研究者業績
基本情報
- 所属
- 国立研究開発法人宇宙航空研究開発機構 宇宙科学研究所 教授国立大学法人総合研究大学院大学 物理科学研究科 宇宙科学専攻 教授
- 学位
- 理学博士(2000年3月 早稲田大学)
- 研究者番号
- 90329029
- ORCID ID
https://orcid.org/0000-0001-5863-4653- J-GLOBAL ID
- 200901056692713269
- researchmap会員ID
- 6000000722
経歴
8-
2022年5月 - 現在
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2016年12月 - 2022年4月
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2007年4月 - 2016年11月
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2012年11月 - 2013年5月
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2006年3月 - 2007年3月
委員歴
2-
2023年4月 - 現在
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2015年3月 - 現在
論文
120-
Proceedings of the International Astronautical Congress, IAC 2023-October 2023年10月A 6U CubeSat “OMOTENASHI” was developed to be the world's smallest moon lander. It was launched by NASA's SLS Artemis-1 on November 16, 2022. However, because of the spacecraft anomaly, the battery was depleted and the communication with the spacecraft had been lost. After we gave up the moon landing experiment, we have been conducting a search and rescue operation till September 2023. But it was unsuccessful, unfortunately. In this article, the mission objective, the spacecraft design, the planed mission scenario, and the in-orbit operation results are presented. Additionally, lessons learned from the development and the in-orbit operation are presented.
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Proceedings of the International Astronautical Congress, IAC 2023-October 2023年10月This paper presents a collaborative effort between NASA and JAXA to make 3-way Doppler data from JAXA tracking stations available to the Artemis 1 navigation team to improve orbit determination. The paper describes the system configuration and concept of operation of this capability. Testing effort at the three JAXA's ground stations - the Uchinoura's 20-m and 34-m antennas and Usuda's 64-m antenna - are discussed. Both aspects of system testing are highlighted, first on the use of Artemis 1 recorded signal to ensure compatibility between ground and flight systems, and second on the tracking with the Lunar Reconnaissance Orbiter, as a substitute for Artemis before launch, to validate other key operational functions such as ephemeris processing, spacecraft tracking capability, data delivery, and interactions among multiple operational teams in different organizations. Results from actual support to Artemis 1's Orion spacecraft in November-December 2022 are also presented.
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Journal of Evolving Space Activities 1 2023年3月 査読有り
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Science (New York, N.Y.) 379(6634) eabo0431 2023年2月24日 査読有りThe near-Earth carbonaceous asteroid (162173) Ryugu is expected to contain volatile chemical species that could provide information on the origin of Earth's volatiles. Samples of Ryugu were retrieved by the Hayabusa2 spacecraft. We measured noble gas and nitrogen isotopes in Ryugu samples and found that they are dominated by presolar and primordial components, incorporated during Solar System formation. Noble gas concentrations are higher than those in Ivuna-type carbonaceous (CI) chondrite meteorites. Several host phases of isotopically distinct nitrogen have different abundances among the samples. Our measurements support a close relationship between Ryugu and CI chondrites. Noble gases produced by galactic cosmic rays, indicating a ~5 million year exposure, and from implanted solar wind record the recent irradiation history of Ryugu after it migrated to its current orbit.
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Publications of the Astronomical Society of Japan 75(1) 199-207 2023年1月11日 査読有り<jats:title>Abstract</jats:title> <jats:p>We present a detection of a bright burst from the fast radio burst (FRB) 20201124A, which is one of the most active repeating FRBs, based on S-band observations with the 64 m radio telescope at the Usuda Deep Space Center/JAXA. This is the first FRB observed by using a Japanese facility. Our detection at 2 GHz in 2022 February is the highest frequency for this FRB and the fluence of &gt;189 Jy ms is one of the brightest bursts from this FRB source. We place an upper limit on the spectral index α = −2.14 from the detection of the S band and non-detection of the X band at the same time. We compare the event rate of the detected burst with those from previous research and suggest that the power law of the luminosity function might be broken at lower fluence and the fluences of bright FRBs are distributed up to over 2 GHz with the power law against frequency. In addition, we show that the energy density of the burst detected in this work is comparable to the bright population of one-off FRBs. We propose that repeating FRBs can be as bright as one-off FRBs and only their brightest bursts might be detected, so some repeating FRBs intrinsically might have been classified as one-off FRBs.</jats:p>
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Science (New York, N.Y.) 379(6634) eabn9057 2023年2月24日 査読有りSamples of the carbonaceous asteroid (162173) Ryugu were collected and brought to Earth by the Hayabusa2 spacecraft. We investigated the macromolecular organic matter in Ryugu samples and found that it contains aromatic and aliphatic carbon, ketone, and carboxyl functional groups. The spectroscopic features of the organic matter are consistent with those in chemically primitive carbonaceous chondrite meteorites that experienced parent-body aqueous alteration (reactions with liquid water). The morphology of the organic carbon includes nanoglobules and diffuse carbon associated with phyllosilicate and carbonate minerals. Deuterium and/or nitrogen-15 enrichments indicate that the organic matter formed in a cold molecular cloud or the presolar nebula. The diversity of the organic matter indicates variable levels of aqueous alteration on Ryugu's parent body.
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Science (New York, N.Y.) 379(6634) eabn9033 2023年2月24日 査読有りThe Hayabusa2 spacecraft collected samples from the surface of the carbonaceous near-Earth asteroid (162173) Ryugu and brought them to Earth. The samples were expected to contain organic molecules, which record processes that occurred in the early Solar System. We analyzed organic molecules extracted from the Ryugu surface samples. We identified a variety of molecules containing the atoms CHNOS, formed by methylation, hydration, hydroxylation, and sulfurization reactions. Amino acids, aliphatic amines, carboxylic acids, polycyclic aromatic hydrocarbons, and nitrogen-heterocyclic compounds were detected, which had properties consistent with an abiotic origin. These compounds likely arose from an aqueous reaction on Ryugu's parent body and are similar to the organics in Ivuna-type meteorites. These molecules can survive on the surfaces of asteroids and be transported throughout the Solar System.
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Science (New York, N.Y.) 379(6634) eabn8671 2023年2月24日 査読有りSamples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed 17 Ryugu samples measuring 1 to 8 millimeters. Carbon dioxide-bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu's parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high pH, and water/rock ratios of <1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate that Ryugu's parent body formed ~2 million years after the beginning of Solar System formation.
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Nature Astronomy 7(2) 170-181 2023年2月 査読有りWithout a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe3+ to Fe2+ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (–OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.
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Advances in Space Research 2023年1月 査読有りIn December 2020, synchronous two-way laser ranging in deep space was successful for the first time over distances up to 6.46 million km. A laser altimeter aboard spacecraft Hayabusa2 was used as an optical transponder, detecting laser pulses emitted from the ground stations on Earth and retransmitting the pulses from the spacecraft to the ground. These retransmitted pulses were successfully detected at the ground station. The experiment was conducted as a demonstration of the deep-space laser ranging. The repetition frequency of the onboard instrument was limited to 0.5 Hz, and there was a lot of background noise because the experiments were carried out in daytime on the ground. Nevertheless, laser detection on the ground was still possible due to the high time-correlation between the detected and the predicted in the presence of random noise. Procedures similar to satellite laser ranging were applied to create residuals by subtracting the orbit predictions of the spacecraft from the observed round-trip time. Furthermore, some of the transponder return pulses were uniquely identified, where the time coherence of the measurements had been recovered from the telemetry data of the laser altimeter.
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Science (New York, N.Y.) 379(6634) eabn7850 2023年2月24日 査読有り<jats:p> Carbonaceous meteorites are thought to be fragments of C-type (carbonaceous) asteroids. Samples of the C-type asteroid (162173) Ryugu were retrieved by the Hayabusa2 spacecraft. We measure the mineralogy, bulk chemical and isotopic compositions of Ryugu samples. They are mainly composed of materials similar to carbonaceous chondrite meteorites, particularly the CI (Ivuna-type) group. The samples consist predominantly of minerals formed in aqueous fluid on a parent planetesimal. The primary minerals were altered by fluids at a temperature of 37 ± 10°C, <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow> <mml:mn>5.2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.7</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> </jats:inline-formula> (Stat.) <jats:inline-formula> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msubsup> <mml:mrow /> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:math> </jats:inline-formula> (Syst.) million years after formation of the first solids in the Solar System. After aqueous alteration, the Ryugu samples were likely never heated above ~100°C. The samples have a chemical composition that more closely resembles the Sun’s photosphere than other natural samples do. </jats:p>
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Science Advances 8(46) 2022年11月 査読有りThe Hayabusa2 spacecraft returned to Earth from the asteroid 162173 Ryugu on 6 December 2020. One day after the recovery, the gas species retained in the sample container were extracted and measured on-site and stored in gas collection bottles. The container gas consists of helium and neon with an extraterrestrial 3He/4He and 20Ne/22Ne ratios, along with some contaminant terrestrial atmospheric gases. A mixture of solar and Earth’s atmospheric gas is the best explanation for the container gas composition. Fragmentation of Ryugu grains within the sample container is discussed on the basis of the estimated amount of indigenous He and the size distribution of the recovered Ryugu grains. This is the first successful return of gas species from a near-Earth asteroid.
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Nature Astronomy 6(10) 1163-1171 2022年10月 査読有りVolatile and organic-rich C-type asteroids may have been one of the main sources of Earth’s water. Our best insight into their chemistry is currently provided by carbonaceous chondritic meteorites, but the meteorite record is biased: only the strongest types survive atmospheric entry and are then modified by interaction with the terrestrial environment. Here we present the results of a detailed bulk and microanalytical study of pristine Ryugu particles, brought to Earth by the Hayabusa2 spacecraft. Ryugu particles display a close compositional match with the chemically unfractionated, but aqueously altered, CI (Ivuna-type) chondrites, which are widely used as a proxy for the bulk Solar System composition. The sample shows an intricate spatial relationship between aliphatic-rich organics and phyllosilicates and indicates maximum temperatures of ~30 °C during aqueous alteration. We find that heavy hydrogen and nitrogen abundances are consistent with an outer Solar System origin. Ryugu particles are the most uncontaminated and unfractionated extraterrestrial materials studied so far, and provide the best available match to the bulk Solar System composition.
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International Journal of Thermophysics 43(7) 2022年7月 査読有り<jats:title>Abstract</jats:title><jats:p>We present a numerical method for simulating a disk-resolved thermal image of an asteroid with small-scale roughness. In our method, we carry out numerical thermal evolution model of a small but rough area taking into account its latitude, shadowing effect, and re-absorption of the thermal radiation by neighbor. By visualization of the resulting temperature distribution for an observation direction, we obtain the thermal flux from the area as a function of the observation direction. Then thermal image of an asteroid with random topography is constructed. The resulting daytime temperature evolution profile is different from the well-known parabolic shape due to the surface roughness, implying that the daytime temperature evolution profile is a diagnostic to evaluate the surface roughness. Although this model is inapplicable to a morphologically complex asteroid such as Itokawa, the target body of Hayabusa2, Ryugu is generally convex and suitable for application of our model. Furthermore, the study presents predictions of the location shift of Ryugu trajectory after one orbital rotation due to the thermal moment caused by the rebound force from thermally emitted photons known as the Yarkovsky effect. This model is thus verifiable by precise calculation of the ephemeris of Ryugu.</jats:p>
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Proceedings of the Japan Academy, Series B 98(6) 227-282 2022年6月10日 査読有り
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Hayabusa2 Asteroid Sample Return Mission 49-72 2022年4月 査読有り
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Hayabusa2 Asteroid Sample Return Mission 137-175 2022年4月 査読有り
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Hayabusa2 Asteroid Sample Return Mission 313-340 2022年4月 査読有り
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Hayabusa2 Asteroid Sample Return Mission 177-187 2022年4月 査読有り
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Hayabusa2 Asteroid Sample Return Mission 241-257 2022年4月 査読有り
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Hayabusa2 Asteroid Sample Return Mission 509-539 2022年4月 査読有り
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Hayabusa2 Asteroid Sample Return Mission 73-94 2022年4月 査読有り筆頭著者責任著者The most challenging aspects for the navigation of Hayabusa2 toward its target asteroid (16273) Ryugu were the limited a priori knowledge of the Ryugu's trajectory and the need for precise orbit determination (OD) during its ion-engine thrusting periods. Ryugu's ephemeris had to be simultaneously determined with the orbit of Hayabusa2 through optical observations from Hayabusa2 because Ryugu had been too far from Earth to be optically observed with ground-based telescopes since 2016. It was difficult for Hayabusa2 to have long coasting (i.e., ballistic flight) periods during asteroid proximity operations because its position-keeping strategy toward Ryugu was hovering, not orbiting, and position-keeping maneuvers with the reaction control system (RCS; i.e., bipropellant chemical thrusters) were inserted almost every day during the period. The conventional Doppler-based OD method cannot provide accurate spacecraft trajectory estimates if the contribution of uncertain perturbation forces (e.g., ion-engine propulsion, frequent RCS maneuvers, or unmodeled gravity field) are dominant, therefore we used a “quasi-kinematic orbit determination method” that simultaneously performed two-dimensional Delta-DOR measurements and two-way Ranging measurements in a short time. This chapter describes the method and how it contributed to the successful arrival of Hayabusa2 to Ryugu and regular updates of the Ryugu ephemeris during asteroid proximity operations.
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Solar Physics 297(3) 2022年3月9日 査読有り
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Science 375(6584) 1011-1016 2022年3月4日 査読有り<jats:p>The Hayabusa2 spacecraft investigated the C-type (carbonaceous) asteroid (162173) Ryugu. The mission performed two landing operations to collect samples of surface and subsurface material, the latter exposed by an artificial impact. We present images of the second touchdown site, finding that ejecta from the impact crater was present at the sample location. Surface pebbles at both landing sites show morphological variations ranging from rugged to smooth, similar to Ryugu’s boulders, and shapes from quasi-spherical to flattened. The samples were returned to Earth on 6 December 2020. We describe the morphology of >5 grams of returned pebbles and sand. Their diverse color, shape, and structure are consistent with the observed materials of Ryugu; we conclude that they are a representative sample of the asteroid.</jats:p>
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Nature Astronomy 6(2) 214-220 2021年12月20日 査読有り<jats:title>Abstract</jats:title><jats:p>C-type asteroids<jats:sup>1</jats:sup> are considered to be primitive small Solar System bodies enriched in water and organics, providing clues to the origin and evolution of the Solar System and the building blocks of life. C-type asteroid 162173 Ryugu has been characterized by remote sensing<jats:sup>2–7</jats:sup> and on-asteroid measurements<jats:sup>8,9</jats:sup> with Hayabusa2 (ref. <jats:sup>10</jats:sup>). However, the ground truth provided by laboratory analysis of returned samples is invaluable to determine the fine properties of asteroids and other planetary bodies. We report preliminary results of analyses on returned samples from Ryugu of the particle size distribution, density and porosity, spectral properties and textural properties, and the results of a search for Ca–Al-rich inclusions (CAIs) and chondrules. The bulk sample mainly consists of rugged and smooth particles of millimetre to submillimetre size, confirming that the physical and chemical properties were not altered during the return from the asteroid. The power index of its size distribution is shallower than that of the surface boulder observed on Ryugu<jats:sup>11</jats:sup>, indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m<jats:sup>−3</jats:sup>, which is lower than that of meteorites<jats:sup>12</jats:sup>, suggesting a high microporosity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements<jats:sup>5,9</jats:sup>. The extremely dark optical to near-infrared reflectance and spectral profile with weak absorptions at 2.7 and 3.4 μm imply a carbonaceous composition with indigenous aqueous alteration, matching the global average of Ryugu<jats:sup>3,4</jats:sup> and confirming that the sample is representative of the asteroid. Together with the absence of submillimetre CAIs and chondrules, these features indicate that Ryugu is most similar to CI chondrites but has lower albedo, higher porosity and more fragile characteristics.</jats:p>
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Advances in Space Research 68(8) 3093-3140 2021年10月15日 査読有りOne of the major challenges in the Hayabusa2 sample-return mission was the second touchdown on the asteroid Ryugu, which was designed to collect subsurface materials near the artificial crater formed by a small carry-on impactor. Due to engineering and scientific requirements, a narrow area with a radius as small as 3.5 m was selected as the target landing site. To achieve pinpoint touchdown at the selected site, an artificial landmark called a target marker (TM) was used for optical navigation. The key to a successful touchdown was precise deployment of the TM in the microgravity environment of the asteroid. This study therefore investigates a viable trajectory for TM deployment, incorporating the uncertainty in the impact and rebound motions of the TM. Following the TM deployment operation, a detailed survey of the landing site around the TM settlement point is performed to assess the terrain conditions. To guarantee the observation quality and spacecraft safety, multi-impulse low-altitude trajectories are introduced in this paper, along with covariance analyses based on the high-fidelity polyhedral gravity model of Ryugu. Subsequently, a pinpoint touchdown trajectory that satisfies various engineering requirements, such as landing accuracy and velocity, is designed, taking advantage of optical TM tracking. The feasibility of the touchdown sequence is further validated by a Monte Carlo dispersion analysis. Consequently, Hayabusa2 successfully touched down within the target site on 11 July 2019. The current research also conducts a post-operation trajectory reconstruction based on the flight data to demonstrate the actual guidance performance in the TM deployment, landing site observations, and pinpoint touchdown.
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Nature Communications 12(1) 2021年10月5日 査読有りRyugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0–3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu’s parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570–670 K (300–400 °C), suggesting that Ryugu’s parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2–2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history.
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Advances in Space Research 68(3) 1533-1555 2021年8月1日 査読有りHayabusa2 is the Japanese Asteroid Return Mission and targeted the carbonaceous asteroid Ryugu, conducted by the Japan Aerospace Exploration Agency (JAXA). The goal of this mission was to conduct proximity operations including remote sensing observations, material sampling, and a Small Carry-On Impact experiment, as well as sample analyses. As of September 2020, the spacecraft is on the way back to Earth with samples from Ryugu with no critical issues after the successful departure in November 2019. Here, we propose an extended mission in which the spacecraft will rendezvous with a small asteroid with ~30 m - ~40 m in diameter that is rotating at a spin period of ~10 min after an additional ~10-year cruise phase. We introduce that two scenarios are suitable for the extended mission. In the first scenario, the spacecraft will perform swing-by maneuvers at Venus once and Earth twice to arrive at asteroid 2001 AV43. In the second scenario, it will perform swing-by maneuvers at Earth twice to reach asteroid 1998 KY26. In both scenarios, the mission will continue until the early 2030s. JAXA recently released the decision that the spacecraft will rendezvous with 1998 KY26. This paper focuses on our scientific assessments of the two scenarios but leaves the decision process to go to 1998 KY26 for future reports. Rendezvous operations will be planned to detail the physical properties and surrounding environments of the target, one of the smallest elements of small planetary bodies. By achieving the planned operations, the mission will provide critical hints on the violent histories of collisions and accumulations of small bodies in the solar system. Furthermore, the established scientific knowledge and techniques will advance key technologies for planetary defense.
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TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES, AEROSPACE TECHNOLOGY JAPAN 19(4) 584-590 2021年7月4日 査読有り
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Nature Astronomy 5(8) 766-774 2021年5月24日 査読有りPlanetesimals—the initial stage of the planetary formation process—are considered to be initially very porous aggregates of dusts1,2, and subsequent thermal and compaction processes reduce their porosity3. The Hayabusa2 spacecraft found that boulders on the surface of asteroid (162173) Ryugu have an average porosity of 30–50% (refs. 4–6), higher than meteorites but lower than cometary nuclei7, which are considered to be remnants of the original planetesimals8. Here, using high-resolution thermal and optical imaging of Ryugu’s surface, we discovered, on the floor of fresh small craters (<20 m in diameter), boulders with reflectance (~0.015) lower than the Ryugu average6 and porosity >70%, which is as high as in cometary bodies. The artificial crater formed by Hayabusa2’s impact experiment9 is similar to these craters in size but does not have such high-porosity boulders. Thus, we argue that the observed high porosity is intrinsic and not created by subsequent impact comminution and/or cracking. We propose that these boulders are the least processed material on Ryugu and represent remnants of porous planetesimals that did not undergo a high degree of heating and compaction3. Our multi-instrumental analysis suggests that fragments of the highly porous boulders are mixed within the surface regolith globally, implying that they might be captured within collected samples by touch-down operations10,11.
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Science 372(6538) 187-190 2021年4月9日 査読有りGiant radio pulses (GRPs) are sporadic bursts emitted by some pulsars that last a few microseconds and are hundreds to thousands of times brighter than regular pulses from these sources. The only GRP-associated emission outside of radio wavelengths is from the Crab Pulsar, where optical emission is enhanced by a few percentage points during GRPs. We observed the Crab Pulsar simultaneously at x-ray and radio wavelengths, finding enhancement of the x-ray emission by 3.8 ± 0.7% (a 5.4σ detection) coinciding with GRPs. This implies that the total emitted energy from GRPs is tens to hundreds of times higher than previously known. We discuss the implications for the pulsar emission mechanism and extragalactic fast radio bursts.
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Icarus 358 114220-114220 2021年4月 査読有り
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TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES, AEROSPACE TECHNOLOGY JAPAN 19(1) 52-60 2021年1月4日 査読有り
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Nature Astronomy 5(3) 246-250 2021年1月4日 査読有りAnalyses of meteorites and theoretical models indicate that some carbonaceous near-Earth asteroids may have been thermally altered due to radiative heating during close approaches to the Sun . However, the lack of direct measurements on the subsurface doesn’t allow us to distinguish thermal alteration due to radiative heating from parent-body processes. In April 2019, the Hayabusa2 mission successfully completed an artificial impact experiment on the carbonaceous near-Earth asteroid (162173) Ryugu , which provided an opportunity to investigate exposed subsurface material and test potential effects of radiative heating. Here we report observations of Ryugu’s subsurface material by the Near-Infrared Spectrometer (NIRS3) on the Hayabusa2 spacecraft. Reflectance spectra of excavated material exhibit a hydroxyl (OH) absorption feature that is slightly stronger and peak-shifted compared with that observed for the surface, indicating that space weathering and/or radiative heating have caused subtle spectral changes in the uppermost surface. The strength and shape of the OH feature suggests that the subsurface material experienced heating above 300 °C, similar to the surface. In contrast, thermophysical modelling indicates that radiative heating cannot increase the temperature above 200 °C at the estimated excavation depth of 1 m, even at the smallest heliocentric distance possible for Ryugu. This supports the hypothesis that primary thermal alteration occurred on Ryugu’s parent body. 1–3 4,5
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Proceedings of the International Astronautical Congress, IAC A3 2021年 査読有りHayabusa2, a Japanese asteroid sample return probe, was launched on December 3, 2014, and arrived at Ryugu, a C-Type asteroid, on June 27, 2018. It left Ryugu on November 13, 2019, after completing its 1.5-year asteroid proximity phase operation, including sample collections and a kinetic impact experiment. The propulsive return cruise with the ion thrusters began on December 3, 2019, and in October 2020, the spacecraft entered the precise guidance phase, in which the trajectory corrections with the chemical thrusters were conducted. After several trajectory correction maneuvers, the spacecraft was precisely guided to the landing point of the sample return capsule, and its reentry capsule entered Earth s atmosphere and successfully landed on December 5, 2020. The capsule recovery team immediately found and retrieved the capsule. This paper describes the results of Hayabusa2 Earth return and capsule reentry, including the cruising and precise guidance phase and capsule release operation.
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Earth, Planets and Space 72(1) 2020年12月1日 査読有り© 2020, The Author(s). The precise orbit of the Hayabusa2 spacecraft with respect to asteroid Ryugu is dynamically determined using the data sets collected by the spacecraft’s onboard laser altimeter (LIght Detection And Ranging, LIDAR) and automated image tracking (AIT). The LIDAR range data and the AIT angular data play complementary roles because LIDAR is sensitive to the line-of-sight direction from Hayabusa2 to Ryugu, while the AIT is sensitive to the directions perpendicular to it. Using LIDAR and AIT, all six components of the initial state vector can be derived stably, which is difficult to achieve using only LIDAR or AIT. The coefficient of solar radiation pressure (SRP) of the Hayabusa2 spacecraft and standard gravitational parameter (GM) of Ryugu can also be estimated in the orbit determination process, by combining multiple orbit arcs at various altitudes. In the process of orbit determination, the Ryugu-fixed coordinate of the center of the LIDAR spot is determined by fitting the range data geometrically to the topography of Ryugu using the Markov Chain Monte Carlo method. Such an approach is effective for realizing the rapid convergence of the solution. The root mean squares of the residuals of the observed minus computed values of the range and brightness-centroid direction of the image are 1.36 m and 0.0270°, respectively. The estimated values of the GM of Ryugu and a correction factor to our initial SRP model are 29.8 ± 0.3 m3/s2 and 1.13 ± 0.16, respectively.[Figure not available: see fulltext.]
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Astrodynamics 4(4) 349-375 2020年11月21日 査読有り© 2020, Tsinghua University Press. The Japanese interplanetary probe Hayabusa2 was launched on December 3, 2014 and the probe arrived at the vicinity of asteroid 162173 Ryugu on June 27, 2018. During its 1.4 years of asteroid proximity phase, the probe successfully accomplished numbers of record-breaking achievements including two touchdowns and one artificial cratering experiment, which are highly expected to have secured surface and subsurface samples from the asteroid inside its sample container for the first time in history. The Hayabusa2 spacecraft was designed not to orbit but to hover above the asteroid along the sub-Earth line. This orbital and geometrical configuration allows the spacecraft to utilize its high-gain antennas for telecommunication with the ground station on Earth while pointing its scientific observation and navigation sensors at the asteroid. This paper focuses on the regular station-keeping operation of Hayabusa2, which is called “home position” (HP)-keeping operation. First, together with the spacecraft design, an operation scheme called HP navigation (HPNAV), which includes a daily trajectory control and scientific observations as regular activities, is introduced. Following the description on the guidance, navigation, and control design as well as the framework of optical and radiometric navigation, the results of the HP-keeping operation including trajectory estimation and delta-V planning during the entire asteroid proximity phase are summarized and evaluated as a first report. Consequently, this paper states that the HP-keeping operation in the framework of HPNAV had succeeded without critical incidents, and the number of trajectory control delta-V was planned efficiently throughout the period.
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Astrodynamics 4(4) 309-329 2020年11月5日 査読有り© 2020, Tsinghua University Press. This paper describes the orbit design of the deployable payload Rover 2 of MINERVA-II, installed on the Hayabusa2 spacecraft. Because Rover 2 did not have surface exploration capabilities, the operation team decided to experiment with a new strategy for its deployment to the surface. The rover was ejected at a high altitude and made a semi-hard landing on the surface of the asteroid Ryugu after several orbits. Based on the orbital analysis around Ryugu, the expected collision speed was tolerable for the rover to function post-impact. Because the rover could not control its position, its motion was entirely governed by the initial conditions. Thus, the largest challenge was to insert the rover into a stable orbit (despite its large release uncertainty), and avoid its escape from Ryugu due to an environment strongly perturbed by solar radiation pressure and gravitational irregularities. This study investigates the solution space of the orbit around Ryugu and evaluates the orbit’s robustness by utilizing Monte Carlo simulations to determine the orbit insertion policy. Upon analyzing the flight data of the rover operation, we verified that the rover orbited Ryugu for more than one period and established the possibility of a novel method for estimating the gravity of an asteroid.
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Astrodynamics 4(4) 289-308 2020年11月2日 査読有り© 2020, Tsinghua University Press. Subsurface exploration is one of the most ambitious scientific objectives of the Hayabusa2 mission. A small device called small carry-on impactor (SCI) was developed to create an artificial crater on the surface of asteroid Ryugu. This enables us to sample subsurface materials, which will provide a window to the past. The physical properties of the resulting crater are also useful for understanding the internal structure of Ryugu. Accurate understanding of the crater and ejecta properties, including the depth of excavation of subsurface materials, requires accurate information on impact conditions. In particular, the impact angle is a critical factor because it greatly influences the size and shape of the crater. On April 5, 2019, the Hayabusa2 spacecraft deployed the SCI at 500 m of altitude above the asteroid surface. The SCI gradually reduced its altitude, and it shot a 2 kg copper projectile into the asteroid 40 min after separation. Estimating the position of the released SCI is essential for determining the impact angle. This study describes the motion reconstruction of the SCI based on the actual operation data. The results indicate that the SCI was released with high accuracy.
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Astrodynamics 4(4) 265-288 2020年11月2日 査読有り© 2020, The Author(s). In late 2018, the asteroid Ryugu was in the Sun’s shadow during the superior solar conjunction phase. As the Sun-Earth-Ryugu angle decreased to below 3°, the Hayabusa2 spacecraft experienced 21 days of planned blackout in the Earth-probe communication link. This was the first time a spacecraft had experienced solar conjunction while hovering around a minor body. For the safety of the spacecraft, a low energy transfer trajectory named Ayu was designed in the Hill reference frame to increase its altitude from 20 to 110 km. The trajectory was planned with the newly developed optNEAR tool and validated with real time data. This article shows the results of the conjunction operation, from planning to flight data.
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Astrodynamics 4(4) 377-392 2020年10月17日 査読有り筆頭著者責任著者© 2020, Tsinghua University Press. The deep-space multi-object orbit determination system (DMOODS) and its application in the asteroid proximity operation of the Hayabusa2 mission are described. DMOODS was developed by the Japan Aerospace Exploration Agency (JAXA) for the primary purpose of determining the trajectory of deep-space spacecraft for JAXA’s planetary missions. The weighted least-squares batch filter is used for the orbit estimator of DMOODS. The orbit estimator supports more than 10 data types, some of which are used for relative trajectory measurements between multiple space objects including natural satellites and small bodies. This system consists of a set of computer programs running on Linux-based consumer PCs on the ground, which are used for orbit determination and the generation of radiometric tracking data, such as delta differential one-way ranging and doppler tracking data. During the asteroid proximity phase of Hayabusa2, this system played an essential role in operations that had very strict navigation requirements or operations in which few optical data were obtained owing to special constraints on the spacecraft attitude or distance from the asteroid. One example is orbit determination during the solar conjunction phase, in which the navigation accuracy is degraded by the effect of the solar corona. The large range bias caused by the solar corona was accurately estimated with DMOODS by combining light detection and ranging (LIDAR) and ranging measurements in the superior solar conjunction phase of Hayabusa2. For the orbiting operations of target markers and the MINERVA-II2 rover, the simultaneous estimation of six trajectories of four artificial objects and a natural object was made by DMOODS. This type of simultaneous orbit determination of multi-artificial objects in deep-space has never been accomplished before.
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Nature Astronomy 5(1) 39-45 2020年9月21日 査読有りThe asteroid (162173) Ryugu and other rubble-pile asteroids are likely re-accumulated fragments of much larger parent bodies that were disrupted by impacts. However, the collisional and orbital pathways from the original parent bodies to subkilometre rubble-pile asteroids are not yet well understood1–3. Here we use Hayabusa2 observations to show that some of the bright boulders on the dark, carbonaceous (C-type) asteroid Ryugu4 are remnants of an impactor with a different composition as well as an anomalous portion of its parent body. The bright boulders on Ryugu can be classified into two spectral groups: most are featureless and similar to Ryugu’s average spectrum4,5, while others show distinct compositional signatures consistent with ordinary chondrites—a class of meteorites that originate from anhydrous silicate-rich asteroids6. The observed anhydrous silicate-like material is likely the result of collisional mixing between Ryugu’s parent body and one or multiple anhydrous silicate-rich asteroid(s) before and during Ryugu’s formation. In addition, the bright boulders with featureless spectra and less ultraviolet upturn are consistent with thermal metamorphism of carbonaceous meteorites7,8. They might sample different thermal-metamorphosed regions, which the returned sample will allow us to verify. Hence, the bright boulders on Ryugu provide new insights into the collisional evolution and accumulation of subkilometre rubble-pile asteroids.
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Icarus 348 113835-113835 2020年9月15日 査読有り
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Astrodynamics 4(2) 137-147 2020年6月1日 査読有り© 2020, Tsinghua University Press. This paper describes the guidance and navigation technique used by Hayabusa2 for the asteroid rendezvous operation to reach Ryugu. The operation results, including the achieved guidance and navigation performance, are also summarized. Multiple assessment and navigation teams worked closely to provide reliable navigation solutions with a short solution delivery cycle. Although the uncertainty of the Ryugu’s ephemeris was considerable before Hayabusa2’s arrival, a combination of radiometric-optical hybrid navigation and a stochastic-constrained optimum guidance method was able to achieve an accuracy of less than 100 m and 1 cm/s, and the arrival was precisely timed.
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Science 368(6491) 654-659 2020年5月8日 査読有り© 2020 American Association for the Advancement of Science. All rights reserved. The near-Earth asteroid (162173) Ryugu is thought to be a primitive carbonaceous object that contains hydrated minerals and organic molecules. We report sample collection from Ryugu’s surface by the Hayabusa2 spacecraft on 21 February 2019. Touchdown images and global observations of surface colors are used to investigate the stratigraphy of the surface around the sample location and across Ryugu. Latitudinal color variations suggest the reddening of exposed surface material by solar heating and/or space weathering. Immediately after touchdown, Hayabusa2’s thrusters disturbed dark, fine grains that originate from the redder materials. The stratigraphic relationship between identified craters and the redder material indicates that surface reddening occurred over a short period of time. We suggest that Ryugu previously experienced an orbital excursion near the Sun.
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TRANSACTIONS OF THE JAPAN SOCIETY FOR AERONAUTICAL AND SPACE SCIENCES, AEROSPACE TECHNOLOGY JAPAN 18(3) 116-122 2020年5月4日 査読有り<p>Hayabusa2 is a Japanese sample return mission from the asteroid Ryugu. The Hayabusa2 spacecraft was launched on 3 December 2014, arrived at Ryugu on 27 June 2018. It will stay in proximity of Ryugu until December 2019 for in situ observation and soil sample collection aiming to return to the Earth in December 2020 with the collected sample. During the 1.5-year asteroid proximity operation, the spacecraft performs several numbers of descent operation to deploy and land rovers, and to touchdown collecting soil sample. To obtain large amount of various data during asteroid proximity operation, data downlink from Hayabusa2 is realized using two high gain antennas (HGAs) which are designated to X-band (8GHz) downlink and Ka-band (32GHz) downlink respectively. This paper summarizes Ka-band capability of Hayabusa2 and its contribution to asteroid proximity operation including the Landing Site Selection (LSS) activity. Also, feedbacks for Ka-band equipped future deep space mission are extracted based on the comparison between X/Ka-band operation results.</p>
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Science 368(6486) 67-71 2020年4月3日 査読有りThe Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble-pile structure. We describe an impact experiment on Ryugu using Hayabusa2’s Small Carry-on Impactor. The impact produced an artificial crater with a diameter >10 meters, which has a semicircular shape, an elevated rim, and a central pit. Images of the impact and resulting ejecta were recorded by the Deployable CAMera 3 for >8 minutes, showing the growth of an ejecta curtain (the outer edge of the ejecta) and deposition of ejecta onto the surface. The ejecta curtain was asymmetric and heterogeneous and it never fully detached from the surface. The crater formed in the gravity-dominated regime; in other words, crater growth was limited by gravity not surface strength. We discuss implications for Ryugu’s surface age.
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Nature 579(7800) 518-522 2020年3月26日 査読有り© 2020, The Author(s), under exclusive licence to Springer Nature Limited. Carbonaceous (C-type) asteroids1 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 chondrites2,3 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. Here we report on global one-rotation thermographic images of the C-type asteroid 162173 Ryugu, taken by the thermal infrared imager (TIR)4 onboard the spacecraft Hayabusa25, indicating that the asteroid’s boulders and their surroundings have similar temperatures, with a derived thermal inertia of about 300 J m−2 s−0.5 K−1 (300 tiu). Contrary to predictions that the surface consists of regolith and dense boulders, this low thermal inertia suggests that the boulders are more porous than typical carbonaceous chondrites6 and that their surroundings are covered with porous fragments more than 10 centimetres in diameter. Close-up thermal images confirm the presence of such porous fragments and the flat diurnal temperature profiles suggest a strong surface roughness effect7,8. We also observed in the close-up thermal images boulders that are colder during the day, with thermal inertia exceeding 600 tiu, corresponding to dense boulders similar to typical carbonaceous chondrites6. These results constrain the formation history of Ryugu: the asteroid must be a rubble pile formed from impact fragments of a parent body with microporosity9 of approximately 30 to 50 per cent that experienced a low degree of consolidation. The dense boulders might have originated from the consolidated innermost region or they may have an exogenic origin. This high-porosity asteroid may link cosmic fluffy dust to dense celestial bodies10.
主要なMISC
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ISASニュース (特集：IKAROSからソーラー電力セイル探査機へ) (420) 20-21 2016年3月 筆頭著者
書籍等出版物
2講演・口頭発表等
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34th International Symposium on Space Technology and Science 2023年6月6日
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34th International Symposium on Space Technology and Science 2023年6月6日
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34th International Symposium on Space Technology and Science 2023年6月5日
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SPACEOPS 2023 the 17th international conference on space operations 2023年3月6日
共同研究・競争的資金等の研究課題
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日本学術振興会 科学研究費助成事業 基盤研究(A) 2021年4月 - 2025年3月
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日本学術振興会 科学研究費助成事業 基盤研究(C) 2015年4月 - 2019年3月
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日本学術振興会 科学研究費助成事業 基盤研究(C) 2012年4月 - 2016年3月
