Hayabusa-2, Japan's second Asteroid Sample Return Mission
Hayabusa-2 is JAXA's (Japan Aerospace Exploration Agency) follow-on mission to the Hayabusa mission, the country's first round-trip asteroid mission that sent the Hayabusa (MUSES-C) spacecraft to retrieve samples of asteroid Itokawa. The initial Hayabusa mission launched in May 2003 and reached Itokawa in 2005; it returned samples of Itokawa — the first asteroid samples ever collected in space — in June 2010. Hayabusa means 'falcon' in Japanese. 1) 2) 3) 4)
The objective of the Hayabusa-2 sample return mission is to visit and explore the C-type asteroid 1999 JU3, a space body of about 920 m in length and of particular interest to researchers, because it consists of 4.5 billion-year-old material that has been altered very little. Measurements taken from Earth suggest that the asteroid’s rock may have come into contact with water. The carbonaceous or C-type asteroid is expected to contain organic and hydrated minerals, making it different from Itokawa, which was a rocky S-type (stony composition) asteroid.
Table 1: Some background on the programmatic exploration of primitive bodies
Table 2: Objectives : Hayabusa vs Hayabusa-2 5)
Figure 1: Overall schedule of the Hayabusa-2 mission (image credit: JAXA)
Detailed information of asteroid 1999JU3 has been obtained by observations of ground-based telescopes. According to observation data of 2008, the diameter of 1999JU3 is estimated to be about 900 m, larger than that Itokawa, and the rotation period is around 7.6 hours. Observation of the reflected sunlight spectrum showed, that it has features of a C-type asteroid. It is rather difficult to determine the spin axis of asteroid 1999JU3 because of its rather spherical shape.
Figure 2 shows the orbit of asteroid 1999 JU3. The orbit is similar to that of Itokawa, and it is orbiting from just inside the orbit of the Earth to just outside the orbit of Mars. The inclination of the orbit is small like the one of Itokawa. Such an orbit is suitable for a small spacecraft like Hayabusa-2 to reach and return to Earth.
Legend to Figure 2: The blue circled lines in the figure illustrate the orbits of Mercury, Venus, Earth and Mars from inside, respectively, Itokawa's orbit is green, while the yellow orbit is that of 1999JU3.
The asteroid was discovered in 1999 by the LINEAR (Lincoln Near-Earth Asteroid Research) project, and given the provisional designation 1999JU3 (it hasn't been named so far). LINEAR is a collaboration of the United States Air Force, NASA, and MIT/LL (Massachusetts Institute of Technology /Lincoln Laboratory) for the systematic discovery and tracking of near-Earth asteroids.
Figure 3: Schematic of the science of Hayabusa and Hayabusa-2 missions (image credit: JAXA)
Project short history:
The Hayabusa-2 mission was proposed in 2006 at first. In this first proposal, the spacecraft was almost same as that of Hayabusa, because the project team wanted to start it as soon as possible. Of course, the team realized that parts had to be modified where trouble occurred in Hayabusa, but there were no major changes. The launch windows to go for launch to asteroid 1999 JU3 were in 2010 and 2011. However, JAXA could not start Hayabusa-2 mission immediately, because no budget was available. Hence, the launch opportunity was missed. The next launch window came up in 2014. Thus, the project postponed the launch date, and continued proposing Hayabusa-2. Since the launch was delayed, the project had time to change the spacecraft a little. New instruments were added, such as a Ka-band antenna and what is called “impactor.” The project even calls Hayabusa-2 a new spacecraft. 6)
In May 2011, the status of Hayabusa-2 project shifted to Phase-B, starting with the design of the spacecraft. In March 2012, the CDR (Critical Design Review) was done, and the team started manufacturing the flight model. The initial integration test started at the beginning of 2013, and the final integration test started at the end of 2013. At present (September 2014), the team has almost finished the final integration test, and the spacecraft will be shipped to the launch site soon.
International collaborations: The Hayabusa-2 mission involves international collaborations with Germany, the United States, and Australia. DLR (German Aerospace Center) and CNES (French Space Agency) are providing the small lander MASCOT. NASA was already a partner in the Hayabusa mission, a similar collaboration is under consideration for Hayabusa-2. The third collaboration is with Australia for capsule reentry as in the case of the Hayabusa mission.
Japan's Hayabusa-2 spacecraft is designed to study asteroid 1999 JU3 from multiple angles, using remote-sensing instruments, a lander and a rover. It will collect surface- and possibly also subsurface materials from the asteroid and return the samples to Earth in a capsule for analysis. The mission also aims to enhance the reliability of asteroid exploration technologies. 7) 8) 9)
In the current plan, the launch window for Hayabusa-2 is in late 2014. With this schedule, Hayabusa-2 will reach the asteroid in the middle of 2018, and return to the Earth at the end of 2020.
Figure 4: Artist's rendition of the Hayabusa-2 spacecraft (image credit: JAXA, Akihiro Ikeshita)
The Hayabusa-2 mission will utilize new technology while further confirming the deep space round-trip exploration technology by inheriting and improving the already verified knowhow established by Hayabusa to construct the basis for future deep-space exploration.
The configuration of Hayabusa-2 is basically the same as that of Hayabusa, with modifications of some parts by introducing novel technologies that evolved after the Hayabusa era. For example:
• The HGA (High Gain Antenna) for Hayabusa featured a parabolic shape, while Hayabusa-2 uses two planar HGAs with a considerably lower mass but with the same performance characteristics. The reason why Hayabusa-2 has two HGAs is that spacecraft has two communication links, Ka-band as well as the X-band links. In daily operations support, the team uses the X-band for data transmission, but for the download of the asteroid observation data, the Ka-band is used to take advantage of the higher data rate of 32 kbit/s, provided by the Ka-band link. The DDOR (Delta-Differential One-way Ranging) technique is used for very accurate plane-of-sky measurements of spacecraft position which complement existing line-of-sight ranging and Doppler measurements.
• The AOCS (Attitude and Orbit Control Subsystem) of Hayabusa-2 was improved, now featuring 4 reaction wheels for a more reliable service in case of need.
- During the cruise phase, Hayabusa-2 controls its attitude with only one reaction wheel to bias the momentum around the Z-axis of the body. This is to save the operating life of reaction wheels for other axes, because the project experienced that two reaction wheels of three equipped on Hayabusa were broken after the touchdown mission. 10)
- In this one wheel control mode, the angular momentum direction is slowly moved in the inertial space (generally called precession) due to the solar radiation torque. This attitude motion caused by the balance of the total angular momentum and solar radiation pressure is known to trace the Sun direction automatically with ellipsoidal and spiral motion around Sun direction. Based on this knowledge of the past, the attitude dynamics model for the Hayabusa-2 mission had been developed before the launch. According to the newly developed attitude dynamics model of Hayabusa-2, the precession trajectory is almost the ellipsoid around the attitude equilibrium point, and this equilibrium point is determined mainly by the phase angle around Z-axis of the body.
- In the actual operation of Hayabusa-2, the spacecraft experienced already the one wheel control mode, and the attitude motion in this mode is nearly corresponding to the expected motion based on the dynamics model developed before the launch. The precession trajectory is ellipsoid around the equilibrium point, and the attitude dynamics model is verified by the actual flight data. In this one wheel operation, the Sun aspect angle is restricted within a certain limit angle in terms of the thermal condition of the spacecraft. Because the precession radius is determined by the initial attitude and the equilibrium point, the Sun aspect angle almost exceeds the limit angle due to the precession without change of the equilibrium point. At this operation, the project executes the attitude maneuver around the Z-axis to change the equilibrium point, in order to reduce the Sun aspect angle - and succeeded. After that, the project executed the maneuver again to change the equilibrium point to a close point in order to make the small precession trajectory (Ref. 10).
• IES (Ion Engine System) has been modified to account for the aging effect during extended support periods. The thrust level of IES was increased by 25%, using the same Xe microwave discharge ion engine system.
IES will be used for orbit maneuvers during the cruising of the Hayabusa-2’s onward journey to the asteroid and return trip to Earth. The engine enables to make the round trip with only one tenth of the power consumption compared to that of chemical propellant.
Major improvements from the Hayabusa mission are:
- Countermeasures to plasma ignition malfunction of one ion source of an ion engine. Carefully coordinating each part of the ion engine to improve both ion source propulsion generation efficiency and ignition stability.
- Countermeasures to degradation and malfunction of three neutralizers that occurred after 10,000 to 15,000 hours of operation. To make the neutralizer’s lifespan longer, the walls of the electric discharge chamber are protected from plasma and the magnetic field has been strengthened to decrease the voltage necessary for electron emission.
- The maximum power was successfully increased to 10 mN per ion engine from the conventional 8 mN.
Figure 5: Photo of the IES assembly (image credit: JAXA)
Figure 7: Bottom view of Hayabusa-2 spacecraft illustrating the various elements of the spacecraft (image credit: JAXA)
The Hayabusa-2 spacecraft has a stowed size of 1.6 m x 1 m x 1.25 m (height). With the solar panels deployed, the 600 kg satellite as a width of 6 m.
Table 3: Spacecraft system parameters
Figure 8: Photo of the Hayabusa-2 flight model, taken in Aug. 31, 2014, before shipping to the launch site, TNSC (Tanegashima Space Center). The SRC (Sample Return Capsule) is mounted to the bottom front side center of the spacecraft (image credit: JAXA)
Launch: The Hayabusa-2 spacecraft was launched on December 03, 2014 (04:22:04 UTC) on a H-IIA vehicle (No. 26) from TNSC (Tanegashima Space Center), Japan. The launch service provider was MHI (Mitsubishi Heavy Industries, Ltd). The launch was nominally and about 1 hr 47 minutes and 21 seconds after liftoff, the separation of the Hayabusa-2 spacecraft into an Earth-escape trajectory was confirmed. 11) 12)
• Shin'en-2, a nanosatellite technology demonstration mission (17 kg) of Kyushu Institute of Technology and Kagoshima University, Japan. The objective is to establish communication technologies with a long range as far as moon. Shin'en-2 carries into deep space an F1D digital store-and-forward transponder which offers an opportunity for earthbound radio amateurs to test the limits of their communication capabilities.
• ArtSat-2 (Art Satellite-2)/DESPATCH (Deep Space Amateur Troubadour’s Challenge), a joint project of of Tama Art University and Tokyo University. DESPATCH is a microsatellite of ~30 kg. The microsatellite carries a “deep space sculpture” developed using a 3D printer, as well as an amateur radio payload and a CW beacon at 437.325 MHz.
• PROCYON (PRoximate Object Close flYby with Optical Navigation) is a microsatellite (67 kg) developed by the ISSL (Intelligent Space Systems Laboratory) of the University of Tokyo and JAXA. The objective is to demonstrate microsatellite bus technology for deep space exploration and proximity flyby to asteroids performing optical measurements. 13)
Orbit: The trajectory of Hayabusa-2 for the whole mission is shown in the sun-earth fixed coordinate in Figure 9. The total cruising time is about 4.5 years, and the asteroid proximity period is about 1.5 years. So the total flight time is about 6 years. The departure C3 is 21 km2/s2, the total impulse of the ion engine is 2 km/s, and the reentry speed of the capsule is 11.6 km/s.
Legend to Figure 9: Hayabusa-2 is equipped with a high-specific impulse ion engine system to enable the round-trip mission. First one year after launch is an interplanetary cruise phase called EDVEGA (Electric Delta-V Earth Gravity Assist).
• May 13, 2020: The 2nd ion engine operation has begun. This is an important operation in the return journey of Hayabusa-2 back to Earth. On May 12, 2020, the ion engine ignited at 07:00 (onboard time, JST) and was confirmed to be operating stably at 07:25 (ground time, JST). Currently, only a single ion engine is operating as the spacecraft is far from the Sun, and receives a low level of solar power with which to operate the ion engines. 14)
- The 2nd ion engine operation will continue until around September this year. At the end of the operation, the spacecraft will be in an orbit that can deliver the capsule to Earth. After that — from October of this year — we will perform precision guidance using the chemical thrusters (Figure 10).
• May 11, 2020: In February and July of 2019, the Hayabusa-2 spacecraft briefly touched down on the surface of near-Earth asteroid Ryugu. The readings it took with various instruments at those times have given researchers insight into the physical and chemical properties of the 1-km-wide asteroid. These findings could help explain the history of Ryugu and other asteroids, as well as the solar system at large. 15)
Figure 11: The Surface of Ryugu. False color map of the surface of Ryugu with craters marked by circles (image credit: 2020 Morota et al.)
- When our solar system formed around 5 billion years ago, most of the material it formed from became the sun, and a fraction of a percent became the planets and solid bodies, including asteroids. Planets have changed a lot since the early days of the solar system due to geological processes, chemical changes, bombardments and more. But asteroids have remained more or less the same as they are too small to experience those things, and are therefore useful for researchers who investigate the early solar system and our origins.
- “I believe knowledge of the evolutionary processes of asteroids and planets are essential to understand the origins of the Earth and life itself,” said Associate Professor Tomokatsu Morota from the Department of Earth and Planetary Science at the University of Tokyo. “Asteroid Ryugu presents an amazing opportunity to learn more about this as it is relatively close to home, so Hayabusa-2 could make a return journey relatively easily.”
- Hayabusa-2 launched in December 2014 and reached Ryugu in June 2018. At the time of writing, Hayabusa-2 is on its way back to Earth and is scheduled to deliver a payload in December 2020. This payload consists of small samples of surface material from Ryugu collected during two touchdowns in February and July of 2019. Researchers will learn much from the direct study of this material, but even before it reaches us, Hayabusa2 helped researchers to investigate the physical and chemical makeup of Ryugu.
- “We used Hayabusa-2’s ONC-W1 and ONC-T imaging instruments to look at dusty matter kicked up by the spacecraft’s engines during the touchdowns,” said Morota. “We discovered large amounts of very fine grains of dark-red colored minerals. These were produced by solar heating, suggesting at some point Ryugu must have passed close by the sun.”
- Morota and his team investigated the spatial distribution of the dark-red matter around Ryugu as well as its spectra or light signature. The strong presence of the material around specific latitudes corresponded to the areas that would have received the most solar radiation in the asteroid’s past; hence, the belief that Ryugu must have passed by the sun.
- “From previous studies we know Ryugu is carbon-rich and contains hydrated minerals and organic molecules. We wanted to know how solar heating chemically changed these molecules,” said Morota. “Our theories about solar heating could change what we know of orbital dynamics of asteroids in the solar system. This in turn alters our knowledge of broader solar system history, including factors that may have affected the early Earth.”
- When Hayabusa-2 delivers material it collected during both touchdowns, researchers will unlock even more secrets of our solar history. Based on spectral readings and albedo, or reflectivity, from within the touchdown sites, researchers are confident that both dark-red solar-heated material and gray unheated material were collected by Hayabusa-2. Morota and his team hope to study larger properties of Ryugu, such as its many craters and boulders.
- “I wish to study the statistics of Ryugu’s surface craters to better understand the strength characteristics of its rocks, and history of small impacts it may have received,” said Morota. “The craters and boulders on Ryugu meant there were limited safe landing locations for Hayabusa-2. Finding a suitable location was hard work and the eventual first successful touchdown was one of the most exciting events of my life” (the paper is provided in Ref. 17).
Figure 12: History of Ryugu. A diagram to show how researchers believe the surface of Ryugu evolved over time (image credit: 2020 Morota et al.)
• May 8, 2020: High-resolution images and video were taken by the Japanese space agency's Hayabusa-2 spacecraft as it briefly landed to collect samples from Ryugu - a nearby asteroid that orbits mostly between Earth and Mars - allowing researchers to get an up-close look at its rocky surface, according to a new report. During the touchdown Hayabusa-2 obtained a sample of the asteroid, which it will bring back to Earth in December 2020. 16)
- The detailed new observations of Ryugu's surface during the touchdown operations help scientists understand the age and geologic history of the asteroid, suggesting that its surface color variations are likely due to rapid solar heating during a previous temporary orbital excursion near the Sun.
- On February 21, 2019, after months of orbital observations to select the target location, the Hayabusa-2 spacecraft descended to the surface of Ryugu to conduct its first sample collection, picking up surface material from the carbon-rich asteroid. Previous Hayabusa-2 observations have shown that Ryugu's surface is composed of two different types of material, one slightly redder and the other slightly bluer.
- The cause of this color variation, however, remained unknown. During Hayabusa-2's touchdown, onboard cameras captured high-resolution observations of the surface surrounding the landing site in exceptional detail - including the disturbances caused by the sampling operation. Tomokatsu Morota and colleagues used these images to investigate the geology and evolution of Ryugu's surface.
- Unexpectedly, Morota et al. observed that Hayabusa-2's thrusters disturbed a coating of dark, fine-grained material that appeared to correspond with the surface's redder materials. By relating these findings with the stratigraphy of the asteroid's craters, the authors conclude that surface reddening was caused by a short period of intense solar heating, which could be explained if Ryugu's orbit took a temporary turn towards the Sun. 17)
• March 16, 2020: Carbonaceous (C-type) asteroids are relics of the early Solar System that have preserved primitive materials since their formation approximately 4.6 billion years ago. They are probably analogues of carbonaceous chondrites and are essential for understanding planetary formation processes. However, their physical properties remain poorly known because carbonaceous chondrite meteoroids tend not to survive entry to Earth’s atmosphere. 18)
• March 16, 2020: The Solar System formed approximately 4.5 billion years ago. Numerous fragments that bear witness to this early era orbit the Sun as asteroids. Around three-quarters of these are carbon-rich C-type asteroids, such as 162173 Ryugu, which was the target of the Japanese Hayabusa2 mission in 2018 and 2019. The spacecraft is currently on its return flight to Earth. Numerous scientists, including planetary researchers from the German Aerospace Center (DLR), intensively studied this cosmic 'rubble pile', which is almost 1 km in diameter. Infrared images acquired by Hayabusa-2 have now been published in the scientific journal Nature. They show that the asteroid consists almost entirely of highly porous material. Ryugu was formed largely from fragments of a parent body that was shattered by impacts. The high porosity and the associated low mechanical strength of the rock fragments that make up Ryugu ensure that such bodies break apart into numerous fragments upon entering Earth's atmosphere. For this reason, carbon-rich meteorites are very rarely found on Earth and the atmosphere tends to offer greater protection against them. 19)
Thermal behavior reveals density
- This investigation of the global properties of Ryugu confirms and complements the findings of the landing environment on Ryugu obtained by the German-French 'Mobile Asteroid Surface SCOuT' (MASCOT) lander during the Hayabusa-2 mission. "Fragile, highly porous asteroids like Ryugu are probably the link in the evolution of cosmic dust into massive celestial bodies," says Matthias Grott from the DLR Institute of Planetary Research, who is one of the authors of the current Nature publication. "This closes a gap in our understanding of planetary formation, as we have hardly ever been able to detect such material in meteorites found on Earth."
- In autumn 2018, the scientists working with first author Tatsuaki Okada of the Japanese space agency JAXA analyzed the asteroid’s surface temperature in several series of measurements performed with the Thermal Infrared Imager (TIR) on board Hayabusa-2. These measurements were made in the 8 to 12 µm wavelength range during day and night cycles. In the process, they discovered that, with very few exceptions, the surface heats up very quickly when exposed to sunlight. "The rapid warming after sunrise, from approximately minus 43 degrees Celsius to plus 27 degrees Celsius suggests that the constituent pieces of the asteroid have both low density and high porosity," explains Grott. About one percent of the boulders on the surface were colder and more similar to the meteorites found on Earth. "These could be more massive fragments from the interior of an original parent body, or they may have come from other sources and fallen onto Ryugu," adds Jörn Helbert from the DLR Institute of Planetary Research, who is also an author of the current Nature publication.
From planetesimals to planets
- The fragile porous structure of C-type asteroids might be similar to that of planetesimals, which formed in the primordial solar nebula and accreted during numerous collisions to form planets. Most of the collapsing mass of the pre-solar cloud of gas and dust accumulated in the young Sun. When a critical mass was reached, the heat-generating process of nuclear fusion began in its core.
- The remaining dust, ice and gas accumulated in a rotating accretion disk around the newly formed star. Through the effects of gravity, the first planetary embryos or planetesimals were formed in this disc approximately 4.5 billion years ago. The planets and their moons formed from these planetesimals after a comparatively short period of perhaps only 10 million years. Many minor bodies – asteroids and comets – remained. These were unable to agglomerate to form additional planets due to gravitational disturbances, particularly those caused by Jupiter – by far the largest and most massive planet.
- However, the processes that took place during the early history of the Solar System are not yet fully understood. Many theories are based on models and have not yet been confirmed by observations, partly because traces from these early times are rare. "Research on the subject is therefore primarily dependent on extraterrestrial matter, which reaches Earth from the depths of the Solar System in the form of meteorites," explains Helbert. It contains components from the time when the Sun and planets were formed. "In addition, we need missions such as Hayabusa2 to visit the minor bodies that formed during the early stages of the Solar System in order to confirm, supplement or – with appropriate observations – refute the models."
A rock like many on Ryugu
- Already in the summer of 2019, results from the MASCOT lander mission showed that its landing site on Ryugu was mainly populated by large, highly porous and fragile boulders. "The published results are a confirmation of the results from the studies by the DLR radiometer MARA on MASCOT," said Matthias Grott, the Principal Investigator for MARA. "It has now been shown that the rock analyzed by MARA is typical for the entire surface of the asteroid. This also confirms that fragments of the common C-type asteroids like Ryugu probably break up easily due to low internal strength when entering Earth’s atmosphere."
- On 3 October 2018, MASCOT landed on Ryugu in free fall at walking pace. Upon touchdown, it 'bounced' several meters further before the approximately 10-kilogram experiment package came to a halt. MASCOT moved on the surface with the help of a rotating swing arm. This made it possible to turn MASCOT on its 'right' side and even perform jumps on the asteroid’s surface due to Ryugu's low gravitational attraction. In total, MASCOT performed experiments on Ryugu for approximately 17 hours.
Samples from asteroid Ryugu on their way to Earth
- Hayabusa-2 mapped the asteroid from orbit at high resolution, and later acquired samples of the primordial body from two landing sites. These are currently sealed in a transport capsule and are traveling to Earth with the spacecraft. The capsule is scheduled to land in Australia at the end of 2020. So far, the researchers assume that Ryugu's material is chemically similar to that of chondritic meteorites, which are also found on Earth. Chondrules are small, millimeter-sized spheres of rock, which formed in the primordial solar nebula 4.5 billion years ago and are considered to be the building blocks of planetary formation. So far, however, scientists cannot rule out the possibility that they are made of carbon-rich material, such as that found on comet 67P/ Churyumov-Gerasimenko as part of ESA's Rosetta mission with the DLR-operated Philae lander. Analyses of the samples from Ryugu, some of which will be carried out at DLR, are eagerly awaited. "It is precisely for this task – and of course for future missions such as the Japanese 'Martian Moons exploration' (MMX) mission, in which extraterrestrial samples will be brought to Earth – that we at DLR's Institute of Planetary Research in Berlin began setting up the Sample Analysis Laboratory (SAL) last year," says Helbert. The MMX mission, in which DLR is participating, will fly to the Martian moons Phobos and Deimos in 2024 and bring samples from the asteroid-sized moons to Earth in 2029. A mobile German-French rover will also be part of the MMX mission.
Hayabusa-1 mission of JAXA (insert)
Some background of Hayabusa-1: This rather important result of the Hayabusa-1 mission of JAXA was inserted in the Hayabusa-2 file due to a lack of a Hayabusa-1 file on the eoPortal. The MUSES-C (Mu Space Engineering Satellite) mission; launch May 9, 2003) with the nickname ”Hayabusa”, was a deep space asteroid sample return mission of JAXA. - In Nov. 2005, the spacecraft performed five descents, among which two touch-down flights were included. Actually Hayabusa made three touch-downs and one long landing on the surface of asteroid Itokawa during those two flights.
- After overcoming multiple serious glitches, and a three-year delay in its round-trip journey of over 5 billion km (or 5 x 109 km), JAXA's Hayabusa sample return canister landed in South Australia (Woomera) on June 13, 2010. The canister separated from the spacecraft about three hours before reaching Earth, and returned to Earth via parachute. The canister has been recovered by JAXA engineers, and will be taken to Japan where scientists will open it to find out if there is anything inside. 20)
- On Nov. 16, 2010, JAXA confirmed that the tiny particles inside the Hayabusa spacecraft‘s sample return container are in fact from the asteroid Itokawa. 21) Based on the results of the scanning electron microscope (SEM) observations and analyses of samples that were collected with a special spatula from sample catcher compartment "A", about 1,500 grains were identified as rocky particles, and most of them were judged to be of extraterrestrial origin, and definitely from Asteroid Itokawa. Their size is mostly less than 10 µm, and handling these grains requires very special skills and techniques. - These are the first samples from an asteroid ever returned to Earth.
• February 28, 2020: Mineralogists from FSU (Friedrich Schiller Universität) Jena, Germany, and Japan discover a previously unknown phenomenon in soil samples from the asteroid ‘Itokawa’: the surface of the celestial body is covered with tiny hair-shaped iron crystals. The team provides an explanation of how these were formed, and why asteroids can be unusually low in sulphur compounds, in the current issue of ‘Nature Communications’. 22) 23)
Itokawa would normally be a fairly average near-Earth asteroid - a rocky mass measuring only a few hundred meters in diameter, which orbits the sun amid countless other celestial bodies and repeatedly crosses the orbit of the Earth. But there is one fact that sets Itokawa apart: in 2005 it became a visit from Earth. The Japanese space agency JAXA sent the Hayabusa probe to Itokawa, which collected soil samples and brought them safely back to Earth — for the first time in the history of space travel. This valuable cargo arrived in 2010 and since then, the samples have been the subject of intensive research.
Figure 13: Asteroid Itokawa, recorded by the Japanese space probe Hayabusa in October 2005 (image credit: JAXA)
A team from Japan and Jena has now succeeded in coaxing a previously undiscovered secret from some of these tiny sample particles: the surface of the dust grains is covered with tiny wafer-thin crystals of iron. This observation surprised Prof. Falko Langenhorst and Dr Dennis Harries of Friedrich Schiller University in Jena. After all, over the last 10 years, research teams all over the world have exhaustively studied the structure and chemical composition of the dust particles from Itokawa, and no one had noticed the iron ‘whiskers’. It was only when Japanese researcher Dr Toru Matsumoto, who is spending a year as a visiting scientist with the Analytical Mineralogy group at the Institute of Geosciences in Jena, examined the particles with a transmission electron microscope that he was able to locate the crystals using high-resolution images.
Figure 14: From left: Prof. Dr Falko Langenhorst, Dr Toru Matsumoto and Dr Dennis Harries (image credit: Anne Günther/FSU)
Solar wind weathers celestial bodies
This discovery is exciting not only because the tiny iron ‘whiskers’ - which have since been shown on other particles from the asteroid as well - had previously been missed. Of particular interest is how they were formed. "These structures are the consequence of cosmic influences on the surface of the asteroid," explains Falko Langenhorst. In addition to rocks, high-energy particles from the solar wind also strike the asteroid’s surface, thus weathering it. An important constituent of the asteroid is the mineral troilite, in which iron and sulphur are bound. "As a result of space weathering, the iron is released from the troilite and deposited on the surface in the form of the needles that have now been discovered," says the mineralogist Langenhorst. The sulphur from the iron sulphide then evaporates into the surrounding vacuum in the form of gaseous sulphur compounds.
Figure 15: Microscopic images in false colors. (a) One of investigated Itokawa grain. The mineral troilite (FeS, violet) is surrounded by silicate minerals (green). (b) Troilite surface (violet) with iron whiskers (blue). (c) An enlarged image of an iron whisker (image credit: Toru Matsumoto)
From the size and number of the ice crystals detected, the researchers can also estimate how quickly the asteroid loses sulphur. "The process is incredibly fast from a cosmic perspective," explains Toru Matsumoto. The crystals he analyzed are up to two-and-a-half micrometers long, which is around one-fiftieth of the thickness of a human hair. "The tiny whiskers have already reached these sizes after around 1,000 years," adds the researcher from Kyushu University in Fukuoka. Over the long term, the analysis of the ice crystals can be used to gain a better understanding of weathering processes on other celestial bodies as well, and to determine their age.
To this end, the researchers already have specific asteroids in their sights. NASA’s OSIRIS-REx probe is currently preparing to take samples from asteroid Bennu, while JAXA’s Hayabusa2 is already on its way back to Earth. The Japanese probe visited the Ryugu asteroid last year and, as with Itokawa, it collected dust particles. The samples should land on Earth at the end of 2020 and the international team of Jena mineralogists and Toru Matsumoto are awaiting them with anticipation
• 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. 24)
- A research group led by Assistant Professor Naoyuki Hirata of the Department of Planetology at Kobe University's Graduate School of Science revealed 77 craters on Ryugu. Through analyzing the location patterns and characteristics of the craters, they determined that the asteroid's eastern and western hemispheres were formed at different periods of time.
- It is hoped that the collected data can be used as a basis for future asteroid research and analysis. These results were first published in the American Scientific Journal 'Icarus' on November 5 2019.
- JAXA's Hayabusa-2 has been used to carry out various missions to increase our understanding of the spinning top-shaped, Near-Earth asteroid Ryugu. Since arriving in June 2018, the unmanned spacecraft has taken samples and a great number of images of the asteroid. It is hoped that these can reveal more about Ryugu’s formation and history.
- This research group focused on using the image data to determine the number and location of impact craters on the asteroid. Impact craters are formed when a smaller asteroid or a comet hits the surface of the asteroid. Analyzing the spatial distribution and the number of impact craters can reveal the frequency of collisions and aid researchers in determining the age of different surface areas.
- First of all, the image data from Hayabusa-2 was analyzed. Hayabusa 2 has many different types of camera including Optical Navigation Cameras (ONC). The ONC team has been able to take around 5000 images of Ryugu, which have revealed many surface features- including impact craters. For this study, image data obtained from the ‘ONC-T’ camera between July 2018 and February 2019 was utilized. The research group had to determine which of these images showed craters. 340 images were used for crater counting, with stereopair images making it easier to identify the craters. A global image mosaic map was constructed from the ONC images and rendered onto the computer model of Ryugu’s shape. Small Body Mapping Tool software was then used to measure the size, latitude and longitude of the craters. A LiDAR (Light Detection and Ranging pulsed laser) was also utilized to determine the overall size of Ryugu.
- The depressions identified on Ryugu were divided into four categories- depending on how evident their circular appearance was. Category I to III depressions were classified as distinct craters. Category IV depressions only had quasi-circular features, therefore it was hard to determine whether they were craters or not. Many craters were filled with boulders or lacked a distinct shape. Depressions that were too vague to determine were left out of the results.
- The research team were able to identify all impact craters over 10 to 20m in diameter on Ryugu’s entire surface- a total of 77 craters. Furthermore, a pattern was discovered in their distribution. The section of the eastern hemisphere near the meridian was found to have the most craters. This is the area near the large crater named Cendrillon - which is one of Ryugu’s biggest. In contrast, there are hardly any craters in the western hemisphere- suggesting that this part of the asteroid was formed later. The analysis also revealed that there are more craters at lower latitudes than at higher latitudes on Ryugu. In other words, there are very few craters in Ryugu’s polar regions.
- The equatorial ridge in the eastern hemisphere was determined to be a fossil structure. When asteroids like Ryugu rotate at high speeds, this can alter their shape. It is thought that this ridge formed in the distant past during a period when it only took Ryugu 3 hours to rotate. As the eastern hemisphere and western hemisphere were formed at different periods of the asteroid’s history- this suggests that there have been at least two instances where Ryugu’s rotational speed has increased.
Figure 16: Size and location of craters on Ryugu (Figure from the Journal paper): The craters are numbered in order of size (image credit: Kobe University) 25)
Figure 17: 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. 26)
- 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. 27)
Asteroid Proximity Operation
- Summary of Asteroid-Proximity Activity: Hayabusa2 arrived at HP (Home Position) on 27 June 2018, when the asteroid proximity phase began. The HP is defined as the position 20 km from the asteroid center toward asteroid-Earth (sub-Earth) line. The HP is always located on the day side of the asteroid, since the Sun-asteroid-Earth angle varies between 0 to 39º during the 1.5 years of the asteroid proximity phase. The Sun-asteroid distance during the asteroid proximity phase varies between 0.96-1.4AU, and the Earth-asteroid distance varies between 2.0-2.4 AU, which corresponds to the round trip light time of 33-40 minutes.
- All the descent operations to lower altitude is defined as “critical operation” in the project, which were managed by a large operation team participated by 20-40 specifically-trained operators. Table 4 shows the list of the descent operations and the other important operations performed by Hayabusa-2. Overall, the asteroid-proximity activity proceeded as planned despite the fact that the environment of Ryugu was found to be unexpectedly severe, and some operations were aborted (i.e. the spacecraft detected a failure and ascent back to HP while descending).
Figure 18: 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 18 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 Figure18 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. 28)
- The landing site candidates for MINERVA-II-1 (Figure 18 bottom-left) and MASCOT (Figure 18 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 18 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 19, the project is to search for a safer region within selected candidate sites by additional observations, and at the same time, to improve the landing accuracy to cope with narrower landing sites.
Figure 19: 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 20). The success of TD1-R3 marked a critical milestone for the project for two reasons, (i) the dropped TM location indicates that our terrain-relative guidance is as good as 15 m, (ii) the autonomous TM tracking capability at very low altitude (~12 m) is confirmed on Ryugu environment with ±1 m TM-relative positioning accuracy. These results opened a possibility toward a TM-relative pin-point touchdown which had not been planned originally.
Critical Operation Achievements
- MINERVA-II-1 Rover Deployment Operation: The two MINERVA-II-1 rovers (Rover-1A named “HIBOU” and Rover-1B named “OWL”, Figure21 top-left) were deployed from the spacecraft at altitude of 55 m at September 21, 4:06 UT. A horizontal ΔV of 0.2 m/s was executed right before the separation to compensate for fast ejection velocity of the rovers, ensuring that the rovers should not exceed the escape velocity of Ryugu. The terrain-relative guidance accuracy at the release was 3.3 m.
- Two rovers were confirmed active immediately after the deployment, and some images captured by the rover onboard cameras were received by Hayabusa-2. On Sol 4 (4th asteroid day), Rover-1A was confirmed doing some autonomous hops, and Rover-1B as well on Sol 7. The final decodable telemetry was received on Sol 114 for Rover-1A and Sol 10 for Rover-1B. The carrier signal of Rover-1B was successfully received again on August 2, 2019 for about 10 minutes at a rather distant location from the original landing point, which indicates that the rover has acted on the Ryugu surface for more than 10 months.
Figure 21: 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 22). The two rovers have become the world’s first mobile landers to successfully act on a small body. The landing site N6 was nicknamed “Tritonis” by the MINERVA-II-1 team.
Figure 22: 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 21 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 23). No horizontal compensation was executed for this release, as the MASCOT separation mechanism deploys the lander very slowly (~5 cm/s). The terrain-relative guidance accuracy at the release was 30 m.
- Different from MINERVA-II rovers, MASCOT is driven by primary battery and can be active for 17 hours from the separation (i.e. 3-4 Sols). The project team prepared the biggest operation organization for this operation. The operation center for the MASCOT was placed in Cologne, Germany, and was connected online to SSOC (Sagamihara Space Operation Center), the operation center for Hayabusa-2 in Japan. A 24 hours continuous coverage was prepared to cover the whole life time of MASCOT using ground stations of JAXA Usuda Deep Space Center, NASA Deep Space Network, and ESA (ESTRACK) Malargüe station. The MASCOT telemetry and command operations were performed through the rover communication component (OME-E) which realizes a simultaneous two-way communication with MASCOT and all the MINERVA-II rovers.
- The MASCOT was confirmed active immediately after release, and landed to MA-6 region a few minutes later after some bounces. On Sol 1, the lander was found to sit on the ground with inappropriate orientation. Hence the MASCOT team decided to command a “hop” on Sol 2, which was successful. Thereafter, extensive scientific observations were performed on Sol 2-3. On Sol 3 a small relocation hop was commanded by the MASCOT team to do some additional observations with different attitude, which was also successful. The last telemetry was received in the dawn of Sol 3, and the battery was estimated to deplete after 17 hours of the successful surface activity. The landing site MA-6 was nicknamed “Alice’s Wonderland” by the MASCOT team.
Figure 23: 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 24) 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 24) adopted for the final touchdown attitude so as to minimize the risk for any part of the spacecraft body other than the sampler horn hitting terrain. By tilting up the attitude by 10 degree from the locally horizontal attitude, additional 10 cm margin has been gained on the minimum distance between the local terrain and the spacecraft body.
Figure 24: 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 25) and a digital elevation map (Figure 26 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 26 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 26 right).
- The touchdown operation 1 (TD1-L08E1) was conducted on February 19-22, 2019. Due to a miss-configuration in the pre-descent activity, the start of descent was delayed by 5 hours, which was recovered by adopting a faster descent rate trajectory to catch up with the original descent path at around the altitude of 6.5 km. The spacecraft was commanded “Go” for autonomous landing at February 21, 21:10 UT at the altitude of about 500 m, and reached the altitude of 45 m at 22:07 UT. Then the spacecraft successfully captured and locked the dropped TM, and started TM-tracking descent to 8.5 m. At 8.5 m, a horizontal maneuver was triggered, and the spacecraft moved about 4 m toward northeast. On converging the horizontal motion, the final free-fall descent was triggered autonomously, and the spacecraft touched the ground at 22:29:10 UT. One of four touch down detection sources (attitude rate limit) was triggered, when a projectile was ejected and an ascent-back ΔV was executed 0.1 seconds later as planned. Four 20 N thrusters on the bottom of the spacecraft were activated for 4.7 seconds to attain the total ascend-back ΔV of 0.64 m/s. The autonomous activity below 45 m was done with non-Earth pointing attitude, limiting the communication link to the Earth only with a Low Gain Antenna. Therefore, the low altitude onboard activity was monitored on ground solely by the carrier signal receive power and the Doppler signals (Figure 27).
- The successful touchdown was confirmed soon through the play-back telemetry of rising temperature of the fired projector at the touchdown, and also of the sequence progressing normally as pre-programmed. There are three container rooms in the Hayabusa-2 sampling system. As we confirmed the touchdown, the sampler room A was commanded “Close” at February 22, 2:20 UT, and the sample collected in the TD1-L08E1 operation has been secured.
Figure 26: Left: Two final candidates for touchdown site L08-B1 and L08-E1. Right: A Monte-Carlo simulation result based on finalized touchdown sequence of Hayabusa-2 for TD1-L08E1 operation (image credit: Hayabusa-2 Team)
Figure 27: 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 28) and CAM-H (a sampler-horn monitor camera, Figure 29). 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 30).
Figure 28: 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 21 bottom-right. It has a mass of 15 kg and is to be separated from the bottom of the spacecraft. SCI is to be ignited by an SCI-onboard timer, and the 2 kg copper bullet is to be ejected at the speed of 2 km/s. For the operation planning purpose, the targeting accuracy from the nominal stand-off distance of 500 m was set to be as conservative as 200 m (3σ), which included the accuracy of SCI acceleration itself as well as the spacecraft attitude and guidance errors. Because of this relatively large dispersion, SCI was planned to be released exactly on the Sub-Earth line, leading to a constraint that the center of the impact target should be on the equatorial ridge of Ryugu.
- The target point of the kinetic impact was called “S01” region, which is located on the equatorial ridge of Ryugu, apart longitude-wise from the L08 landing site (Figure 31). 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 21 bottom-left) at 2:14 UT, which successfully observed the impact event (Figure 32). The impact occurred on 5 April 5 at 2:37 UT.
- As the result of the CRA2 operation conducted on April 13-16, the artificial crater generated by SCI was found (Figure 33). The diameter of the crater was measured to be 13 m, and was located approximately 25 m off from the aim point, which is far better targeting accuracy than anticipated. The SCI crater was then nicknamed “Omusubi-Kororin (meaning “rolling rice ball” from a Japanese fairly tail).
Figure 32: Impact event captured by two cameras equipped on DCAM3. Left: DCAM3-A (640 x 480 pixels analog camera), 2 seconds after impact. Right: DCAM3-D (2000 x 2000 pixels digital camera), 3 seconds after impact (image credit: Hayabusa-2 Team)
Figure 33: 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 34). 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 35). 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 29), optical lenses have been covered by dust and the sensitivity was reduced by approximately 70%. To compensate for this optical degradation, the altitude for entering the full-autonomous mode was lowered to 30 m from the original 45 m. This required better guidance accuracy above 30 m to appropriately “lock” the TM by the onboard camera with narrower FOV coverage, and also finer altitude control precision below 30 m to guarantee collision-safe maneuver against closer terrain.
Figure 34: 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 36). The landing site C01-Cb was then nicknamed “Uchideno-kozuchi” (meaning “Treasure Hummer”) by the Hayabusa-2 project team.
Figure 35: Left: Landing dispersion analysis. Green dots indicate the landing dispersion obtained by a Monte-Carlo simulation. Blue region indicates safe for landing, and red region indicates unsafe. Right: High resolution image taken in PPTD-TM1B operation. Red ellipsoids indicate boulders marked for special safety caution with their estimated height. “TM” indicates the location of the target maker dropped in PPTD-TM1A operation, and “C01-Cb” indicates the landing target of the PPTD operation (image credit: Hayabusa-2 Team)
- Target Markers/MINERVA-II-2 Orbiting Operation: Two more semi-critical operations were conducted after PPTD. One is the Target Markers (TMs) orbiting operation (TM-ORB), in which the two TMs were released from the spacecraft at the terrain altitude of 1 km with fine-tuned velocity. The TM-ORB operation was planned to provide an additional contribution to the gravity science, and was also prepared as a rehearsal for the following MINERVA-II-2 orbiting operation. The first TM was inserted onto an equatorial orbit at 16:17:40 UT on 16 September 16 2019, and the second TM was inserted onto a polar orbit at 16:24:20 UT on the same day. The TMs were then observed from the spacecraft holding at 20 km sub-solar point using ONC-T camera from the separation until 23 September successfully.
- The last semi-critical operation was the MINERVA-II-2 orbiting operation (MNRV-ORB), in which the MINERVA-II-2 rover, developed by Tohoku University and the Japanese University Consortium, was released from the spacecraft at the terrain altitude of 1 km onto an equatorial orbit. The main computer of the MINERVA-II-2 rover had been found to be nonfunctional, and therefore the objective of the mission was changed to more focus on the behavior of the rover from separation until landing rather than post-landing activity. The separation occurred at 15:57:20 UT on 2 October 2019 and the orbiting MINERVA-II-2 rover was observed from the spacecraft’s ONC-T/ONC-W1 camera at the altitude of 8-10 km from the separation until 8 October. The radio link between Hayabusa-2 and MINERVA-II-2 was also confirmed. These measurements are to be also used for the gravity science.
- In summary, Hayabusa-2 arrived at an unexplored C-type asteroid Ryugu, and conducted all the planned missions successfully. Fundamental scientific questions about Ryugu have been answered through successful scientific activities, and more fundamental questions have been raised. From the engineering point of view, during one and a half year of the Ryugu-proximity phase, Hayabusa-2 has achieved seven “world’s firsts”, that are (1) mobile activity of rovers on small body, (2) multiple rover deployment on a small body, (3) 60 cm-accuracy landing and sampling, (4) artificial crater forming and observation of impact process, (5) multiple landing on extra-terrestrial planet, (6) subsurface material sampling, (7) Smallest-object constellation around extra-terrestrial planet. Hayabusa-2 will leave Ryugu in the middle of November 2019 and is scheduled to return to Earth in November-December, 2020.
• July 15, 2019: Ryugu and other asteroids of the common ‘C-class’ consist of more porous material than was previously thought. Small fragments of their material are therefore too fragile to survive entry into the atmosphere in the event of a collision with Earth. This has revealed the long-suspected cause of the deficit of this meteorite type in finds on Earth. Researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) have come to this conclusion in a scientific paper published in the journal Nature Astronomy. The results are based on high-resolution measurements of the surface temperature with the DLR radiometer MARA on board the German-French Mobile Asteroid Surface Scout (MASCOT) lander. On 3 October 2018, as part of the Japanese Hayabusa-2 mission, MASCOT descended onto the almost 1 km diameter asteroid Ryugu and sent spectacular images and physical measurements from the surface back to Earth. 29) 30)
- “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." 31)
- The fridge-sized probe made its second landing on the asteroid around 10:30 am (01:30 GMT), with officials from JAXA (Japan Aerospace Exploration Agency) breaking into applause and cheers as initial data suggested the touchdown had been a success.
- Confirmation of the landing came only after Hayabusa2 lifted back up from the asteroid and resumed communications with the control room.
- Research director Takashi Kubota told reporters that the touchdown operation was "more than perfect." - And Tsuda, with a grin, said he rated it "1000 points out of 100."
- "The probe moved perfectly and the team's preparation work was perfect," he said.
Figure 37: Officials from Japan's space agency celebrated news of Hayabusa-2's successful second touchdown on the asteroid Ryugu (image credit: JAXA)
- The brief landing Thursday is the second time Hayabusa2 has touched down on the desolate asteroid Ryugu, some 300 million km from Earth.
- Ryugu, which means "Dragon Palace" in Japanese, refers to a castle at the bottom of the ocean in an ancient Japanese tale.
- The complex multi-year Hayabusa-2 mission has also involved sending rovers and robots down to the surface.
- Thursday's touchdown was intended to collect pristine materials from beneath the surface of the asteroid that could provide insights into what the solar system was like at its birth, some 4.6 billion years ago.
- To get at those crucial materials, in April an "impactor" was fired from Hayabusa-2 towards Ryugu in a risky process that created a crater on the asteroid's surface and stirred up material that had not previously been exposed to the atmosphere.
- Hayabusa-2's first touchdown was in February, when it landed briefly on Ryugu and fired a bullet into the surface to puff up dust for collection, before blasting back to its holding position.
- The second touchdown required special preparations because any problems could mean the probe would lose the precious materials already gathered during its first landing.
Figure 38: 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. 32)
- Images were acquired with ONC-W1 (Optical Navigation Camera – Wide angle). Immediately after touchdown, we captured images with the ONC-W1. Here are two bulletin images from this camera.
Figure 39: This image was taken on 11 July 2019 at 10:06:32 JST (onboard time, 01:06:32 UTC) with the ONC-W1 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST)
Figure 40: This image was taken on 11 July 2019 at 10:08:53 JST (onboard time, 01:08:53 UTC) with the ONC-W1 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST)
The following images were taken with CAM-H (Small Monitor Camera-Head). CAM-H operated before and after touchdown, capturing images 4 seconds before touchdown, the moment of touchdown and 4 seconds after touchdown. (CAM-H is the camera that was developed and installed on Hayabusa-2 through public donations. The field of view is downwards beside the sampler horn.)
Figure 41: This image was taken 4 seconds before touchdown with CAM-H (image credit: JAXA)
Figure 42: This image was acquired at the moment of touchdown with CAM-H (image credit: JAXA)
Figure 43: 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. 33) — Hayabusa-2 is operating nominally.
- As a result of checking the images captured by the Optical Navigation Camera - Telescopic (ONC-T) onboard the asteroid explorer Hayabusa-2, we have concluded that a crater was created by the SCI.
Figure 44: ONC-T images. The left image was acquired on 22 March 2019. The right image was acquired on 25 April 2019. By comparing the two images, we have confirmed that an artificial crater was created in the area surrounded by dotted lines. The size and depth of the crater are now under analysis (image credit: JAXA, The University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, The University of Aizu, AIST)
- After the start of the operation, the camera (DCAM3) separated from Hayabusa-2 captured an image that shows ejection from Ryugu’s surface, which implies that the SCI had functioned as planned. The SCI was released from the spacecraft, carrying a plastic explosive charge that shot a 2.5 kg copper projectile at the surface of the 900 m diameter Ryugu asteroid at a velocity of around 2 km/s. The objective is to uncover subsurface material to be brought back to Earth for detailed analysis.
Figure 45: 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. 36) 37)
- "The ground shook. My heart pounded. The clock counted. 3... 2... 1... Liftoff!" regaled Professor Seiji Sugita of the University of Tokyo's Department of Earth and Planetary Science. "I've never felt so excited and nervous at the same time, that wasn't just another science experiment on top of that rocket. That was the culmination of my life's work and the hopes and dreams of my entire team."
- On Wednesday, 3rd December 2014, an orange and white rocket over 50m tall weighing almost 300 tons launched from Tanegashima Space Center in South West Japan and successfully sent the Hayabusa2 spacecraft hurtling into space. Its carefully calculated trajectory swung Hayabusa2 round the Earth to pick up speed so it could reach its destination in the asteroid belt between Mars and Jupiter. The target was the asteroid Ryugu and Hayabusa2 arrived on schedule on Wednesday 27 June 2018.
- Since then the spacecraft has used a wide range of cameras and instruments to collect images and data about Ryugu which it continually sends to researchers back on Earth. It has even made a brief soft landing in preparation for a second where it will collect loose surface material — regolith — to return to Earth. We'll have to wait until 2020 before that sample returns, but researchers are far from idle in the meantime.
- "Just a few months after we received the first data we have already made some tantalizing discoveries," said Sugita. "The primary one being the amount of water, or lack of it, Ryugu seems to possess. It's far dryer than we expected, and given Ryugu is quite young (by asteroid standards) at around 100 million years old, this suggests its parent body was much largely devoid of water too."
- According to colleagues of Sugita writing in a companion paper, various instruments on Hayabusa2 including a visible-light camera and a near-infrared spectrometer confirm the lack of water. This fact is important as it's thought all of Earth's water, including that comprising 70% of you, came from local asteroids, distant comets and the nebula or dust cloud that became our sun. The presence of dry asteroids in the asteroid belt would change models used to describe the chemical composition of the early solar system. But why does this matter?
- "Life," explained Sugita. "This has implications for finding life. There are uncountably many solar systems out there and the search for life beyond ours needs direction. Our findings can refine models that could help limit which kinds of solar systems the search for life should target."
- But there's more to this than water; other compounds crucial to life exist in asteroids and Ryugu has some surprises here too. To understand why, it's important to know that Hayabusa-2 is not the only terrestrial robot out there exploring asteroids right now. In 2016 NASA launched OSIRIS-REx which arrived at its target asteroid Bennu on 3 December 2018, four years to the day from the launch of Hayabusa-2.
- The two projects are not in competition but actively share information and data which could help one another. Researchers compare their asteroids to learn even more than would be possible if they could only probe one. Although alike in most ways, Bennu and Ryugu differ significantly in some areas. They are both extremely dark, have spinning-top-like shapes and are covered in large boulders, but Ryugu contains far less water. This discrepancy has researchers scratching their heads.
- "I hoped the surface of Ryugu had more variety as previous ground-based observations had suggested. But every surface feature and boulder on Ryugu seems to be like every other, showing the same scarcity of water," said Sugita. "However, what felt limiting is now enlightening; Ryugu's homogeneity demonstrates the capacity of our instruments to capture nuanced data. It also serves as a necessary constant to compare subsequent data against. So much of science is about controlling variables and Ryugu does this for us."
- As Hayabusa-2 continues to explore our little rocky neighbor researchers gradually piece together its history, which is entwined with our own. Sugita and his colleagues believe Ryugu comes from a parent asteroid several tens of kilometers wide, most likely in the asteroid families Polana or Eulalia.
Figure 46: 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 47: 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. 38)
Figure 48: Schematic of the TD1 (Touchdown 1)-L08E1 operation (image credit: JAXA)
Figure 49: 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. 39)
- At the 45-meter hold point, Hayabusa-2 oriented itself for landing and turned its high-gain antenna away from Earth, shutting off the flow of telemetry in the process. From there, mission controllers could only watch for Doppler shifts in the signal from Hayabusa-2's low-gain antenna, indicating the spacecraft had pushed its sample horn into Ryugu and was starting to ascend.
- That shift occurred around 22:49 UTC.
Figure 50: Hayabusa-2 photo of Ryugu during sampling descent. Hayabusa-2 took this photo with its optical navigation camera at an altitude of about 180 meters, before it entered its final descent to grab a sample from asteroid Ryugu, on 21 February 2019 (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST)
- A cheer erupted at JAXA's mission control center.
- Hayabusa-2 began to ascend, and just minutes later was able to swing its high-gain antenna back toward Earth. Mission controllers confirmed that the spacecraft was healthy and the command to fire the tantalum bullet executed as expected.
- The next step will be for JAXA to download imagery, particularly from the camera on the sample horn, and further confirm the touchdown sequence went as planned.
Figure 51: 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. 40)
- 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 54), but the final area where the touchdown is planned is a region of radius 3 m within L08-E1 as shown in Figure 52.
Figure 52: 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 52, 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 53: Figure 2 shows an animation of the area in Figure 52 in three dimensions (3D) using a DEM (Digital Elevation Map). You can see the elevation of the area surrounding the touchdown location (video credit: JAXA)
- Prior to Hayabusa's arrival at Ryugu, the project planned to touchdown in a flat area about 100 m wide, but ultimately, we selected a region with a diameter of about 6 m (radius 3m). We were able to improve the necessary landing accuracy using a technique called “pinpoint touchdown”. Pinpoint touchdown was originally planned for touching down around the artificial crater generated with the onboard small carry-on impactor (SCI), but the environment we discovered on the asteroid surface has made this method necessary from the start.
• February 18, 2019 updated: On December 28 —the last day of work in 2018— the sampler team conducted an important experiment. As a final test before touchdown (TD), the team fired an identical bullet to that onboard Hayabusa-2 into a simulated soil of the surface of Ryugu to test how much sample would be ejected. 41) 42)
- 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 55).
- The original purpose of this experiment was to confirm that one month before the TD operation, the flight spare projector was operating normally after it had been stored for a long period of four years.
- As we now know, the expected topography of a powdery fine regolith was not found on the surface of Ryugu. But cm-sized or larger gravel was observed by the MASCOT and MINERVA-II1 rovers that landed on the asteroid surface. This is quite different from the prediction before launch, so it took time to investigate the safety of the spacecraft during TD. Additionally, it was necessary to review whether sample material would still be released from the asteroid surface as originally assumed.
- Therefore, we decided to use the flight spare projector to perform an operation confirmation test, whereby we examined what happens if a bullet identical to that onboard Hayabusa2 is fired into a target that simulates the observed surface of Ryugu.
Figure 55: 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. 43)
- 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 56 shows a map of Ryugu with the place names labelled. Additionally, Figure57 shows the location of the places on images of Ryugu taken from four different directions. In these figures, the north pole of Ryugu is at the image top. Please keep in mind that the north pole of Ryugu is in the same direction as the south pole on Earth, as Ryugu rotates in the opposite direction. Table 5 shows a list of the place names.
Figure 56: Map of Ryugu showing the place names. Trinitas and Alice’s Wonderland are nicknames of the MINERVA-II1 and MASCOT landing sites, respectively, and not place names recognized by the IAU (image credit: JAXA naming team)
Figure 57: The location of place names on Ryugu. Trinitas and Alice’s Wonderland are nicknames of the MINERVA-II1 and MASCOT landing sites, respectively, and not place names recognized by the IAU (image credit: JAXA naming team)
- As it is difficult to get a feel for how the place names were chosen from just a list, we will introduce the story behind the main choices below.
- The asteroid name “Ryugu” comes from the Japanese fairy tale of Taro Urashima. In the story, Urashima is a fisherman who rescues a sea turtle from the cruelty of a group of children. The turtle takes Urashima to the underwater palace of Ryugo-jo (Dragon Palace), where he meets the princess, Otohime. After 3 years, Urashima wishes to return home and is given a treasure box (tamatebako) by Otohime with instructions never to open it. But when Urashima returns to the surface, he discovers everything he knew has changed as 300 years has actually past. In confusion, Urashima opens the treasure box and is engulfed in white fog. When it clears, he has become an old man, as the box contained his age.
- With the name of the asteroid being Ryugu, there was a strong desire from the Project to use other names that appear in Urashima’s story for major asteroid topography. However, place names cannot be common nouns so words such as “sea bream”, “flounder” and “turtle” do not work and we were limited to names such as Taro Urashima, Otohime etc.
- Therefore, “Urashima” was chosen for the biggest crater on Ryugu and “Otohime” for the largest boulder near the south pole. Both of these are very important features for deciphering the formation history of Ryugu. However, Otohime had already been used! Venus (whose place theme uses the names of goddesses) had already a location named Otohime Tholus. Otohime was therefore initially refused by the IAU when it was proposed. But Otohime is an extremely important person in the story of Taro Urashima and how can we collect the tamatebako if Otohime is not on Ryugu?! (That was a joke, but we did want to use such a relevant name.) Since the name was important to the Project, the place name core members refined the proposal to the IAU, explaining why Otohime should be one of the main topological features on Ryugu and this was accepted.
- A defining feature of Ryugu is that the shape is similar to a spinning top or abacus bead. This shape is the combination of two cones which appear almost circular when seen from the north pole. The ridge where they join was named “Ryujin”, after the ruler of the Dragon Palace who is the father of princess Otohime. This name came from the Place Name Core Members who felt the ridge resembled a dragon coiling around the asteroid or an ouroboros (the image of the serpent or dragon that swallows its own tail).
- On either side of Otohime saxum there are large grooves extending in the equatorial direction. In the story of Taro Urashima, Otohime lives in this mysterious place at the bottom of the ocean which is sometimes depicted as a different world in the various retellings of the tale. This world is often called “Horai”, “Tokoyo” or “Niraikanai”. The grooves adjacent to Otohime saxum were therefore named Horai fossa and Tokoyo fossa.
- There is a reasonably big boulder to the southeast of the Urashima crater. According to one version of the tale, the place where Taro Urashima helped the turtle and left to travel to Ryugu-jo is the place “Ejima”, which gave the boulder its name Ejima saxum.
- There are also large craters on both sides of Urashima crater. In particular, there are two craters stuck together along the north-south direction to the west. This state reminded us of the kibidango (Japanese dumplings) in another Japanese fairy tale called Momotaro. The northern crater of the pair was therefore named “Momotaro crater” and the southern crater became “Kibidango crater”. To the east of the Urashima crater, there is a crater with big black boulder inside. This reminded us of the Japanese tale of Kintaro, a boy with super strength who carried a broad-axe, and so was named “Kintaro crater”.
- Ryugu also has topological names derived from children’s stories from outside Japan. For example, while you might not immediately recognize the name of the Cendrillion crater, the name is from the original French name for the familiar fairy tale, “Cinderella”. The name of the Brabo crater is derived from the name of the hero of a Netherlands tale, which was proposed by the overseas project members. The Kolobok crater and Catafo saxum were both names proposed by the IAU WG. They are taken from Russian and Cajun (famous for Cajun cuisine in the USA) folktales.
- These are the place names formally recognized by the IAU WG. In addition, there are two nicknames shown in Figures 56 and 57; 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. 44)
• 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. 45)
- 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 58 shows an animation of the trajectory of the spacecraft in the Hill coordinate system. In this coordinate system, the Sun is always to the left and outside the figure. The black dot on the right is Ryugu. From late November to the end of December, the spacecraft will travel along the blue line. The red dots are points where a trajectory control maneuver will be performed.
• December 21, 2018: Until now, “astrodynamics” has been one of the less frequently reported operations for Hayabusa2. In space engineering, the movement, attitude, trajectory and overall handling of the flight mechanics of the spacecraft is referred to as “astrodynamics”. For example, astrodynamics played an active role in the gravity measurement descent operation in August 2018. While this was a short time ago, let’s look at a few of the details. 46)
- From August 6 - 7, 2018, the “Gravity Measurement Descent Operation” was performed to estimate the strength of asteroid Ryugu’s gravity. Hayabusa-2 initially descended from the home position at an altitude of 20 km to an altitude of 6100 m. Orbital control was then temporarily stopped to allow the spacecraft to “free-fall” towards Ryugu, moving due to the gravitational pull of the asteroid alone. When the altitude decreased to about 850 m, the thrusters were instantaneously fired to give the spacecraft an upward velocity, whereupon Hayabusa-2 performed a “free-rise” to an altitude of about 6100 m (the spacecraft’s movement here is similar to throwing a ball vertically upwards).
- From the spacecraft’s motion during the free-fall and free-rise, the strength of Ryugu’s gravity could be measured and the mass of the asteroid obtained. As a result of this measurement, the mass of Ryugu was calculated to be about 450 million tons.
- The shape and volume of Ryugu are known thanks to the construction of the three-dimensional shape model (article on July 11). Using this volume and the measured mass of Ryugu from the gravity measurement descent operation, the average density of the asteroid can be calculated. The average density and shape of Ryugu could then be used to find the gravitational strength (gravitational acceleration) on the surface of Ryugu, which was found to have the following distribution:
- The gravitational acceleration on the surface of Ryugu is approximately 0.11~0.15 mm/s2, which is about eighty thousandths (~ 1/80000th) the strength of the Earth’s gravity and a few times stronger than that of Itokawa. We can additionally see that the gravity near the poles of Ryugu is stronger than near the asteroid’s equator. This is due to the equatorial ridge protruding from the surface.
- The information on the asteroid’s gravitational acceleration obtained through this method has been used for operations that approach close to the surface of Ryugu. Of course, it will also be used during touchdown. The gravity measurement descent operation described here is one application of astrodynamics. The astrodynamics team for Hayabusa-2 uses a variety of similar methods to estimate the trajectory of the spacecraft and Ryugu, and to evaluate the dynamic environment for operating around Ryugu.
Figure 59: 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 60). 47)
- The image resolution is about 4.6 mm/pixel. This is the highest resolution image that Hayabusa-2 has taken so far and even small rocks with a diameter of 2 – 3 cm are clearly visible. The maximum resolution of AMICA –the camera at the time of the first Hayabusa mission— was 6 mm/pixel, so even its resolution has now been exceeded. As the image captured of the asteroid surface from the spacecraft, it will be one of the highest resolution to be taken of Ryugu (MINERVA-II1 and MASCOT which landed on the surface, have captured even higher resolution images).
- A feature from the image is the lack of regolith (sandy substance). This was suspected to be true from the images obtained so far, but it is more clearly seen in this high resolution photograph. There is also a collection of pebbles with different colors, which may be evidence that the surface material of Ryugu is mixed.
- It is a landmark for the mission that such high resolution images were captured by the spacecraft before landing. Such a detailed image that can be used to visually recognize anything above about 1cm in size is extremely useful in analyzing the surface photographs returned from the MINERVA-II1 rovers and MASCOT lander and also for understanding the microanalysis from the sample once it is returned to Earth.
Figure 60: The surface of Ryugu photographed on October 15 at 22:40 JST using the Optical Navigation Camera – Telescopic (ONC-T). The altitude here is about 42m. (image credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST)
Figure 61: 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 60 (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). 48)
Figure 62: 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 63. 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 64.
Figure 64: 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 65 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. 49)
- In order to construct this shape, the orbit of the spacecraft must be accurately predicted. The accuracy is currently (in early August) several hundred meters. That the position of a spacecraft 300 million km away from Earth can be determined to within hundreds of meters is impressive, but this error is still too big to describe the shape of the 900 m diameter Ryugu. The project is therefore improving the trajectory of the spacecraft using the distance measurement data from the laser altimeter. The improved orbital data of the spacecraft also aids the other equipment teams on Hayabusa-2 and is being used to select the best landing site.
Figure 65: 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. 50)
- 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. 51)
Figure 66: 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. 52)
- "This success was possible thanks to state-of-the-art robotic technology, long-term planning and intensive international cooperation between the scientists and engineers of the three space nations Japan, France and Germany," says Hansjörg Dittus, DLR Executive Board Member for Space Research and Technology about this milestone in Solar System exploration. "We are proud of how MASCOT was able to master its way across the asteroid Ryugu over boulders and rocks and send so much data about its composition back to Earth," says DLR Chair Pascale Ehrenfreund.
- MASCOT had no propulsion system and landed in free fall. Six minutes after separating from Hayabusa-2, and following the end of a ballistic trajectory, the landing module made its first contact with asteroid Ryugu. On the surface, MASCOT moved through the activation of a tungsten swing arm accelerated and decelerated by a motor. This made it possible for MASCOT to be repositioned to the 'correct' side or even perform hops across the asteroid's surface. The gravitational attraction on Ryugu is just one 66,500th of the Earth's, so the little momentum provided was enough: a technological innovation for an unusual form of mobility on an asteroid surface used for the first time in the history of space travel as part of the Hayabusa-2 mission.
Through a rock garden full of rough boulders and no flat surfaces
- To reconstruct MASCOT's path across the surface of Ryugu, the cameras aboard the Hayabusa-2 mother probe were aimed at the asteroid. The Optical Navigation Cameras (ONC) captured the lander's free fall in several images, detected its shadow on the ground during the flight phase, and finally identified MASCOT directly on the surface in several images. The pattern of the countless boulders distributed on the surface could also be seen in the direction of the respective horizon in oblique photographs of the lander's DLR MASCAM camera. The combination of this information unlocked the unique path traced by the lander.
- After the first impact, MASCOT smoothly bounced off a large block, touched the ground about eight times, and then found itself in a resting position unfavorable for the measurements. After commanding and executing a specially prepared correction maneuver, MASCOT came to a second halt. The exact location of this second position is still being determined. There, the lander completed detailed measurements during one asteroid day and night. This was followed by a small ‘mini-move’ to provide the MicrOmega spectrometer with even better conditions for measuring the composition of the asteroid material.
- Finally, MASCOT was set in motion one last time for a bigger jump. At the last location it carried out some more measurements before the third night on the asteroid began, and contact with Hayabusa-2 was lost as the spaceship had moved out of line of sight. The last signal from MASCOT reached the mother probe at 21:04 CEST. The mission was over. "We were expecting less than 16 hours of battery life because of the cold night, says MASCOT project manager Tra-Mi Ho from the DLR Institute of Space Systems. "After all, we were able to operate MASCOT for more than one extra hour, even until the radio shadow began, which was a great success." During the mission, the team named MASCOT's landing site (MA-9) 'Alice's Wonderland', after the eponymous book by Lewis Carroll (1832-1898).
Figure 67: MASCOT's approach to Ryugu and its path across the surface (image credit: JAXA/U. Tokyo/Kochi U/Rikkyo U/Nagoya U/Chiba Inst Tech/Meiji U/U Aizu/AIST)
A true wonderland
- Having reconstructed the events that took place on asteroid Ryugu, the scientists are now busy analyzing the first results from the acquired data and images. "What we saw from a distance already gave us an idea of what it might look like on the surface," reports Ralf Jaumann from the DLR Institute of Planetary Research and scientific director of the MASCOT mission. "In fact, it is even crazier on the surface than expected. Everything is covered in rough blocks and strewn with boulders. How compact these blocks are and what they are composed of, we still do not know. But what was most surprising was that large accumulations of fine material are nowhere to be found – and we did not expect that. We have to investigate this in the next few weeks, because the cosmic weathering would actually have had to produce fine material," continues Jaumann.
- "MASCOT has delivered exactly what we expected: an 'extension' of the space probe on the surface of Ryugu and direct measurements on site," says Tra-Mi Ho. Now there are measurements across the entire spectrum, from telescope light curves from Earth to remote sensing with Hayabusa2 through to the microscopic findings of MASCOT. "This will be of enormous importance for the characterization of this class of asteroids," emphasizes Jaumann.
- Ryugu is a C-type asteroid – a carbon-rich representative of the oldest bodies of the four-and-a-half-billion year-old Solar System. It is a 'primordial' building block of planet formation, and one of 17,000 known Near-Earth asteroids.
- On Earth, there are meteorites with a composition that could be similar to Ryugu's, which are found in the Murchison Range, Australia. However, Matthias Grott from the DLR Institute of Planetary Research and responsible for the radiometer experiment MARA is skeptical as to whether these meteorites are actually representative of Ryugu in terms of their physical properties: "Meteorites such as those found in Murchison are rather massive. However, our MARA data suggests the material on Ryugu is slightly more porous. The investigations are just beginning, but it is plausible to assume that small fragments of Ryugu would not survive the entry into the Earth's atmosphere intact."
Table 6: MASCOT's 17 hours and 7 minutes on Ryugu
• October 11, 2018: JAXA told reporters the Hayabusa-2 probe is now expected to touch down on the Ryugu asteroid in "late January" at the earliest, rather than at the end of this month as initially expected. JAXA project manager Yuichi Tsuda said they needed more time to prepare the landing as the latest data showed the asteroid surface was more rugged than expected. 53)
- "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. 54) 55)
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 68: 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. 56) 57)
- The moment of separation was one of the risks of the mission: If MASCOT had not successfully separated from Hayabusa-2 as planned and often tested, the lander’s team would hardly have had the opportunity to solve this problem. But everything went smoothly: Already during the descent on the asteroid, MasCam (MASCOT Camera) took 20 pictures, which are now stored on board the Japanese space probe. "The camera worked perfectly," says Ralf Jaumann, DLR planetary scientist and scientific director of the camera instrument. "The team's first images of the camera are therefore safe." The magnetometer team was also able to recognize in the data sent by MASCOT that the MASMAG instrument had switched on and performed measurements prior to the separation. "The measurements show the relatively weak field of the solar wind and the very strong magnetic disturbances caused by the spacecraft," explains Karl-Heinz Glaßmeier from the Technical University of Braunschweig. "At the moment of the separation, we expected a clear decrease of the interference field – and we were able to recognize this clearly."
- MASCOT came to rest on the surface approximately 20 minutes after the separation. Now, the team is analyzing the data that MASCOT is sending to Earth to understand the events occurring on the asteroid Ryugu. The lander should now be on the asteroid’s surface, in the correct position thanks to its swing arm, and have started to conduct measurements independently. There are four instruments on board: a DLR camera and radiometer, an infrared spectrometer from IAS ( Institut d'Astrophysique Spatiale, Orsay, France) and a magnetometer from the TU Braunschweig. Once MASCOT has performed all planned measurements, it is expected to hop to another measuring location. This is the first time that scientists will receive data from different locations on an asteroid. "With MASCOT, we have the unique opportunity to study the Solar System’s most primordial material directly on an asteroid," emphasizes DLR planetary researcher Ralf Jaumann. With the data acquired by MASCOT and the samples that Hayabusa2 brings to Earth from Ryugu in 2020, scientists will not only learn more about asteroids, but more about the formation of the Solar System. "Asteroids are very primordial celestial bodies."
Figure 69: Shadow of MASCOT on asteroid Ryugu during descent: DLR's MasCam on board MASCOT acquired this image as it descended to the the asteroid Ryugu 3.5 minutes after separating from its mothercraft Hayabusa-2. In the image, the lander is ~20 m above the asteroid's surface, and MASCOT's shadow can be seen at the top right (image credit: MASCOT/DLR/JAXA)
Figure 70: Artist's rendition of MASCOT during the landing approach (image credit: DLR (CC-BY 3.0))
Figure 71: Left: Illustration of MASCOT separating from Hayabusa-2. Right: Illustration of MASCOT landing on the surface of Ryugu (image credit: JAXA, Ref. 57)
- For the landing site for MASCOT, a region in the southern hemisphere of Ryugu was selected (Figure 72). This location was selected based on criteria that ensured no overlap between the landing sites for the touchdown of Hayabusa-2, MINERVA-II1 and MASCOT, the time to be able to communicate with Hayabusa-2, the duration of sunlight exposure and expectation of scientifically meaningful exploration.
Figure 72: 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. 58)
Figure 73: These images were taken by Rover-1B on 23 September 2018; confirmation of Rover-1B hop. Observation time (JST): Left: 09:50; Center: 09:55; Right: 10:00 (image credit: JAXA)
Figure 74: This image was captured immediately before a hop of Rover-1B on 23 September 2018 at 09:46 JST (image credit: JAXA)
Figure 75: Surface image from Rover-1B after landing, taken on 23 September at 10:10 JST (image credit: JAXA)
Figure 76: Surface image taken from Rover-1A on September 23 2018 at 09:43 JST (image credit: JAXA)
Figure 77: 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. 59)
- The Hayabusa-2 spacecraft is operated by JAXA (Japanese Space Agency) and supported in part by ESA's Estrack Malargüe deep-space tracking station in Argentina. The spacecraft carries four small landers that will investigate the asteroid’s surface, all four designed to gently fall onto the surface of the rocky boulder, taking advantage of its low gravity environment.
- Around the time this remarkable picture was taken, the spacecraft released its two MINERVA-II1 rovers which have since successfully landed and demonstrated an ability to hop around this rock-strewn body.
- "I cannot find words to express how happy I am that we were able to realize mobile exploration on the surface of an asteroid" enthused Yuichi Tsuda, Hayabusa-2 Project Project Manager, "I am proud that Hayabusa-2 was able to contribute to the creation of this technology for a new method of space exploration by surface movement on small bodies."
- The next stage will see the Mobile Asteroid Surface Scout (MASCOT) lander released onto the asteroid’s surface. Developed by the German Aerospace Center (DLR) in cooperation with the French Space Agency (CNES) MASCOT has enough power for a 12-hour mission, in which it will analyze the asteroid’s surface at two different sites.
- The Hayabusa-2 spacecraft itself will collect three samples from Ryugu, bringing them back to Earth in December 2020. These strange specimens will provide insights into the composition of this carbonaceous asteroid — a type of space rock expected to preserve some of the most pristine materials in the Solar System.
- As well as hopefully shining light on the origin and evolution of the inner planets, and the sources of water and organic compounds on Earth, this knowledge should help in efforts to protect our planet from marauding masses that come too close for comfort to our home planet.
- Understanding the composition and characteristics of near-Earth objects is vital to defending ourselves from them, if one were to head in our direction. ESA’s proposed Hera mission to test asteroid deflection is an ambitious example of how we can get to know these ancient bodies better, all in the name of planetary defence.
Figure 78: 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)