by MASCOT that the MASMAG instrument had switched on and performed measurements prior to the separation. "The measurements show the relatively weak field of the solar wind and the very strong magnetic disturbances caused by the spacecraft," explains Karl-Heinz Glaßmeier from the Technical University of Braunschweig. "At the moment of the separation, we expected a clear decrease of the interference field – and we were able to recognize this clearly."
- MASCOT came to rest on the surface approximately 20 minutes after the separation. Now, the team is analyzing the data that MASCOT is sending to Earth to understand the events occurring on the asteroid Ryugu. The lander should now be on the asteroid’s surface, in the correct position thanks to its swing arm, and have started to conduct measurements independently. There are four instruments on board: a DLR camera and radiometer, an infrared spectrometer from IAS ( Institut d'Astrophysique Spatiale, Orsay, France) and a magnetometer from the TU Braunschweig. Once MASCOT has performed all planned measurements, it is expected to hop to another measuring location. This is the first time that scientists will receive data from different locations on an asteroid. "With MASCOT, we have the unique opportunity to study the Solar System’s most primordial material directly on an asteroid," emphasizes DLR planetary researcher Ralf Jaumann. With the data acquired by MASCOT and the samples that Hayabusa2 brings to Earth from Ryugu in 2020, scientists will not only learn more about asteroids, but more about the formation of the Solar System. "Asteroids are very primordial celestial bodies."
Figure 63: Shadow of MASCOT on asteroid Ryugu during descent: DLR's MasCam on board MASCOT acquired this image as it descended to the the asteroid Ryugu 3.5 minutes after separating from its mothercraft Hayabusa-2. In the image, the lander is ~20 m above the asteroid's surface, and MASCOT's shadow can be seen at the top right (image credit: MASCOT/DLR/JAXA)
Figure 64: Artist's rendition of MASCOT during the landing approach (image credit: DLR (CC-BY 3.0))
Figure 65: Left: Illustration of MASCOT separating from Hayabusa-2. Right: Illustration of MASCOT landing on the surface of Ryugu (image credit: JAXA, Ref. 47)
- For the landing site for MASCOT, a region in the southern hemisphere of Ryugu was selected (Figure 66). This location was selected based on criteria that ensured no overlap between the landing sites for the touchdown of Hayabusa-2, MINERVA-II1 and MASCOT, the time to be able to communicate with Hayabusa-2, the duration of sunlight exposure and expectation of scientifically meaningful exploration.
Figure 66: MASCOT landing site candidate region (light blue area). Since MASCOT is expected to bounce several times after first touching down, a reasonably wide region was selected (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST, CNES, DLR)
- MASCOT does not use solar cells but is equipped with a lithium primary battery built into the lander. The duration of the battery is about 16 hours (about two rotations of Ryugu), allowing MASCOT to operate continuously for two Ryugu days after a successful separation. Operation of the lander will end when the battery runs out.
• September 27, 2018: The MINERVA-II1 rovers were deployed on September 21 to explore the surface of asteroid Ryugu. Here is the second report on their activities, following our preliminary article at the start of this week. We end this report with a video taken by one of the rovers that shows the Sun moving across the sky as seen from the surface of Ryugu. Please take a moment to enjoy “standing” on this new world. 48)
Figure 67: These images were taken by Rover-1B on 23 September 2018; confirmation of Rover-1B hop. Observation time (JST): Left: 09:50; Center: 09:55; Right: 10:00 (image credit: JAXA)
Figure 68: This image was captured immediately before a hop of Rover-1B on 23 September 2018 at 09:46 JST (image credit: JAXA)
Figure 69: Surface image from Rover-1B after landing, taken on 23 September at 10:10 JST (image credit: JAXA)
Figure 70: Surface image taken from Rover-1A on September 23 2018 at 09:43 JST (image credit: JAXA)
Figure 71: Rover-1A captured the shadow of its own antenna and pin. Surface image taken from Rover-1A MINERVA-II1 successfully on 23 September 2018 at 09:48 JST (image credit: JAXA)
The pins on the MINERVA-II rovers have three roles:
1) To increase friction when hopping
2) Protect the solar cells during landing
3) A few of the pins also have a temperature sensor, so surface temperature can be measured directly.
• On 21 September 2018, 180 million miles ( 290 million km) from Earth, a roughly 1.5 m2 cube descended towards a primitive space rock. After years of planning and 4 years in flight, this tiny spacecraft captured this ‘shadow selfie’ as it closed in on asteroid Ryugu, just 80 m from the remnant of our Solar System’s formation, 4.6 billion years ago. 49)
- The Hayabusa-2 spacecraft is operated by JAXA (Japanese Space Agency) and supported in part by ESA's Estrack Malargüe deep-space tracking station in Argentina. The spacecraft carries four small landers that will investigate the asteroid’s surface, all four designed to gently fall onto the surface of the rocky boulder, taking advantage of its low gravity environment.
- Around the time this remarkable picture was taken, the spacecraft released its two MINERVA-II1 rovers which have since successfully landed and demonstrated an ability to hop around this rock-strewn body.
- "I cannot find words to express how happy I am that we were able to realize mobile exploration on the surface of an asteroid" enthused Yuichi Tsuda, Hayabusa-2 Project Project Manager, "I am proud that Hayabusa-2 was able to contribute to the creation of this technology for a new method of space exploration by surface movement on small bodies."
- The next stage will see the Mobile Asteroid Surface Scout (MASCOT) lander released onto the asteroid’s surface. Developed by the German Aerospace Center (DLR) in cooperation with the French Space Agency (CNES) MASCOT has enough power for a 12-hour mission, in which it will analyze the asteroid’s surface at two different sites.
- The Hayabusa-2 spacecraft itself will collect three samples from Ryugu, bringing them back to Earth in December 2020. These strange specimens will provide insights into the composition of this carbonaceous asteroid — a type of space rock expected to preserve some of the most pristine materials in the Solar System.
- As well as hopefully shining light on the origin and evolution of the inner planets, and the sources of water and organic compounds on Earth, this knowledge should help in efforts to protect our planet from marauding masses that come too close for comfort to our home planet.
- Understanding the composition and characteristics of near-Earth objects is vital to defending ourselves from them, if one were to head in our direction. ESA’s proposed Hera mission to test asteroid deflection is an ambitious example of how we can get to know these ancient bodies better, all in the name of planetary defence.
Figure 72: Hayabusa-2 took this selfie, i.e. its shadow projected onto Ryugu when the tiny spacecraft was closing in on the ancient rocky body, around the time when the two MINERVA-II1 rovers were released (image credit: JAXA)
• On 21 September 2018, the small compact MINERVA-II1 rovers separated from the Hayabusa-2 spacecraft (time of separation was 13:06 JST). The MINERVA-II1 consists of two rovers, Rover-1A and Rover-1B. We have confirmed both rovers landed on the surface of asteroid Ryugu. The two rovers are in good condition and are transmitting images and data. Analysis of this information confirmed that at least one of the rovers is moving on the asteroid surface. 50)
- MINERVA-II1 is the world’s first rover (mobile exploration robot) to land on the surface of an asteroid. This is also the first time for autonomous movement and picture capture on an asteroid surface. MINERVA-II1 is therefore “the world’s first man-made object to explore movement on an asteroid surface”. We are also delighted that the two rovers both achieved this operation at the same time. "Each of the rovers is operating normally and has started surveying Ryugu's surface," JAXA said in a statement.
- Taking advantage of the asteroid's low gravity, the rovers will jump around on the surface — soaring as high as 15 m and staying in the air for as long as 15 minutes — to survey the asteroid's physical features. 51)
- "I am so proud that we have established a new method of space exploration for small celestial bodies," said JAXA project manager Yuichi Tsuda.
Figure 73: Image captured by Rover-1A on September 21 at around 13:08 JST. This is a color image taken immediately after separation from the spacecraft. Hayabusa-2 is at the top and the surface of Ryugu is bottom. The image is blurred because the shot was taken while the rover was rotating (image credit: JAXA)
Figure 74: Image captured by Rover-1B on September 21 at around 13:07 JST. This color image was taken immediately after separation from the spacecraft. The surface of Ryugu is in the lower right. The colored blur in the top left is due to the reflection of sunlight when the image was taken (image credit: JAXA)
Figure 75: Image captured by Rover-1A on September 22 at around 11:44 JST. Color image captured while moving (during a hop) on the surface of Ryugu. The left-half of the image is the asteroid surface. The bright white region is due to sunlight (image credit: JAXA)
The MINERVA-II1 cameras can shoot in color. In Figure 73, although the image is blurred due to the rover rotating, you can clearly see the body of Hayabusa-2 and the paddle of the solar cells. The solar paddle appears blue. In Figure 75, the image was taken during a hop on the surface and you can feel this dynamic movement.
Operation of MINERVA-II1 will continue from now on. We are planning to acquire more data for analysis.
Legend to Figure 76:
- dV, ΔV: acceleration of the spacecraft
- free fall: The speed of the spacecraft is not controlled, but falls due to the pull of Ryugu’s gravity.
- GCP-NAV: Ground Control Point Navigation, tool for image guided navigation during the descent operation.
- HP: home position
- LIDAR: laser altimeter
- ONC-T: Optical Navigation Camera Telescopic (installed on the bottom of the spacecraft).
- ONC-W1: Optical Navigation Camera Wide angle (installed on the bottom of the spacecraft).
- ONC-W2: Optical Navigation Camera Wide angle (installed on the side of the spacecraft).
- Attitude scan: change the attitude of the probe to photograph MINERVA-II1
- constant descent velocity: the spacecraft descends at a nearly constant speed. At first, this speed is about 40 cm/s. From an altitude of about 5 km, the speed is about 10 cm/s.
- Trigger Altitude: when this altitude is reached, the spacecraft’s speed will be controlled.
- Additional free fall for preventing plume contamination: the spacecraft will not use thrusters during the period shown in the figure when MINERVA-II1 is in free fall. This is to avoid thruster gas hitting MINERVA-II1.
• August 23, 2018: Landing sites of the Hayabusa-2 mission on the asteroid Ryugu determined: 52)
Figure 77: The MASCOT lander is set to land on the blue-marked ellipse MA-9 on Ryugu. The Japanese Hayabusa-2 probe will approach the asteroid surface at location L07 (L08 and M04 are substitute landing sites), where it will take samples. The MINERVA rovers will be dropped off at the red-marked landing site N6 (image credit: JAXA, University of Tokyo & collaborators)
• July 25, 2018: Hayabusa-2 arrived at asteroid Ryugu on June 27, after which the spacecraft remained at a distance of about 20 km (the Home Position) to continue to observe the asteroid. During this time, the spacecraft was maintaining a hovering altitude of 20 km above the asteroid surface. — In the week of July 16, operations were begun to lower this orbiting altitude, eventually bringing the spacecraft to less than 6 km from the asteroid surface. One of the images taken at that time is shown in Figure 78. 53) 54)
- The resolution in Figure 78 is about 3.4 times higher than the images taken from the Home Position so far. One pixel in Figure 78 corresponds to about 60 cm. The largest crater on the surface of Ryugu is situated near the center of the image and you can see that it has a shape like a "mortar". You can also see that the surface of Ryugu is covered with a large number of boulders. This picture will provide important information as we choose the landing site.
Figure 78: Asteroid Ryugu from an altitude of 6 km. The image was captured with ONC-T (Optical Navigation Camera - Telescopic) on July 20, 2018 at around 16:00 JST (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST)
- The activity at this lowered altitude is called a "BOX-C operation". In the Hayabusa-2 Project, we define three regions for spacecraft operations above Ryugu; BOX-A, BOX-B and BOX-C. A diagram of these operation regions is shown in Figure 79.
- BOX-A corresponds to the Home Position, with an altitude of about 20 km. BOX-B is at same altitude as BOX-A, but the spacecraft can now move ±10 km forwards, backwards, left and right. BOX-C has the same dimensions at BOX-A front-to-back and left-to-right, but this region contains an altitude down to about 5 km from the asteroid surface. We define the coordinate system here as the Home Position coordinate system. To understand this system, take a look at Figure 80. The important point is that the Z-axis always points in the direction away from the asteroid towards the Earth.
- During the last week, the project operated the spacecraft within BOX-C, descending slowly towards the asteroid from July 17. Hayabusa-2 orbited at the lowest altitude for one day from July 20. After conducting observations of Ryugu during this time, Hayabusa-2 began to ascend on July 21 and returned to BOX-A on July 25.
Figure 79: Definition of the operation BOX. XHP, YHP, ZHP are the coordinates of the Home Position. The YHP -axis is perpendicular to this page (or screen) and faces towards the reverse side of the page (image credit: JAXA)
• August 14, 2018: In early October 2018, the Mobile Asteroid Surface Scout (MASCOT) lander is expected to be in operation for approximately 16 hours on the Ryugu asteroid. The selection of the landing site will take place this August. The ideal site must firstly offer the MASCOT team engineers excellent conditions for a safe landing and stable operation on the asteroid, while providing the researchers with a wealth of new and productive measurements. The lander made its way to the asteroid on board the Japanese Hayabusa-2 space probe, carrying four instruments that will examine the celestial body directly on the surface. The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR) is contributing the MASCOT Camera (MasCam), which will acquire the first images during the descent to the asteroid and is intended to investigate the area around the landing site, in addition to the fine structure of the surface. DLR is also providing the MARA radiometer, which will measure the temperature on the asteroid's surface, among other things. The Technical University of Braunschweig will use the MasMag magnetometer to research the magnetization of the asteroid, while the French space agency CNES (Centre National d'Etudes Spatiales) will analyze the minerals and rocks on Ryugu using the MicrOmega spectrometer. Ralf Jaumann from the DLR Institute of Planetary Research is supervising the MasCam camera experiment and is the Principal Investigator of the MASCOT lander. In this interview, he explains what a suitable landing site may look like and what the scientists expect to find on the asteroid. 55)
• June 28, 2018: After a 42-month journey, Japan's Hayabusa-2 spacecraft arrived at asteroid 162173 Ryugu, 300 million km from Earth, on 27 June at 00:35 GMT. This remarkable achievement was confirmed when the spacecraft closed to just 20 km from the 1 km-diameter asteroid's surface, having entered a critical phase of this ambitious mission. 56)
- The confirmation of the Hayabusa-2 rendezvous, made at 9:35 a.m. (Japan Standard Time, JST), is based on the following data analyses: 57)
a) The thruster operation of Hayabusa-2 occurred nominally
b) The distance between Hayabusa-2 and Ryugu is approximately 20 kilometers
c) Hayabusa-2 is able to maintain a constant distance to asteroid Ryugu
d) The status of Hayabusa-2 is normal
- From this point, JAXA is planning to conduct exploratory activities in the vicinity of the asteroid, including scientific observation of asteroid Ryugu and surveying the asteroid for sample collection.
Figure 81: Image of Ryugu, captured with ONC-T (Optical Navigation Camera – Telescopic) at 12:50 p.m. (JST), June 26, 2018 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST)
Figure 82: The location of Hayabusa-2 thrusters: A set of 12 thrusters is loaded with the Hayabusa-2 spacecraft as circled in red. Propulsion at 2.9 cm/s in the plus z-axis proceeded for the rendezvous (image credit: JAXA, Ref. 57)
- Hayabusa-2 aims to study Ryugu in detail, deposit a European and a series of Japanese landers on the surface and return a sample of ancient rock to Earth in 2020.
- "Together with all of you, we have become the first eyewitnesses to see asteroid Ryugu. I feel this amazing honor as we proceed with the mission operations," said Yuichi Tsuda, project manager of JAXA (Japan Aerospace Exploration Agency).
- In 2014-17, during Hayabusa-2's cruise phase from Earth to the asteroid, ESA's deep-space ground station at Malargüe, Argentina - part of the Agency's worldwide Estrack network - provided crucial communication support to the mission.
- In July this year, Malargüe will resume support, providing one communication contact session per week together with ESA's Cebreros station in Spain. The Malargüe station will also support the ESA-JAXA BepiColombo mission, due for launch in the autumn (Ref. 56).
• June 15, 2018: Hayabusa-2, JAXA's asteroid explorer, and the MASCOT lander, developed by the German Aerospace Center (DLR) and the French space agency (CNES) have been travelling through space since December 2014. They are finally closing in on their destination asteroid – Ryugu. As of 14 June 2018, the distance between Hayabusa-2 and Ryugu is less than 770 km and the closing speed is 2.1 m/s. 58) 59)
- On 13 June 2018, the 'Optical Navigation Camera – Telescopic' (ONC-T) acquired an image of Ryugu, where the asteroid extends to 10 pixels. The short exposure time means that the background star field is invisible.
Figure 83: Left: An image of Ryugu acquired by the ONC-T on board Hayabusa-2 at around 13:50 JST (06:50 CEST, 04:50 UTC) on 13 June 2018. The distance to the asteroid is approximately 920 km and the field of view is 6.3º x 6.3º; the exposure time was 0.09 seconds. Right: enlarged section of left-hand image [image credit: JAXA, Kyoto University, Japan Spaceguard Association, University of Seoul, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, and AIST (National Institute of Advanced Industrial Science and Technology)]
- Earlier, the ONC-T also acquired an image with a much longer exposure time in which the background stars are visible.
Figure 84: Image of Ryugu acquired by the ONC-T on board Hayabusa-2 on 13 June 2018 at 12:50 JST. The distance to the asteroid is about 920 km and the field of view is 6.3º x 6.3º; the exposure time was 178 seconds. Ryugu, which is visible against the constellation Gemini, is sufficiently bright that it has partially saturated the ONC-T sensor [image credit: JAXA, Kyoto University, Japan Spaceguard Association, University of Seoul, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, and AIST (National Institute of Advanced Industrial Science and Technology)]
- Arrival at Ryugu is scheduled for June 27.
Figure 85: Overview of the Hayabusa-2 mission schedule (image credit: JAXA)
• On February 26, 2018, Hayabusa-2 saw its destination —asteroid Ryugu— for the first time! The photographs were captured by the ONC-T (Optical Navigation Camera - Telescopic) imager onboard the spacecraft (Figure 86). Images were taken between noon JST on February 26th and 9:00am the following morning, with about 300 shots taken in total. Data for nine of these images were transmitted from the spacecraft on February 27th, allowing us to confirm that Ryugu had indeed been seen. The animation shows these nine consecutive frames. 60) 61)
- Ryugu's brightness from Hayabusa-2 is about magnitude 9, which would be impossible to see with the naked eye but visible with the ONC-T. Looking at the image above, you can see how the position of the surrounding stars relative to Ryugu appears to change as Hayabusa-2 moves towards the asteroid. The distance between Ryugu and Hayabusa-2 when the images were taken is about 1.3 million km. Ryugu as seen from Hayabusa-2 is in the direction of the constellation Pisces.
- Ryugu was photographed when the Sun, Hayabusa-2 and Ryugu were almost in a line. This configuration can be seen in Figure 87, which shows a snapshot of the header from the Hayabusa-2 website on February 26, 2018, which continuously updates to show the position of Hayabusa-2. If you were to stand on Ryugu, Hayabusa-2 would be seen in the direction of the Sun.
- Hayabusa-2 is currently using its ion engine to make adjustments to its course. This makes it difficult to alter the orientation of the spacecraft. However, at the alignment shown in Figure 87, the ONC-T camera can image Ryugu without needing to make significant changes to the spacecraft's orientation. This made February 26th the perfect time to try and capture Ryugu's image with the ONC-T. From the data, Ryugu was observed to be exactly at the expected location based on Hayabusa-2's estimated position. This tells us that Hayabusa-2 is flying on the planned course.
- "Now that we see Ryugu, the Hayabusa-2 project has shifted to the final preparation stage for arrival at the asteroid. There are no problems with the route towards Ryugu or the performance of the spacecraft, and we will be proceeding with maximum thrust," explains Project Manager, Yuichi Tsuda.
- The remaining images will be transmitted back to Earth from the spacecraft and allow us to further confirm the asteroid and spacecraft location. Although we can currently see Ryugu only as a point, it is very exciting for the whole project team to catch sight of the destination!
• As of September 2017, two of three IES (Ion Engine System) sessions have been completed. The first and second IES session, 798 hours and 2558 hours of IES operation was executed, respectively. The total IES operation time is about 3,900 hours and a velocity increment of about 580 m/s has been obtained. 62)
- In December 2017, the last IES session will be started and will continue until June 2018. After the completion of the third IES operation, the Hayabusa-2 spacecraft will arrive in the vicinity of Ryugu. Since there are large uncertainties on the asteroid ephemeris, the Hayabusa-2 spacecraft will perform an optical navigation campaign to reduce the relative position error to the asteroid. The direction of the asteroid will be determined by the ONC-T (Optical Navigation Camera - Telescopic) imager; if required, a series of guidance maneuvers with the RCS will be used to correct the trajectory toward the asteroid. A month of an optical navigation campaign is scheduled for this final approach phase.
- Hayabusa-2 will arrive at the asteroid vicinity in the summer of 2018. Hayabusa-2 will explore Ryugu for 1.5 years and return to the Earth in the winter of 2020. The 1.5 year allocation for proximity phase operations is much larger than the 3 months of Hayabusa's proximity operation, however the schedule is marginal. Three touchdown operations need to be planned within the limited amount of time to satisfy the mission success criteria.
- There are three main uncertainty parameters, which impact the schedule. Those are: gravity, thermal environment, and spin axis orientation of Ryugu.
- Firstly, the Ryugu gravity limits the number of descent operations. This is because the limitation of the fuel onboard and the fuel consumption per descent operation. The smaller the gravity constant, the less fuel requirement - thus, more descent attempts can be done. Currently, the porject assumes the gravity coefficient between 11 to 92 m3/s2. The "worst case" for the scheduling is the largest number of the descent operations, derived by the smallest gravity. The maximum number of descent operations is considered as a slot in the nominal schedule.
- Secondly, the thermal environment of the asteroid impacts the schedule. This constraint is the solar distance constraint for the TD (Touch Down) operation. There are upper constraints in the solar distance because the spacecraft temperature becomes too high when the spacecraft approaches the asteroid surface and receives the thermal inputs both from the asteroid as well as from the Sun. The current best estimate of Ryugu's thermal environment derived resulted in a touch down epoch requirement between October 2018 and May 2019; this corresponds with a 1.25 AU sun distance. This constraint will be updated after the Ryugu in-situ observation of Hayabusa-2 in the Summer of 2018.
- The third uncertainty parameter is the spin axis orientation of the asteroid. Although there is series of light curve observation of Ryugu from ground based telescope, the spin axis orientation is still uncertain. The spin axis impacts the Hayabusa-2 visibility of the asteroid. Since the Hayabusa-2 spacecraft stays in the vicinity of the Earth-asteroid line, the limited visible latitude is given as a function of spin axis. Assuming that the asteroid is a single spinner, the visible latitude varies due to the asteroid orbital motion around the Sun. The orbital motion changes the geometry between the asteroid and the Earth, therefore the visible latitude changes. — The accessible region for the descent operation also depends on the spin axis orientation. Due to the GNC limitation, the descent operation need to be done within 200 m from the asteroid-Earth line. This restriction describes the accessible latitude band as a function of time. Figure 88 illustrates the schematic image of the change of the visible area and accessible latitude band. The relative position in HP (Home Position) frame will change due to the orbital motion around the Sun.
- Figure 89 describes the latitude band and visibility of a sample spin axis orientation. The blue region is the touch down or MASCOT release point accessible latitude band. The gray zone during the end of 2018 is the conjunction phase and neither observation nor critical operation can be performed. In this example, the accessible latitude before and after the conjunction have a large difference. 63)
- Considering all three points, the nominal schedule is conducted as shown in Figure 90. The TD1 is allocated just before the conjunction (with margin). All the previous operations are derived by the TD1 operation. Especially, the landing site selection is the most important input for the operation.
Legend to Figure 90: LSS =Landing Site Selection, SCI =Small Carry-on Impactor.
• As of September 2016, 1346 hours of the ion engine operation have been achieved as planned. Three touch downs/sample collections, one kinetic impact/crater generation, four surface rovers deployment and many other in-situ observations are planned in the asteroid proximity phase. The operation team will perform extensive operation practice/rehearsal using a hardware-in-the-loop simulator in the year 2017 to be ready for the asteroid arrival in the summer 2018. 64)
- Hayabusa-2 adopts a novel attitude control technique called “Solar Sail Mode” for reliable and fuel-efficient attitude operation. 65) It is used in ballistic cruise periods and realizes a coarse Sun pointing attitude using one RW only and actively utilizing SRP (Solar Radiation Pressure). The last 1 month of the commissioning phase from January 24 until March 2 was dedicated to the SRP torque characterization and Solar Sail Mode attitude control testing/demonstration, which was completed successfully.
- Forward Cruise Operation: Hayabusa-2 started the forward cruise phase (cruise from Earth to Ryugu) on March 3, 2015, following the completion of the commissioning phase. Total of approximately 7000 hours IES operation is planned to reach Ryugu. Table 7 shows the history of the IES operation from launch until present. Figure 91 shows the trajectory from EGA to Ryugu arrival with the IES thrust vector history.
- Ballistic flight phase (cruise without IES operation) is effectively utilized for precise navigation, regular onboard instruments checkout/calibrations, and engineering demonstration/testing. For example, Mars observation campaign was conducted from May 28-June 9, 2016 at the closest distance (41 million km) for ONC/TIR/NIRS3 calibration purpose. The Ka-band operation between Hayabusa-2 and an ESA’s deep space station (ESTRACK Malargue station in Argentina) was conducted successfully on July 5-8, 2016. The Ka-band DDOR was tested several times using NASA(DSN) and ESA(ESTRACK) stations and achieved the world-first successful inter-agencies operation of the Ka-band DDOR under the CCSDS (Consultative Committee for Space Data Systems) DDOR operation standard. Besides testing, X-band DDOR is regularly used for precise orbit determination in ballistic cruise periods.
- EGA (Earth Gravity Assist) operation: Hayabusa-2 performed the EGA on December 3, 2015. Four TCMs (Trajectory Correction Maneuvers) were planned and three were actually conducted. The final TCM was canceled because the spacecraft had already been guided accurately with the three TCMs. Table 7 shows the list of TCMs for the gravity assist with the guidance accuracy results.
- The Earth closest approach occurred at 10:08:07 UT, December 3, 2015 over the Pacific Ocean (Figure 92). The closest distance was 3090 km (from the Earth surface) and the flyby deflection angle (angle between incoming and outgoing velocity vector in ECI frame) is 83º. The interplanetary velocity increment by this EGA is 1.6 km/s. The spacecraft experienced an eclipse for 20 minutes, which is the longest battery-powered operation for Hayabusa-2 throughout its mission.
- The operation team is now preparing for operation practice/rehearsals using the hardware-in-the-loop simulator in the year 2017 to be ready for the asteroid arrival in the summer 2018.
• July 14, 2016: MASCOT (Mobile Asteroid Surface Scout) of DLR has been travelling on board the Japanese Hayabusa-2 spacecraft for the last 1.5 years, and is currently at approximately 65 million km from Earth. On 14 July 2016, DLR (German Aerospace Center) engineers in the LCC (Lander Control Center) in Cologne switched the shoebox-sized lander and its four German and French-built instruments back on, and will spend the next few days finding answers to two questions: How is MASCOT's state of health? And how are the experiments on board? "We do a check-up once a year to find out whether all system components and instruments are still in good working order," explains Christian Krause from the Lander Control Center team at DLR. 66)
- The Hayabusa-2 spacecraft by JAXA (Japan Aerospace Exploration Agency) set off on its mission on 3 December 2014, carrying the French-German MASCOT lander. One year later, the duo zipped round the Earth to gain momentum and sent back photos from our planet before continuing on toward the asteroid Ryugu. The spacecraft will venture deeper into space until the summer of 2018, when it will enter orbit around the celestial body that DLR planetary researcher Ralf Jaumann refers to as a 'beautifully primitive object'. "During this mission, we will be investigating primordial material from the solar nebula; it has remained practically unchanged in its 4.5 billion years of existence." Then, while the Hayabusa-2 spacecraft measures and analyses the asteroid from its position in orbit, MASCOT will descend to its surface to conduct scientific measurements. The Japanese spacecraft will also take on soil samples that it will bring back to Earth in 2020. "This is a complete package. There has never been anything like this before: we will be observing and mapping remotely, measuring the asteroid, analyzing its surface and bringing the samples back to Earth." But this complete package requires the concerted efforts of engineers and scientists from Germany, France and Japan who have joined forces in an international cooperation.
• December 25, 2015: Before and after the Earth swing-by, the laser altimeter (LIDAR) on Hayabusa-2 attempted to receive laser light from the SLR (Satellite Laser Ranging) ground stations. 67)
- After the swing-by, the Mt. Stromlo station at SERC (Space Environment Research Center Australia) in the suburbs of Canberra, Australia, transmitted laser light towards Hayabusa-2. The spacecraft successfully received the beam using the onboard LIDAR that can send and receive laser signals to accurately establish the range of objects from the spacecraft. At the time of the transmission from Mt. Stomlo, Hayabusa-2 was 6,700,000 km from Earth. This success established the one-way 'up link' of the optical connection.
• December 14, 2015: JAXA confirmed that the Asteroid Explorer “Hayabusa-2” is cruising on its target orbit after measuring and calculating the post-Earth-swing-by orbit. With the swing-by results, the explorer’s orbit turned by about 80 degrees and its speed increased by about 1.6 km/s to about 31.9 km/s (against the sun), thus the orbit achieved the required target velocity (Figure 93). According to the operational support provided by the NASA DSN (Deep Space Network) stations and the ESA (European Space Agency) deep space ground station Malargüe in Australia, the Hayabusa-2 is in good health. 68) 69)
- At 9:00 UTC on Dec. 14, 2015, the Hayabusa-2 is at a distance of ~4.15 million km from Earth, and about 144.85 million km from the sun. Its cruising speed is 32.31 km/s (against the sun). Hayabusa-2 is increasing its speed under the influence of the sun’s gravity after the swing-by.
- After the swing-by, Hayabusa-2 took images of the Earth using its onboard ONC-T (Optical Navigation Camera - Telescopic). The ONC-T can shoot color images using seven filters. The image of Figure 94 is composed by using three of these filters. One can see the Australian continent and Antarctica in the image. The South Pole is not lit by the sun during the(northern hemisphere) summer, and the meteorological satellites also do not cover the Antarctic continent to take their imagery, hence the shot this time is precious.
Figure 94: Shot at 22:09 (UTC) on Dec. 4, 2015, about 340,000 km from the center of the Earth One can see the Australian continent on the upper right, and Antarctica on the lower right (image credit: JAXA)
• December 3, 2015: JAXA performed an Earth swing-by operation of the Asteroid Explorer "Hayabusa-2 " in the evening of December 3 (Thursday), 2015 JST (Japan Standard Time). The Hayabusa-2 flew closest to the Earth at 10:08 (UTC) and passed over Hawaii at an altitude of about 3,090 km. - The Hayabusa-2 project team is currently measuring and calculating the post-swing-by orbit. It will take about a week to confirm if the explorer entered the target orbit. We will report the result once it is determined. The Hayabusa-2 is in good health. 70)
- ESA's deep-space Malargüe ground station in Australia lent a helping ear as Japan’s Hayabusa-2 asteroid mission visited Earth on Thursday. 71)
• October 5, 2015: The asteroid 1999 JU3, a target of the Asteroid Explorer “Hayabusa-2,” was named “Ryugu”. 72)
- JAXA conducted a naming proposal campaign between July 22 and August 31, 2015.
- Selection reasons: In the Japanese ancient story “Urashima Taro”, the main character, Taro Urashima, brought back a casket from the Dragon’s palace, or the “Ryugu” Castle, at the bottom of ocean. The Hayabusa-2 will also bring back a capsule with samples, thus the theme of “bringing back a treasure” is common.
- Rocks containing water are expected to exist on asteroid 1999 JU3. The name “Ryugu” also reminds us of water, as “Ryugu Castle” is under the ocean. The name is not similar or identical with any other already existing names of planets or asteroids, and there were many entries for this name among suggested names that are related to mythology.
- According to the naming rule stipulated by the International Asteroid Union (IAU), the name “is preferably from mythology” and the “Ryugu” fits that rule. Also, there is little concern of infringing the Trademark Law or any other third party trademarks.
• June 8, 2015: The Hayabusa-2 spacecraft has been continuously operating its ion engine for the second time since June 2, and successfully completed its operations at 0:25 a.m. on June 7 (Japan Standard Time.) The second continuous operation lasted for 102 hours as scheduled. 73)
- The ion engine boost was needed in preparation for the Earth swing-by planned in December 2015, and the total hours of the first and second operations (409 hours and 102 hours respectively) reached 511 hours.
• April 29, 2015: ESA is set to support Japan’s ‘touch-and-go’ Hayabusa-2 spacecraft, now en route to a little-known asteroid, helping to boost the scientific return from this audacious mission. A flawless launch last December marked the start of a six-year round-trip for Japan’s Hayabusa-2, which is on course to arrive at the carbon-rich asteroid 1999 JU3 in June 2018. 74)
- In the first such support provided to a Japanese deep-space mission, ESA’s 35 m diameter dish at Malargüe, Argentina, will provide up to 400 hours of tracking, establishing radio contact as the asteroid arcs through the Solar System between 135 million to 210 million km from the Sun. Telecommands from mission controllers at JAXA (Japan Aerospace and Exploration Agency) will be fed to the station via ESA/ESOC in Darmstadt, Germany.
- ESA is using its ESTRACK network with deep space stations at Malargüe (Argentina), Cebreros (Spain) and New Norcia (Australia). On 22 April, ESA completed a live, inflight compatibility test, linking the Malargüe station with the Japanese spacecraft, demonstrating the network's readiness to provide tracking for the Hayabusa-2 mission.
• April 10, 2015: Hayabusa-2 is stably flying in space. The new fiscal year has just started in Japan, and JAXA is taking a new step as we became a 'National Research and Development Agency' from the previous independent administrative agency. The Hayabusa-2 project is also taking a fresh step with a new team, including handing the baton over to a new project manager, namely from Hitoshi Kuninaka to Yuichi Tsuda. All members of the project are engaged in the mission with a fresh mindset. 75)
• March 18, 2015: LIDAR (LIght Detection And Ranging) is one of the satellite buses onboard Hayabusa-2. Basically LIDAR is used to measure the distance between the satellite and the target asteroid, 1999JU3. The distance is important information to control the satellite in descending operation such as close-up observation from low altitude and separation of rover, lander, and small carry on impactor. Especially an accurate measurement of the distance is required during touch-down and sampling operations. Thus, the LIDAR onboard Hayabusa-2 is required to measure about 4 orders of magnitude variety of distance, from 30m to 25km. To satisfy this requirement, the LIDAR has two types of receiver optics, one is for near range (nearer than 1km) and another is for far range (farther than 1km). 76) 77)
- The distance from the target asteroid, 1999JU3, is one of the fundamental information for operational and scientific observations. For example, the gravitational acceleration is calculated from the change rate of the distance from the target asteroid during free fall operation. Construction of a shape model requires a characteristic length scale in camera image. And also, the measurement of the absolute reflectance from camera image and/or spectrometer data requires an absolute distance from the target.
- The dust count mode is one of the operational modes of the LIDAR in which the LIDAR detects faint scattered light from dust grains on the line of sight. The distribution of dust grains along the line of sight is estimated from the profile of received light and the effective reflectance of dust cloud. Its function was tested both electrically and optically, and it seemed working properly.
Figure 95: Schematic view of dust count mode. Thick curve in the bottom figure represents hypothetical profile of received light. Black-and-white line at the bottom represent an output of dust count mode observation (white=0, black=1), image credit: Hayabusa-2 team
• March 5, 2015: Hayabusa-2, launched on Dec. 3, 2014, completed its initial functional confirmation period of about three months. The explorer was moving to the cruising phase on March 3 while heading to the asteroid 1999 JU3. 78)
- The Hayabusa-2 is in good health. It will be under preparatory operation including speed increase by continuous operation of the ion engines for an Earth swing-by scheduled in Nov. or Dec., 2015.
- The project plans to increase the cruising speed of the explorer (60 m/s) by operating two ion engines twice (in total about 600 hours or 25 days) until the Earth swing-by. For the first operation, the project will gradually increase the time duration of continuous ion-engine operation from March 3 onwards, and will operate the engines for about 400 hours within March. The second operation is scheduled in early June.
• Feb. 3, 2015: Hayabusa-2 is now undergoing the initial functional confirmation. Basic operations and performance of onboard instruments and ground systems have been tested one by one as of the end of January. Some examples: 79) 80)
- Ion engine test operation (one unit at a time). Four ion engines were being operated one by one. A thrust of 7-10 mN was generated on the orbit for the first time.
- Establishing communication by Ka-band communication equipment (between Jan. 5 to 10, 2015). Communication was successful between the Hayabusa-2 and NASA DSN stations to establish deep-space Ka-band communication for the first time for a Japanese space explorer. Ka-band communication will be used to send observation data during the mission for the Hayabusa-2 to stay near the asteroid.
- Ion engine can autonomously operate for 24 hours. Long duration of autonomous operation with two or three ion engines was tested, and 24-hour continuous operation was attained. The maximum thrust was confirmed to be about 28 mN, which is the expected value.
• Dec. 5, 2014: JAXA) confirmed the completion of a sequence of the important operations for the Asteroid Exploration Hayabusa-2 mission including the deployment of the horn part of the sampler that captures samples from the asteroid’s surface, the release of the locks for the launch that ratchet the gimbal that controls the direction of the ion engine, and functional verification of the three-axis stabilization controls and the ground precision orbit determination system. With this confirmation, the critical operation phase of the Hayabusa-2 was completed. 81)
• JAXA received the first signals of Hayabusa-2 on December 3, 2014 at 6.44 UTC on Dec. 3, 2014, acquired by the NASA DSN (Deep Space Network) at Goldstone in California; it was confirmed that its initial sequence of operations including the solar array paddle deployment and sun acquisition control have been performed normally. - The explorer is now in a stable condition. 82)
Sensor complement: (SMP, SCI, NIRS3, TIR, MINERVA-II-1A/1B/2, DCAM3, ONC-T, MASCOT, SRC)
SMP (Sampler Horn):
The SMP, also referred to as “sampler mechanism”, will collect samples from the surface of the asteroid. The basic design is the same of that aboard the Hayabusa mission; thus, the same mechanism will be used, which is a small projectile to be shot as soon as the tip of a cylinder-shaped horn touches the asteroid surface, then materials ejected from the surface will be collected in a catcher (storage).
Major improvements from the Hayabusa:
• The seal performance has been improved for Hayabusa-2, and a newly developed metal seal method is applied, so that volatile gas such as rare gas can be brought back thanks to high airtightness.
• The number of compartments for the catcher that will store captured materials is increased by one to three compartments compared to two for the Hayabusa.
• For the Hayabusa-2, the edge of the sampler horn was folded back inward as you can see in the left figure so that sand gravel can be hung on the cuff (gravel of 1 to 5 mm can be captured.) If the explorer makes an emergency stop during its ascending, gravel will keep going up to be stored in the catcher. This mechanism is a backup for sample retaining. 83)
Figure 96: Photo of the SMP assembly (image credit: JAXA)
SCI (Small Carry-on Impactor):
SCI is a new payload, an impactor caller “Liner”, which is featured on Hayabusa-2. It consists of a small box, about 30 cm in size, which will be released a few hundred meters above the surface of the asteroid. The 2 kg copper liner will be shot at high speed (2 km/s) into the surface of the asteroid to create a small artificial crater by collision. The size of the crater may be about 2 or 3 m in diameter. The purpose of this impact experiment is not only to study the physical characteristics of the asteroid surface, but also to reveal the subsurface material, of which Hayabusa-2 will try to recover samples. It is expected that the fresh crater samples are less weathered by the space environment or heat. — The impactor (Liner) is made of pure copper to be able to easily identify the sample from other materials on the asteroid.
Figure 97: SCI pyrotechnics: Left: A conical shape structure filled with explosives. The “Liner” will be ejected forward at a high speed by explosive power. Right: The flying “Liner” at 2 km/s (image credit: JAXA)
NIRS3 (Near InfraRed Spectrometer, and 3 comes from “3µm”) and TIR (Thermal Infrared Imager)
The Hayabusa-2 will be orbiting asteroid 1999JU3 at an altitude of ~20 km, and observe the asteroid by remote sensing. Two kinds of infrared observations will play an important role in this remote sensing.
• One is the near infrared spectrometer “NIRS3” to investigate mineral and water metamorphism or reciprocal chemical action of minerals and water by spectroscopic observations with near infrared rays.
• The other is the TIR (Thermal Infrared Imager) to study the temperature and thermal inertia of the asteroid, by capturing images of thermal radiation from the asteroid. In other words, it will find particles of the soil and the porosity of the mass of rock that influence the temperature.
Figure 98: Photo of the NIRS3 (left) and of the TIR (right) instruments (image credit: JAXA)
TIR (Thermal Infrared Imager) is a two-dimensional thermal infrared imager on Hayabusa-2 to explore C-type near-earth asteroid 162173 Ryugu. During the cruise phase, TIR observed the deep sky with setting the instrument at various temperatures, and also observed the Earth and the Moon before and after the Earth swing-by. Asteroid proximity phase operations are planned for TIR. 84)
TIR performances: TIR is basically the same as the LIR on Akatsuki Venus climate orbiter, based on a two-dimensional uncooled micro-bolometer array with 328 x 248 effective pixels, and covers 8 to 12 am range. TIR has been proven during the pre-flight tests and in-flight operations to confirm the temperature range from 150 to 460 K. This implies that not only the sunlit areas of Ryugu but even the nighttime areas could be observed if the thermal inertia is larger than 50 tiu. The FOV (Field of View) of TIR covers 16.7° x 12.7° in horizontal and vertical directions with IFOV is 0.051° per pixel. This corresponds to ca. 17 m per pixel from the Home Position (20 km altitude from asteroid) and to ca. 85 cm per pixel from 1 km altitude. Closest views of asteroid surface by TIR will be ca. 1 cm per pixel from 10 m altitude just before the final free fall to the surface for touchdown.
MINERVA-II (Rover payload)
There are three small MINERVA-II rovers in total. They are of the heritage of MINERVA of Hayabusa, and they move on the surface of the asteroid by hopping. Hayabusa had only one MINERVA rover which failed to land it on the surface of Itokawa. Hayabusa-2 has two kinds of rovers, MINERVA-II-1 which separates into two (A and B), and MINERVA-II-2, so there are three small rovers in total. The rovers have small sensors (cameras, thermometers) onboard to convey their information to the orbiting Hayabusa-2 spacecraft.
Figure 99: Schematic view of the MINERVA-II rovers (image credit: JAXA)
All the rovers have hopping mobile systems to explore over the surface after the deployment from the mother spacecraft at the vicinity of the target asteroid. 85)
Table 8: Specification of the twin rovers (Rover-1A and Rover-1B)
DCAM3 (Deployable Camera)
DCAM3 is a miniaturized optical camera with the objective to observe the artificial impact of SCI ( Small Carry-on Impactor) on the asteroid. DCAM3 takes images of the spreading ejecta motion on the asteroid, which provides information of surface physical properties and ejecta behavior under microgravity. 86)
Collisions between primitive planetary bodies are one of the most important physical processes in the planetary accretion from planetesimals to planets. DCAM3 is a detachable camera inherited from DCAM-1,2 of the Japanese IKAROS mission. A separable instrument is necessary to obtain close up views of the impact, because the mother ship (Hayabusa-2) will be hiding in a safe region far from the impact point to avoid a risk that the mother ship encounters high-speed ejecta from the asteroid during the impact operation.
The impact observation part in the DCAM3 system consist of a separable small camera cylinder, its detachment instrument, and receiving antennas on the mother ship (Figure 100). After separation in the SCI impact operation, DCAM3 starts its observation at visible wavelength and sends image data on one-way communication to the mother ship. The design of DCAM3 is similar to IKAROS DCAM1,2 (Figure 101), but has an additional high-resolution camera in the body.
The DCAM3 instrumentation is provided by a Japanese University Consortium in cooperation with JAXA. The University Consortium consists of: University of Tokyo, Kashiwa; Kobe University, Kobe; JAXA (Japan Aerospace Exploration Agency), Sagamihara; Planetary Exploration Research Center, Chiba Institute of Technology, Narashino; Kochi University, Kochi; The Graduate University for Advanced Studies, Sagamihara; and the University of Occupational and Environmental Health, Kitakyusyu.
The scientific observations are performed by DCAM3-D, a wide-angle high-resolution camera and its fast digital transmission component in the DCAM3 body. The scientific objectives of this camera are summarized as (1) clarifying the subsurface structure, and (2) constructing the impact scaling rule applicable to the surface of asteroid 1999JU3. The observation objects of the camera are ejecta and a subsequent crater of the SCI impact, and a relative position of the SCI to the asteroid when it is launched. DCAM3-D can determine the size and the angle of the ejecta curtain, and the speed of the ejecta spreading or fragment spattering, which are the key information for the above objectives. In addition, low-speed ejecta (dust) spreading will possibly be observed around the DCAM3 in a few hours after the impact.
The DCAM3-D CMOS detector produces 2000 x 2000 pixels 8 bit monochromatic images with a 74º x 74º wide-angle optics. It takes 1 frame/s sequential images at maximum. Figure 102 shows a virtual image of spreading ejecta on asteroid 1999JU3 taken by the DCAM3-D in an ideal position. The optical camera has enough-high space resolution and bright resolution for its sciences. DCAM3 continues to produce data for a few hours until the batteries run out or the DCAM3 falls and crashes on the asteroid. The digital communication device can send the image data to the mother ship with 4 Mbit/s at maximum. Instruments in the mother ship store all data taken by the deployed camera. The total volume of image data is estimated to be approximately 5 Gbit after compression.
Figure 102: A virtual image of asteroid 1999JU3 and ejecta cone taken by the DCAM3-D camera. The blue oval sphere shows a potential region in which SCI exists. The green cone shows a possible trajectory of the projectile (image credit: University Consortium, JAXA)
ONC-T (Optical Navigation Camera-Telescopic) imager
The ONC-T was developed under collaboration between JAXA, the University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, The University of Aizu, the National Institute of Advanced Industrial Science and Technology (AIST).
The ONC system onboard the Hayabusa-2 spacecraft consists of one telescopic camera (T) and two wide-angle cameras (W1 and W2). ONC-T is a telescopic camera with seven band-pass filters in the visible and near-infrared range. These filters are placed on a wheel, which rotates to put a selected filter for different observations, enabling multiband imaging. 87)
The main objective of this instrument is to optically navigate the spacecraft to asteroid Ryugu (1999 JU3) and to conduct multi-band mapping the asteroid for choosing touchdown candidate sites and understanding the nature of this asteroid.
Figure 103: Photo of the ONC-T imager (image credit: JAXA)
MASCOT (Mobile Asteroid surface SCOut)
MASCOT is a small lander provided by DLR (German Aerospace Center) in collaboration with CNES (French Space Agency). Building upon the successful joint development of the Philae lander of the Rosetta mission, DLR and CNES have studied this lander since 2008, and its realization was formally decided in early 2012. 88) 89) 90) 91)
After Hayabusa 2’s arrival to its target – asteroid 1999JU3 - in 2018, MASCOT will be dropped to the surface where it will perform in-situ investigations for about 15 hours. A hopping mechanism of MASCOT will enable measurements at several locations. The payload of MASCOT consists of four scientific instruments, a wide angle camera, an imaging IR spectrometer (MicrOmega), a radiometer (MARA) and a magnetometer. MASCOT will significantly enhance the overall scientific return of the Hayabusa-2 mission by providing context measurements to support the interpretation of analyses of the returned samples. 92)
The general concept of MASCOT is to provide a small landing system intended to be deployed from a main spacecraft (or “mother-ship”) on an asteroid sample return mission. MASCOT has been specifically designed to be compatible with JAXA Hayabusa-2 spacecraft and the environment given by the target asteroid 162173 Ryugu to fulfill the following scientific objectives: 93) 94)
- In-situ observations of undisturbed materials and microscopic scale observations, not possible from the main spacecraft, to determine the asteroid regolith structure, texture and composition
- Magnetic properties measured during the descent, during the hopping maneuvers and on the surface on more than one site
- Temperatures of one asteroid rotation and thermal properties on more than one site
- Help the selection of the sampling spot(s) by the main spacecraft.
MASCOT is a 11 kg lander with a 3 kg payload, of 0.3 x 0.3 x 0.2 m3 volume and a single energy source (of 220 Wh) allowing about a 12 hours of mission duration. The design for MASCOT is based on the following subsystems:
Figure 104: Photo of the MASCOT flight model (image credit: DLR, Telespazio VEGA)
• Structure & Accommodation: The lander baseline design features a highly integrated carbon-fiber composite structure with a middle wall as main load bearing element. A common electronics box, called E-box, houses all the electronics. The lander is connected with the main spacecraft Hayabusa-2 via MESS (Mechanical and Electronics Support Structure), which comprises the PRM (Preload Release Mechanism) and the separation device. An umbilical connector supplies the power during the cruise phase.
• Thermal Control: The thermal control concept is based mainly on passive means such as MLI (Multi-Layer Insulation) and color coatings. Thus, it relies also on a day-night-cycle on the asteroid’s surface to balance the heat-load build-up. This constraint drives the selection of the operating asteroid site for MASCOT. An exception to the passive concept are the heaters used for the thermal control of batteries and MicrOmega during the cruise and commissioning phase. The heater is controlled by Hayabusa-2.
Figure 105: Overview of MASCOT elements (image credit: DLR, Telespazio VEGA)
• Power: The power and energy supply is maintained by a primary battery, provided by the SAFT company (France), based on an earlier development for Philae. The unregulated bus voltage is converted into auxiliary voltages for MASCOT electronics by an internal PCDU (Power Control and Distribution Unit) located in the E-box. During the cruise and commissioning phase, the power will be supplied by the Hayabusa-2 spacecraft via a regulated power line. The power subsystem is a French contribution to MASCOT.
- During the cruise phase, the power is supplied by the Hayabusa-2 spacecraft via a dedicated power line, through the MESS connector. After separation, the power is supplied by a primary battery. Its capacity is dimensioned for an operating time of two asteroid days, ~16 hours nominal. Both the Hayabusa-2 power and the battery power line is converted into auxiliary voltages for MASCOT electronics by an internal PCDU (Power Control and Distribution Unit), located in the common E-box. The PCDU is made of two similar units forming a redundant architecture, with redundant power converters for the secondary voltages to redundant and non-redundant equipment. The data link with the OBC is via two UART RS422 lines for each PCDU unit.
• GNC (Guidance and Navigation) sensors: The GNC sensors are used to determine the MASCOT motion state and orientation on the asteroid’s surface. They are distributed all over the MASCOT body and are acquired via the hot redundant A/D converter of the OBC. The algorithms to determine the motion status, the illumination status and the MASCOT side in contact with the soil (side2soil) are implemented in the MASCOT OBSW. The navigation sensors consist of:
- OPS (Optical Position Sensors): 5 optical distance sensors are mounted on the outer skin of MASCOT on each side, except -Z, the nominal side at surface contact. The optical position sensors use an IR LED and a photodiode. The IR LED is switched on/off by the OBC to perform a differential measurement and detect the presence of a surface. With these sensors it is also possible to detect if MASCOT is moving or is lying still, the side on which MASCOT is lying and the distance to the soil.
- PECs (Photo-Electric Cells): These sensors are used for redundancy to the OPS. One PEC is mounted on each side of MASCOT. There are a total of 6 sensors. The output of the PEC is a voltage proportional to the cosine of angle between the sun vector and the cells normal vector.
- Separation sensor: Break-wire contacts on the power connector from Hayabusa-2, integrated in the MESS, will detect the separation.
• OBC (On-Board Computer): The MASCOT OBC is a dual redundant computer composed by 2 CPU boards (CPU-M, CPU-R) to execute the OBSW (On-Board Software) and 2 I/O boards (IOM-M, IOM-R) to provide interfaces with all MASCOT subsystems. The 4 PCBs are accommodated in the common E-box. The two CPU boards work in cold redundancy, while the I/O boards work in warm redundancy, cross-strapped with the CPU boards via SpaceWire links. Each I/O board uses a radiation tolerant FPGA to implement:
- the I/O controllers for the external UART serial interfaces
- the I/O controllers to handle the analog section and bi-level I/O interfaces
- a NAND Flash controller featuring a page level Reed - Solomon encoder / decoder
- the CPU switch-over logic driving the reconfiguration of the CPU from main to redundant or vice versa
- a local timer to centrally maintain the MOBT (MASCOT On-Board Time) independent from the CPU
- a small RAM area, EDAC protected, used as Safe Guard Memory (SGM) to propagate the OBSW context in case of CPU reset or switch-over.
These devices are accessible by the two CPU board via SpaceWire interface. The I/O board also contains the NAND Flash mass memory device, the analog section circuits, the RS422 transceivers, the SpaceWire LVDS transceivers and other interface drivers/receivers needed for the external interfaces. The CPU board uses an Aeroflex GR712RC SOC (System On a Chip) clocked at 40 MHz, embedding a dual LEON3FT CPU core and a few other cores, like SpaceWire controllers, timers, DSU (Debug Support Unit) etc. The board also accommodates the program and data RAM (implemented as a SRAM multi chip module), a reprogrammable non-volatile memory PROM (implemented as a MRAM device), the Watch Dog and Reset circuits and the SpaceWire LVDS and other transceivers needed for the external interfaces. The SOC DSU JTAG interface is routed to the E-box test connector through dedicated transceivers. The SpaceWire serial interfaces to the MicrOmega and CAM instruments are implemented by the CPU board.
In terms of processing and memory resources, each OBC section (1 CPU + 1 IOM) provides:
- CPU speed: minimum 40 DMIPS, 10 MFLOPS
- RAM (SRAM): 16 MByte EDAC protected
- PROM (MRAM): 256 kByte / 204 kByte with EDAC enabled
- NAND Flash: 1 GByte / 860 MByte with Reed-Solomon encoding.
• RF communications: The communication architecture is based on a redundant CCOM (Child -COM) transceiver provided by JAXA; it is identical with the communications subsystem of the MINERVA small landers. The data link is based on CCSDS TM/TC packets and the transmission protocol between the MASCOT CCOM and the Hayabusa-2 PCOM (Parent-COM) ensures one-to-one delivery of the CCSDS packets by automatic retransmission requests in both directions in case of errors. MASCOT is equipped with two CCOM units working in cold redundancy. Each CCOM unit has two RF ports connected to two patch antennas via an RF coupler. One antenna located on top (+Z) and the other on the bottom (-Z) of MASCOT ensure almost omnidirectional coverage.
The maximum bit rate possible is ~37.04 kbit/s for telemetry packet downlink; ~1.7 kbit/s for telecommand packet uplink. An UART RS422 TM/TC umbilical interface is foreseen through the E-box test connector, to be used as alternative to the CCOM RF channel only during ground AIV activities. After the Hayabusa-2 launch, only the CCOM RF channel is used for TM/TC with MASCOT, via the Hayabusa-2 data handling unit (DHU) and the lander’s computer,OME (On-board Minerva Equipment).
MASCOT antenna: For cruise phase operations, a dedicated MESS antenna is required whereas for on asteroid operations, the OME-A Hayabusa antenna is used. Two omnidirectional antennas are positioned on the top and bottom sides of MASCOT (Figure 106). The communication subsystem (CCOM transceiver excepted) is a French contribution to MASCOT.
Inside MASCOT a redundant set of JAXA-provided Child-Communication-transceivers (CCOM) is used to communicate with its counterpart - the Parent-Communication transceiver (PCOM) - on board of Hayabusa-2 based on a half-duplex communication and time division multiple access (TDMA) methods. The whole inter-spacecraft communication chain is shown in Figure 108.
Figure 107: MASCOT communication subsystems overview (image credit: DLR, Telespazio VEGA)
• MASCOT Mobility Mechanism: It fulfils two functions, namely up-righting MASCOT into the correct attitude after landing, and providing a hopping capability to relocate MASCOT onto a different site. The motion is generated by driving an off-centered mass to provide the adequate momentum. The mass is driven by an electric motor, which is controlled by a dedicated, dual, cold-redundant electronic unit called MMC (Mobility Mechanism Controller). Each MMC branch is connected to the OBC via a UART RS422 line.
Figure 109: Sketch of separation & surface operation concept (image credit: DLR)
The OPS (Optical Position Sensors), designed and manufactured by Cosine Research B.V., Leiden, The Netherlands, is a small sensor (32:6 mm x 27 mm x 21:6 mm) with a mass of about 28 g. It consists of an infrared LED and an appropriate photodiode (Figure 110). The light omitted by the LED is reflected by any object in the FOV (Field of View) of the sensor. Nearer objects reflect more light and light from objects further away than 12 cm can not detected any more. This gives the opportunity to detect objects in proximity in a certain direction. In addition the LED is on/off modulated by the OBC and the signal from the photodiode is correlated with this modulation. With this method one can distinguish between reflected light from the LED and background illumination e.g. from sunlight. Figure 111 shows the output voltage of the OPS as function of the distance from the object.
Five of these OPS systems are used on MASCOT, mounted on 5 different sides of the cuboid. Only the instrument side is not equipped with such a sensor. This space is needed for the scientific instruments. As one can see in Figure 111, the output voltage decreases again for distances smaller than 10 mm. To prevent such small distances, the sensors are not mounted flat to the surface but shifted inside the body of MASCOT.
Figure 112: Photo of the Mascot EQM, mounted on the Hayabusa-2 Flight Model at JAXA (image credit: DLR, CNES)
MASCOT instruments: (MicrOmega, CAM, MAG, MARA)
The payload consists of a suite of instruments which fit into the payload compartment of the structure (Figure 113). Each instrument has its own electronic unit, which provides the signal conditioning to the instrument sensors/detectors and, where present, to the actuators, converts the acquired measurements into digital format and transmits them to the OBC via dedicated serial lines. The OBC is in charge of configuring the instruments, driving their acquisition sequence and performing data processing realizing instrument specific data acquisition modes. The instruments are not designed to be fault tolerant, except for the data lines, which are connected to both OBC main and redundant sections.
As an in situ Science Landing Package, MASCOT will augment the science capabilities of the Hayabusa-2 mission on three levels:
1) ‘Context Science’ – By coordinated and complementary observations of the instruments on board MASCOT, on the main spacecraft and in combination with the lab analysis of the returned samples, MASCOT will ensure ground truth, down to the microscopic scale, for the acquired scientific results of the other mission elements. The goal is to obtain a cross-scale link with respect to surface and subsurface science , combining all three data sets to obtain a profound knowledge of the asteroid.
2) ‘Stand-alone Science’ – MASCOT can perform unique investigations of major science importance including those that only a landed package can do, such as geophysics. Direct in situ measurements by the MASCOT instruments will also enable the analytical characterization of the elemental, isotopic and molecular (organic and mineral) composition of 1999JU3’s surface and its near-surface material for samples in their natural state. Since 1999JU3 is a C-type asteroid, the astrobiological relevance of such measurements is significant. In addition, complementing visual documentation at the level of microscope scale would place the analytical results in context to the sample setting. MASCOT will investigate several locations on the asteroid to determine compositional and structural homogeneity and heterogeneity.
3) ‘Reconnaissance & Scouting’ - By supporting Hayabusa-2’s task of obtaining samples relevant to the top-level science objectives, MASCOT can serve as a ‘scouting vehicle’ for assessing candidate sampling sites before the Hayabusa-2 samples are acquired; in other words, if enough lead time for MASCOT deployment is accounted for in mission operations, the final selection of the sampling sites can be guided by the results from the MASCOT instruments.
MicrOmega is a near infrared spectrometer and a hyperspectral infrared microscope for in situ mineralogical analyses of the ground, developed by IAS (Institut d'Astrophysique Spatiale, Orsay). The objective is to characterize the composition of surface samples at their grain scale. Supported by CNES, MicrOmega/MASCOT has been developed within the frame of the Pasteur payload of ExoMars’ rover. It is also derived from a first MicrOmega model which was developed for Phobos Grunt. MicrOmega/MASCOT will enable the first in situ microscopic characterization of a C-type asteroid.
MicrOmega/MASCOT acquires monochromatic images of samples, 128 x 128 pixels of 20 µm2 each. The samples are illuminated by an AOTF-based dispersive system, onto a 2D HgCdTe array, cooled by a dedicated cryocooler. By scanning the illumination wavelength, over the spectral range 0.9 to 3.5 µm with steps of 20 cm-1, 3D (x,y,λ) image cubes are built, in about 10 mn.
The overall instrument, including its driving electronics, has a mass of 2 kg. The detector assembly, the illuminator and the analog electronic box of MicrOmega are shown in Figure 114.
The camera is provided by DLR (Figure 115) is mounted inside the lander slightly tilted, such that the center of its FOV (Field of View) is aimed at the surface at an angle of 22º with respect to the surface plane. This implies that both the surface close to the lander at a distance of 15 cm and the horizon are in the FOV. The optics are designed according to the Scheimpflug principle, which ensures that the entire scene is in focus.
This wide angle camera uses LED illumination for nighttime operation. It is equipped with a 1024 x 1024 pixel CMOS sensor sensitive in the 400-1000 nm wavelength range, and has a FOV of 60° x 60°. The instantaneous field of view is 2.1 mrad/pixel. The camera images will be high priority and involve five pictures per day and four per night. Images will also be made during descent and hopping.
Each photo has a volume of 14.7 Mbit. The instrument has heritage from ExoMars, Rosetta/Philae and the ISS.
MAG is provided by TU Braunschweig, Germany. As indicated by meteorite composition, the asteroid material is magnetic, but how is the magnetic field arranged? In order to answer the question about the magnetization state of asteroids, MASCOT MAG shall:
- Observe magnetic field profile during descent and hopping
- Determine global and local magnetization of the asteroid.
This will provide new data on asteroid formation and history, and provide deeper insight on early planetary formation stages. MAG (Figure 4) is a three-axial fluxgate vector compensated magnetometer based on a long heritage (Themis, Rosetta, VeX, BepiColombo).Its main performance parameters are :
- Dynamic range is ±50000 nT
- Sensor noise (@1Hz) 10 pT/√Hz
- Resolution 6 pT
- Sampling rate 10 Hz
Figure 116: Schematic view of MAG (image credit: TU Braunschweig)
MARA (MASCOT Radiometer):
MARA is a multispectral instrument, provided by DLR, to measure the radiative flux emitted from the asteroid’s surface using thermopile sensors. Six individual filters will be employed to measure the flux in the wavelength bands between 5.5-7, 8-9.5, 9.5-11.5, 13.5-15.5, 8-14, and 5-100 µm. The primary scientific goal of the MARA instrument is the determination of the asteroid’s thermal inertia, the secondary goal is the characterization of the surface mineralogy.
To determine the surface thermal inertia, MARA will measure the temperature of the asteroid’s surface over the period of a full rotation using the long wavelength channel from 5 - 100 µm. In addition, the emissivity of the surface can be determined from the flux in the bandpass filters from 5.5-7, 8-9.5, 9.5-11.5, 13.5-15.5, and 8-14 µm. Thermal inertia can then be determined from an investigation of the surface radiative energy balance.
The mineralogy of the surface can be characterized from an investigation of the radiative flux in the same bandpass channels, as rock forming minerals like olivine and pyroxene have characteristic absorption features in the channels covered by MARA. In addition, the 8 to 14 µm filter is identical to the filter used by the thermal mapper on the Hayabusa-2 spacecraft, such that measurements by MARA can be directly compared to the results obtained from the spacecraft. In this way, MARA can provide ground truth at small scales, thus providing context for the spacecraft measurements.
Figure 117: Photo of the MARA sensor head flight model (image credit: DLR)
MARA is mounted in between the infrared spectrometer and the camera, such that the FOV of MARA and the camera overlap. MARA consists of the following functional elements:
- Sensor head
- MARA electronics
- In-flight calibration surface.
Overview of the MASCOT operational concept:
In the context of Hayabusa-2 mission, the following major flight operational phases are foreseen for MASCOT:
1) Cruise phase (~4 years): MASCOT is normally off controlled in temperature by Hayabusa-2. At agreed times, MASCOT is activated from the ground to perform checkouts and instrument calibrations; Special operations, like PRM activation, instrument commissioning, on-board software maintenance are also foreseen.
2) Asteroid approach and global mapping: This phase is to characterize the asteroid’s surface and select the target landing site based on scientific relevance and lander’s constraints; Operational preparation for landing will occur: Initial on-board parameter tuning, science command sequences upload, on-asteroid operations rehearsal.
3) Pre-separation: Final on-board parameter tuning, final science command sequences upload, battery depassivation, full MASCOT check-out, lander pre-heating.
4) SDL (Separation, Descent and Landing) and on-asteroid operations: During an Hayabusa-2 descent maneuver, MASCOT will be deployed towards the asteroid’s surface. Primary batteries will supply the power for the mission. Due to the short lifetime (10-16 hrs depending on the mission profile) all operations for science data collection, processing, downlink to Hayabusa-2, uprighting and hopping maneuvers will be fully autonomous, driven by the MAM (Mission Autonomy Manager), an OBSW package. This phase will last up to EOL (End-Of-Life), corresponding to the discharge of the primary batteries.
Autonomous Operations during SDL and On-Asteroid Phase:
After the global mapping and pre-separation phase operations, MASCOT will be released towards the asteroid’s surface: Autonomous operations driven by the MAM will start at this point, after the separation detection. Manual commanding from the ground is still possible in parallel to MAM, but its use is only in case of unforeseen contingencies, not handled on-board. MAM commanding can be paused if necessary to avoid interference in the commanding.
MASCOT deployment will occur either
during a dedicated Hayabusa-2 descent or during one of the Hayabusa-2
sampling touchdown rehearsals. This maneuver foresees Hayabusa-2 to
descent to a separation altitude less than 100 m at which point MASCOT
will get ejected via a spring mechanism. As MASCOT has no propulsion
system, its descent to the surface of the asteroid will be completely
passive, under the effects of the weak asteroid gravity field, followed
by a bouncing period, before it comes to rest in
During descent, MASCOT will measure the asteroid magnetic field by the MAG instrument. The MAM will also schedule several image takings of the asteroid by the CAM instrument (a specific CAM image processing algorithm is implemented in the OBSW to discard dark - night sky - or saturated - sun facing - pictures).
During descent, MASCOT will measure the asteroid magnetic field by the MAG instrument. The MAM will also schedule several image takings of the asteroid by the CAM instrument (a specific CAM image processing algorithm is implemented in the OBSW to discard dark - night sky - or saturated - sun facing - pictures).
Once at rest on the surface, the MAM will start an upright maneuver, in case MASCOT is not laying with the -Z side (bottom) in contact with the surface: This is necessary for CAM, MARA and MicrOmega instrument science operations to characterize the asteroid soil. The upright maneuver may take several attempts and a specific FDIR algorithm has been devised to maximize the chance of success, to cope against adverse surface morphology and possible on-board failures.
Landing is currently foreseen at around the asteroid midday, so the MAM will schedule a day and a night science measurement cycle on the first landing location. Afterwards, the mobility mechanism will be activated by the MAM to send MASCOT in an uncontrolled hop across the asteroid’s surface at a varying distance of up to a few 10 m. Further scientific activities will take place, and then, depending on the power and data download status, a second hop will be considered.
After MASCOT’s release, Hayabusa-2 will return to its home position (HP), hovering 20 km above the asteroid’s surface (Figure 118). During the ascent phase, pictures will be taken from Hayabusa-2 of the MASCOT landing site, using the asteroid camera.
The selected landing site will have to ensure the following characteristics:
- daylight duration between 50% and 70% of the asteroid's rotation period, due to thermal and scientific reasons (one asteroid day is estimated to be ~7.6 hours)
- the duration per asteroid rotation period of visibility of the landing site from Home Position must be over 40% (TM/TC link constraint)
- the velocity at touchdown must be smaller than half the escape velocity
- average surface temperature between -50ºC and +25ºC (under re-assessment) to grant acceptable landing conditions.
SRC (Sample Return Capsule)
The SRC is designed to deliver the asteroid samples to Earth. A series of major TCM (Trajectory Correction Maneuvers) are to be carried out starting a month before the reentry (R) day. The spacecraft is to be inserted into the reentry orbit targeting the landing center at the final TCM on about R-3 day. A schematic view of the reentry sequence is shown in Figure 119. - The SRC is separated from the mothership about 8 hours before the reentry and the spacecraft will enter the escape trajectory after orbital maneuvering by the RCS while the SRC enters Earth's atmosphere at a velocity of about 12 km/s. After passing through the severe aerodynamic environment, SRC deploys the parachute at the altitude of about 5 km and it descends slowly with beacon signal emitted for localization until landing on the ground. 95)
Figure 120 shows the aerodynamic heating environment of the Hayabusa-2 SRC when compared with that of Hayabusa-1 as a function of the flight path angle and the entry velocity. The main parameters dominating the reentry flight environment are the flight path angle and the entry velocity while the effect of the entry position (latitude/longitude) and the flight azimuth direction is secondary, so that we can neglect them at the present discussion. A maximum heat flux of 13 MW/m2 is predicted on the stagnation heat transfer rate with total heat input of 270 MJ/m2.
Design and verification tests for the Hayabusa-2 SRC:
There are strong demands of short-term development and cost reduction as well as improvement of the reliability for the Hayabusa-2 project. Therefore, the basic design concept for the Hayabusa-2 SRC is not to change the design from the former SRC except for the following items:
1) During the period of over 10 years since the development of the Hayabusa-1 SRC, some components have gone out of production. Since the many electronics parts, including FPGAs, used as the core processors of the SRC instrument module are discontinued, the onboard electronics of the instrument module need to be redesigned.
2) The SRC design needs to be improved based upon the experience of Hayabusa-1 operation. Examples of this case include employing a new HK telemetry for monitoring of the on-board clock accuracy and a backup timer for generating the parachute deployment trigger for the case of an acceleration sensor malfunction.
3) The items judged by their design shall be changed to be compatible with the change of the design condition such as environmental requirements and the design standard of Hayabusa-2 system. Examples of this case include a change of the criterion on the tensional force of the Marman Clamp band of the SRC separation mechanism.
4) The items judged by their design need to be improved to ensure higher reliability based on the result of the design review including the evaluation of the reliability analysis such as FMEA, FTA and SPFA. Examples of this case include a change of an ignition circuit design, strengthening of the puller used for the SRC separation mechanism, and an adoption of the additional O-ring to prevent the inflow of high-temperature gas during reentry operation.
Overview of the SRC Subsystem:
Hayabusa-2 SRC is designed with the following functions in its compact body:
• Sampler Interface: SRC has an interface with the sampler-container system for collecting and storing asteroid sample. The sampler-container is fixed to the inside of the SRC at the time of the launch. The sampler-catcher is held in the sampler-mechanism. After Hayabusa-2 approaches the target asteroid and collects a “sample”, the sampler-catcher is conveyed into the sampler container and is fixed to it with the latch mechanism. The SRC conducts the Earth reentry in the configuration of Figure 121.
• Mothership Interface / SRC Separation Mechanism: In addition to its mechanical I/F, the SRC has electric and thermal I/F with the mother ship, such as HK telemetry, command, heater control, and external power supply from the mothership. The SRC separation mechanism holds the SRC on the mothership during the orbital flight. The mechanism deploys the Merman Clamp band and ejects the SRC from the mothership with proper velocity and spin rate given by the helical coil spring.
• Heatshield and Thermal Protection Subsystem: In order to protect the internal instruments from the aerodynamic heating environment during reentry operation, the SRC is wrapped with an ablator made of carbon phenolic resin. The ablator consists of four parts; the fore body ablator, the aft ablator, the support ablator and the sampler ablator. With the aim of avoiding soakback from the ablator, the instrument box in the SRC throws the fore body ablator and the aft ablator away after passing through the excessively severe aerodynamic environment (Figure 122).
• Descending Subsystem: After the separation of the ablators, the instrument box connected to a parachute starts slow descending. The descending system consists of the cross-type parachute, the parachute deployment mechanism and the ignition circuit (IG-BOX). The instrument box is also equipped with the parachute anchor release mechanism in order to avoid breakage due to dragging by strong ground-winds after landing.
• Beacon Transmission: The beacon antenna is pulled out and hanged from the bottom of the instrument module (I/M) by a small mass at the time of the parachute deployment. And simultaneously, I/M starts transmitting the beacon signal for the localization from the ground stations. The beacon is transmitted even after the instrument box lands on the ground according to the prescribed sequence.
• Reentry Flight Measurement: Hayabusa-2 has a new capability to measure acceleration, attitude motion (angular rate) and the temperature of each part inside the SRC during reentry operation. The measurement data is stored to a memory and is retrieved after recovery. This function is implemented in a new instrument: REMM (Reentry Flight Measurement Module).
REMM is an instrument which was newly developed to add the function of flight environment measurement to SRC. REMM measures the heat shield temperatures, the acceleration and the attitude motion of the SRC during the reentry and records the measurement data to a non-volatile memory. Since there is no structural design change for Hayabusa-2 SRC, only very limited space and mass can be allocated for the REMM. A COTS based design was adopted in design of REMM to meet the requirement under this very strict constraints.
Table 9: Specification of REMM
Figure 123: REMM functional block diagram (image credit: JAXA)
Figure 124: REMM FM (Flight Model) and its configuration in SRC (image credit: JAXA)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).