Minimize Hayabusa-2

Hayabusa-2, Japan's second Asteroid Sample Return Mission

Spacecraft   Launch   Mission Status   Sensor Complement   MASCOT
Sample Return Capsule   References

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.

Primitive bodies are celestial bodies which contain information of the birth or the following evolution of our solar system. In larger bodies such as the Earth, the pristine materials inside were once melted and we cannot access older information. On the other hand, hundreds of thousands of asteroids and comets discovered ever, though they are very small, retain memories of when and where they were formed in the solar system. Exploration of such primitive bodies will provide us an essential clue for how our solar system was formed and has grown, and how the primary organic materials of life on the Earth were composed and evolved, which will be important knowledge also in investigating formation and evolution of exoplanets.

Among primitive bodies, asteroids can be classified into several groups according to the spectroscopic observation by telescopes. Inside the asteroid belt between the Mars and Jupiter, it is known that distribution of each group varies according to the distance from the Sun.

In the region close to the Sun, we can observe many 'S-type asteroids' whose primary component is expected to be stony. These will give us hints about components of stony planets such as the Earth and Mars. S-type asteroids have been expected to be the birthplace of 'ordinary chondrite', the most common meteorites on the Earth. This hypothesis was extensively confirmed by exploration of S-type asteroid "Itokawa" by JAXA's asteroid probe Hayabusa.

Most commonly distributed around the midst of the asteroid belt are 'C-type asteroids' expected to contain substantial organic or hydrated minerals. This type is expected to be the birthplace of 'carbonaceous chondrite', and an important target for investigation of origin of life on earth. The Hayabusa-2 mission, following Hayabusa, is planning a sample-return from C-type asteroids.

Around the dark and cold outer edge of the belt, closer to Planet Jupiter rather than to Mars, there are many P-type or D-type asteroids, expected to be more primitive than the S- or C-type ones. Trojan groups sharing their orbits with Jupiter are repositories of D-type asteroids. Active comets abundant in volatile components, born in further space and changed its orbit relatively recently to come closer to the Sun, or "dormant comet nuclei" depleting gases or dusts and difficult to distinguish from asteroids, are also quite essential targets. Because meteorites from D-, P-type asteroids or comet nuclei have been scarcely discovered on the Earth, the surface materials and constructions of these distant bodies are entirely unknown. Materials yet to be acquired, holding the earliest information at the birth of the solar system, may be discovered. The successive mission after "Hayabusa-2" is discussed to fetch samples from such bodies.

In this respect, JAXA will conduct, not only random single missions, but programmatic and systematic mission series successively, by Hayabusa, Hayabusa-2 and post -Hayabusa-2 for sample-return from typical primitive bodies. This will allow a unified understanding of various primitive bodies, revealing of components and construction of the whole solar system, and elucidation of the mystery behind its origin and evolution. This successive sequence is directed to the more distant and more primitive bodies from the scientific view, with more sophisticated technologies.

Table 1: Some background on the programmatic exploration of primitive bodies

Hayabusa →Visit to S-type Asteroid

Hayabusa-2 →Visit to C-type Asteroid

Technological demonstrator:
- Round-trip to asteroid
- Sample return

1) Science:
-
Origin and evolution of the solar system
- Organic matter, H2O

Engineering:
- Ion engine
- Autonomous navigation
- Sample collection
- Reentry capsule

2) Engineering:
- Technology: more reliable and robust
- New challenge : ex) impactor

Science: Origin and evolution of the solar system
-
Remote sensing observation
-
Sample analysis

3) Exploration:
- Extend the area that humanity can reach
- Spaceguard, Resources, Research for manned mission, etc.

Table 2: Objectives : Hayabusa vs Hayabusa-2 5)

Hayabusa2_Auto77

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.

Hayabusa2_Auto76

Figure 2: Illustration of the asteroid 1999JU3 orbit within the solar system (image credit: JAXA)

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.

Hayabusa2_Auto75

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.




Spacecraft:

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.

Hayabusa2_Auto74

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.

Hayabusa2_Auto73

Figure 5: Photo of the IES assembly (image credit: JAXA)

Hayabusa2_Auto72

Figure 6: Top view of the deployed Hayabusa-2 spacecraft illustrating the various elements of the spacecraft (image credit: JAXA)

Hayabusa2_Auto71

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.

Spacecraft structure

Size: 1.6 m×1.0 m×1.4 m(height) box structure with two fixed SAPs
Mass: 609 kg(wet), 493 kg(dry)

Data handling subsystem

- COSMO16 based DHU-PIM bus system
- Onboard autonomous command generation feature
- 1GB data recorder

AOCS (Attitude and Orbit Control Subsystem)

- HR5000S based processor, double redundant
- 4 Reaction Wheels, 2 IRUs, 2 Star Trackers (STT), 4 CSAS Coarse Sun Aspect Sensors), 4 ACS (Accelerometers)
- Sensors for Proximity Operation: LIDAR, LRF, 5 Target Makers (TM), Flash lamp (FLASH)
- ONC (Optical Navigation Cameras): Wide: ONC-W1, ONC-W2, (FOV 54º x 54º, 1Mpix); Telescopic: ONC-T (FOV 5.4º x5.4º, 1Mpix, 5 band filter)

Propulsion subsystems

- RCS (Reaction Control System): Bi-propellant hydrazine system, 20 N thruster x 12
- IES (Ion Engine System): Xe microwave discharge ion engine system; Maximum thrust 28 mN, Isp=2800 s; 4 thruster heads on gimballed stage; 3 operative at once (4/3 redundant)

Power subsystem

SAP (Solar Array Paddle): -1.4 kW@1.4 AU, 2.6 kW@1AU; BAT: Li-ion battery 13.2 Ah; power bus: SSR (Series Switching Regulator) system, 50 V bus.

Communication subsystem

- X-band TT&C (coherent X-up/X-down), data rate: 8 bit/s-32 kbit/s, double redundant.
- Ka-band telemetry (coherent X-up/Ka-down), data rate: 8 bit/s-32 kbit/s
- Normal/Regenerative Ranging System
- DDOR or ΔDOR (Delta-Differential One-way Ranging) support
- 1 X-HGA, 1 Ka-HGA, 1 two-axis gimballed X-MGA, 3 X-LGA

Table 3: Spacecraft system parameters

Hayabusa2_Auto70

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)

Secondary payloads:

• 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.

Hayabusa2_Auto6F

Figure 9: Round-trip trajectory design for Hayabusa-2 (J2000EC Sun-Earth line-fixed coordinates), image credit: JAXA

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).




Mission status:

• 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.

Research Methodology

- 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.

Research 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.

Hayabusa2_Auto6E

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)

Hayabusa2_Auto6D

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).

Year

Date

Event

Status

2018

June 3
June 27
June 17-24
July 31-August 2
August 5-7
August 18-September 7
September 11-12
September 20-21
October 2-5
October 14-15
October 23-25
November 23-December 29

Approach Guidance Start (Dist.=3100 km)
Home Position Arrival (Alt.=20 km)
Box-C Descent Observation (Alt.=6.5 km)
[MID] Mid. Altitude Descent (Alt.=5 km)
[GRV] Gravity Measurement Descent (Alt.=1 km)
Box-B Obs. (Alt.=20 km, Dawn-Dusk Obs.)
[TD1-R1] Touch Down Rehearsal (Alt.=600 m)
[MNRV1] MINERVA-II-1 Deployment (Alt.=55 m)
[MSC] MASCOT Deployment (Alt.=51m)
[TD1-R1A] Touch Down Rehearsal (Alt.=22 m)
[TD1-R3] TD Rehearsal/TM Release (Alt.=12 m)
Conjunction Orbit Operation (Max Alt.=110 km)

Complete
Complete
Complete
Complete
Complete
Complete
Aborted
Success
Success
Complete
Complete
Complete

2019

February 20-22
March 6-8
March 20-22
April 3-6
April 23-25
May 14-16
May 28-30
June 11-13
July 8-11
September 12-22
September 28-October 8
November 12-18
December 2

[TD1-L08E1] Touch Down Operation 1
[DO-S01] TD Candidate Descent Observation (Alt.=22 m)
[CRA1] Pre-Impact Scan Observation (Alt=1.6 km)
[SCI] Kinetic Impact Operation (Alt.=500 m)
[CRA2] Post-Impact Scan Observation (Alt=1.6 km)
[PPTD-TM1] TM Release for PPTD (Alt.=50 m)
[PPTD-TM1A] TM Release Retrial (Alt.=9 m)
[PPTD-TM1B] Crater/PPTD Target Observation (Alt.=9 m)
[PPTD] Touch Down Operation 2
[TM-ORB] TM Orbiting (Alt=1.0 km)
[MNRV-ORB] MINERVA-II-2 Orbiting (Alt=1.0 km)
Leaving Home Position (Asteroid Departure)
Return Ion Engine Cruise Start

Success
Complete
Complete
Success
Complete
Aborted
Complete
Complete
Success
Success
Success
Plan
Plan

Table 4: Asteroid Proximity Operation History

Hayabusa2_Auto6C

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.

Hayabusa2_Auto6B

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.

Hayabusa2_Auto6A

Figure 14: Narrowed touchdown candidate site L08-B and the Target Marker (indicated by the arrow) dropped in the TD1-R3 operation (image credit: Hayabusa-2 Team and the DLR/CNES MASCOT Team)

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.

Hayabusa2_Auto69

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.

Hayabusa2_Auto68

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.

Hayabusa2_Auto67

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.

Hayabusa2_Auto66

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).

Hayabusa2_Auto65

Figure 19: Boulder distributions around the dropped TM. The ellipses indicate identified big boulders with the numbers indicating their estimated height in centimeter (image credit: Hayabusa-2 Team)

- 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.

Hayabusa2_Auto64

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)

Hayabusa2_Auto63

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).

Hayabusa2_Auto62

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.

Hayabusa2_Auto61

Figure 23: A series of images captured by CAM-H before (a-c) and after (d-f) the touchdown (image credit: Hayabusa-2 Team)

Hayabusa2_Auto60

Figure 24: Post analysis result of the touchdown accuracy (left), and the sample point (right) for TD1-L08E1 operation (image credit: Hayabusa-2 Team)

- 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.

Hayabusa2_Auto5F

Figure 25: Impact target for SCI kinetic impact operation with 200m dispersion radius (image credit: Hayabusa-2 Team)

- 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).

Hayabusa2_Auto5E

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)

Hayabusa2_Auto5D

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.

Hayabusa2_Auto5C

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.

Hayabusa2_Auto5B

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)

Hayabusa2_Auto5A

Figure 30: Post analysis result of the touchdown accuracy (left), and the sample point (right) for 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.

Hayabusa2_Auto59

Figure 31: Officials from Japan's space agency celebrated news of Hayabusa-2's successful second touchdown on the asteroid Ryugu (image credit: JAXA)

Pristine samples

- 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.

Hayabusa2_Auto58

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.

Hayabusa2_Auto57

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)

Hayabusa2_Auto56

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.)

Hayabusa2_Auto55

Figure 35: This image was taken 4 seconds before touchdown with CAM-H (image credit: JAXA)

Hayabusa2_Auto54

Figure 36: This image was acquired at the moment of touchdown with CAM-H (image credit: JAXA)

Hayabusa2_Auto53

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.

Hayabusa2_Auto52

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)

• On 5 April 2019, JAXA separated the SCI (Small Carry-on Impactor) onboard the asteroid explorer of Hayabusa-2 for deployment to Ryugu and put the SCI into operation. 24) 25)

- 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.

Hayabusa2_Auto51

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.

Hayabusa2_Auto50

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)

Hayabusa2_Auto4F

Figure 42: Schematic of the TD1 (Touchdown 1)-L08E1 operation (image credit: JAXA)

Hayabusa2_Auto4E

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.

Hayabusa2_Auto4D

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.

Hayabusa2_Auto4C

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.

Hayabusa2_Auto4B

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.

Hayabusa2_Auto4A

Figure 48: Touchdown candidate sites L08-B1 (red) and L08-E1 (green). The white circle indicates the area of L08-B, while the cross is the location of the landed target marker (image credit: JAXA)

• 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.

Hayabusa2_Auto49

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.

Hayabusa2_Auto48

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)

Hayabusa2_Auto47

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)

Name

Type

Terrain description

Original story

Country

Origin of name

Ryujin

Dorsum

Equatorial ridge

Urashima Taro

Japan

Dragon god who is Princess Otohime’s father

Urashima

Crater

Biggest crater on Ryugu

Urashima Taro

Japan

Fisherman who rescued the turtle

Cendrillon

Crater

One of the biggest craters outside the equatorial ridge

Cinderella

France

The French name for Cinderella

Kolobok

Crater

Typical crater on the equatorial ridge

Kolobok

Russia

A small round bread that ran away from home

Brabo

Crater

Typical crater on the equatorial ridge

Brabo and the giant

Netherlands

The brave young man who defeated a giant

Kintarou

Crater

5th largest crater on Ryugu

Kintarou

Japan

The child with super strength who grew up on Mt Ashigara

Momotarou

Crater

4th largest crater on Ryugu

Momotarou

Japan

The boy born from a peach who fought against an ogre

Kibidango

Crater

6th largest crater on Ryugu

Momotarou

Japan

Food that Momotarou gave his friends

Tokoyo

Fossa

Ryugu’s largest groove-shaped depression

Urashima Taro

Japan

Tokoyo, a faraway land across the sea, the land of eternal life

Horai

Fossa

Ryugu’s 2nd largest groove-shaped depression

Urashima Taro

Japan

Horai, the utopia in the sea

Catafo

Saxum

Boulder that denotes the prime meridian on Ryugu

Cajun Folktales

America

Boy who cleverly marked a route to avoid losing his way

Otohime

Saxum

Ryugu’s biggest boulder

Urashima Taro

Japan

The princess who lived in Ryugu castle and entertained Urashima and who gave him the treasure box (tamatebako)

Ejima

Saxum

One of the boulders that holds the key to Ryugu’s formation history

Urashima Taro

Japan

Where Urashima rescued the turtle and left for Ryugu Palace

Table 5: List of names places on the surface of Ryugu

- 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.

Figure 52: Spacecraft position during the conjunction transition orbit between November 23 and December 29, 2018 (video credit: JAXA)

• 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.

Hayabusa2_Auto46

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.

Hayabusa2_Auto45

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)

Hayabusa2_Auto44

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)

Hayabusa2_Auto43

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.

Hayabusa2_Auto42

Figure 57: The sampler horn on Hayabusa-2 captured with the Small Monitor Camera on August 14, 2018 (image credit: JAXA)

Hayabusa2_Auto41

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.

Hayabusa2_Auto40

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)

Hayabusa2_Auto3F

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).

Hayabusa2_Auto3E

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."

Time (CEST)

Event

03:57:21

At an altitude of 51 m above asteroid Ryugu, MASCOT is pushed out of the Hayabusa2 supporting frame by means of a spring
mechanism behind a push-off plate at about 4 cm/s and starts to descend to Ryugu without control of its ground station.

04:03

Approximately six minutes later, MASCOT touches down on Ryugu, making contact with a block of rocks about three to four meters
high. The Optical Navigation Camera (ONC) from Hayabusa-2 records the process in high-resolution images. At the same time,
DLR’s MASCAM acquires 20 images of the asteroid during the descent. MASCOT lands on the designated landing site MA-9
(Alice's Wonderland). It is located at about 300 degrees east and 30 degrees south.

Around 04:34
First location

After another approximately 31 minutes and several contacts with the surface, MASCOT reaches its first resting position. It is
asteroid day at the landing site and the surface measurements start.

Around 06:30

At the DLR Control Center in Cologne it is recognized that MASCOT is lying on an unfavorable side and thus cannot carry out
its planned experiments. All systems and experiments are operating as intended.

Around 09:20

An unscheduled command from the Earth is sent to Hayabusa-2 and from there to MASCOT to activate the swing arm to turn
the lander into its intended position for the experiments. The command time to Ryugu, which is about 300 million kilometers
away, is about 18 minutes one way.

Around 09:52

MASCOT has completed its first day-and-night cycle. The second day on Ryugu begins.

Around. 10:30
Second location

The maneuver has brought the desired result. MASCOT is in the right position, is now operational and automatically starts
to perform its four experiments again.

Around 12:51

The second daylight phase on Ryugu is slowly coming to an end, and MASCOT turns with Ryugu into its second asteroid night.

Around 17:28

MASCOT’s third day on Ryugu begins.

Around 18:29
Third location

MASCOT successfully executes a mini-move. This maneuver was commanded by the operations team in Cologne to optimize
the position of the sensors of the experiments. Further scientific investigations are carried out.

Around 20:04
Fourth location

The last jump is commanded to MASCOT and the lander goes into the End of Life phase. Further scientific investigations
are carried out.

21:04

It is the end of the third day for MASCOT on Ryugu. In the meantime, more than 16 hours have passed – the lander’s expected
maximum battery life. Contrary to the calculations, the battery still provides some power before the contact with MASCOT
breaks off by the entry of a radio shadow and the upcoming night. Instead of 16 hours, the experiments were able to work for
17 hours and 7 minutes.

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."

Hayabusa2_Auto3D

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)

Hayabusa2_Auto3C

Figure 64: Artist's rendition of MASCOT during the landing approach (image credit: DLR (CC-BY 3.0))

Hayabusa2_Auto3B

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.

Hayabusa2_Auto3A

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)

Hayabusa2_Auto39

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)

Hayabusa2_Auto38

Figure 68: This image was captured immediately before a hop of Rover-1B on 23 September 2018 at 09:46 JST (image credit: JAXA)

Hayabusa2_Auto37

Figure 69: Surface image from Rover-1B after landing, taken on 23 September at 10:10 JST (image credit: JAXA)

Hayabusa2_Auto36

Figure 70: Surface image taken from Rover-1A on September 23 2018 at 09:43 JST (image credit: JAXA)

Hayabusa2_Auto35

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.

Hayabusa2_Auto34

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.

Hayabusa2_Auto33

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)

Hayabusa2_Auto32

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)

Hayabusa2_Auto31

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.

Hayabusa2_Auto30

Figure 76: MINERVA-II1 schematic showing the deployment operation sequence (image credit: JAXA)

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)

Hayabusa2_Auto2F

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.

Hayabusa2_Auto2E

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.

Hayabusa2_Auto2D

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)

Hayabusa2_Auto2C

Figure 80: Definition of the Home Position coordinate system (image credit: JAXA)