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We observed a stellar occultation by (3200) Phaethon, which occurred in western Japan on 2021 October 3 (UTC). This observation was requested by the DESTINY+ mission team, which plans to conduct a flyby of asteroid Phaethon in 2028. Overall, this research effort contributes towards a large-scale observation campaign with a total of 72 observers observing from western Japan to southern Korea. 36 stations were established, and stellar occultation by the asteroid Phaethon was detected in 18 of them. This is the first time that this many multiple chord observations have been made for such a small asteroid (it has a diameter of 5–6 km). Observational reductions show that the apparent cross-section of Phaethon at the time of the occultation could be approximated using an ellipse with a major diameter of 6.12 ± 0.07 km and a minor diameter of 4.14 ± 0.07 km, and a position angle of 117.°4 ± 1.°5. As can be seen from the small error bars of the fitted ellipse, we have succeeded in estimating the shape and size of the asteroid with an extremely high degree of accuracy. Our observation results, together with other observations, will be used to create a 3D model of Phaethon and to improve its orbit. The instruments that we used for this observation are commonly used by many amateur astronomers in Japan for occultation observations and are not difficult to obtain. This paper describes the method and results of our observations using a CMOS camera and a GPS module, so that many people can participate in occultation observations in the future.

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.

Presented here are the observations and interpretations from a comprehensive analysis of 16 representative particles returned from the C-type asteroid Ryugu by the Hayabusa2 mission. On average Ryugu particles consist of 50% phyllosilicate matrix, 41% porosity and 9% minor phases, including organic matter. The abundances of 70 elements from the particles are in close agreement with those of CI chondrites. Bulk Ryugu particles show higher δ18O, Δ17O, and ε54Cr values than CI chondrites. As such, Ryugu sampled the most primitive and least-thermally processed protosolar nebula reservoirs. Such a finding is consistent with multi-scale H-C-N isotopic compositions that are compatible with an origin for Ryugu organic matter within both the protosolar nebula and the interstellar medium. The analytical data obtained here, suggests that complex soluble organic matter formed during aqueous alteration on the Ryugu progenitor planetesimal (several 10’s of km)

International audience; INTRODUCTION: Organic compounds in asteroids and comets contain information about the early history of the Solar System. They could also have delivered organic material to early Earth. The Hayabusa2 spacecraft visited the carbonaceous asteroid Ryugu and collected samples of its surface materials, which were brought to Earth in December 2020. RATIONALE: We investigated the macromolecular organic matter in the Ryugu samples, measuring its elemental, isotopic, and functional group compositions along with its small-scale structures and morphologies. Analytical methods used included spectro-microscopies, electron microscopy, and isotopic microscopy. We examined intact Ryugu grains and insoluble carbonaceous residues isolated by acid treatment of the Ryugu samples. RESULTS: Organic matter is abundant in the Ryugu grains, distributed as submicrometersized organic grains and as organic matter dispersed in matrix. The Ryugu organic matter consists of aromatic carbons, aliphatic carbons, ketones, and carboxyls. The functional group

C-type asteroids1 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 sensing2–7 and on-asteroid measurements8,9 with Hayabusa2 (ref. 10). 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 Ryugu11, indicating differences in the returned Ryugu samples. The average of the estimated bulk densities of Ryugu sample particles is 1,282 ± 231 kg m−3, which is lower than that of meteorites12, suggesting a high microporosity down to the millimetre scale, extending centimetre-scale estimates from thermal measurements5,9. 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 Ryugu3,4 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.

An altimeter is a critical instrument in planetary missions, for both safe operations and science activities. We present required specifications and link budget calculations for light detection and ranging (LIDAR) onboard the Martian Moons Exploration (MMX) spacecraft. During the mission phase, this LIDAR will continuously measure the distance between the spacecraft and its target. The time-series distance provides important diagnostic information for safe spacecraft operations and important information for geomorphological studies. Because MMX is a sample return mission, its LIDAR must accommodate physical disturbances on the Martian satellite surface. This resulted in changes to the optical system design. Graphical abstract

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.

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 measured the mineralogy and bulk chemical and isotopic compositions of Ryugu samples. The samples are mainly composed of materials similar to those of 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, about 5.2 − 0.7 + 0.8 million (statistical) or 5.2 − 2.1 + 1.6 million (systematic) years after the 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 that of the Sun’s photosphere than other natural samples do.

C-type asteroids likely formed in the outer Solar System and were then scattered inwards during giant planet migration (Walsh et al., 2011). They may have transported volatiles to the inner Solar System and created the conditions suitable for life on Earth(Alexander, 2017). Carbonaceous chondrites are fragments from C-type asteroids and provide evidence that these generally organic-rich (Garvie and Buseck, 2007) bodies experienced extensive aqueous alteration early in Solar System history (Alexander et al., 2014). On 6th December 2020, ~5.4g of material was delivered to Earth from the C-type asteroid 162173 Ryugu by the Hayabusa2 spacecraft (Yada et al., 2021). Here we present the results of an integrated bulk and micro-analytical study of Ryugu particles, which provides a unique insight into the interrelationship between aliphatic-rich organics and surrounding hydrous minerals at a sub-micrometer scale. This dataset has clear implications for better understanding the origin and early evolution of Solar System organic matter and demonstrates that Ryugu particles are among the most uncontaminated extraterrestrial materials so far studied.

C-type asteroids are considered to be primitive small Solar-System bodies enriched in water and organics, providing clues for understanding 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 and on-asteroid measurements with Hayabusa2, but further studies are expected by direct analyses of returned samples. Here we describe the bulk sample mainly consisting of rugged and smooth particles of millimeter to submillimeter size, preserving physical and chemical properties as they were on the asteroid. The particle size distribution is found steeper than that of surface boulders11. Estimated grain densities of the samples have a peak around 1350 kg m-3, which is lower than that of meteorites suggests a high micro-porosity down to millimeter-scale, as estimated at centimeter-scale by thermal measurements. The extremely dark optical to near-infrared reflectance and the spectral profile with weak absorptions at 2.7 and 3.4 microns implying carbonaceous composition with indigenous aqueous alteration, respectively, match the global average of Ryugu, confirming the sample’s representativeness. Together with the absence of chondrule and Ca-Al-rich inclusion of larger than sub-mm, these features indicate Ryugu is most similar to CI chondrites but with darker, more porous and fragile characteristics.

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 Sun1–3. 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) Ryugu4,5, 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. Hayabusa2 created an artificial crater on Ryugu to analyse the subsurficial material of the asteroid. Results show that the subsurface is more hydrated than the surface. It experienced alteration processes that can be traced back to Ryugu’s parent body.

In this study, we determined the alignment of the laser altimeter aboard Hayabusa2 with respect to the spacecraft using in-flight data. Since the laser altimeter data were used to estimate the trajectory of the Hayabusa2 spacecraft, the pointing direction of the altimeter needed to be accurately determined. The boresight direction of the receiving telescope was estimated by comparing elevations of the laser altimeter data and camera images, and was confirmed by identifying prominent terrains of other datasets. The estimated boresight direction obtained by the laser link experiment in the winter of 2015, during the Earth’s gravity assist operation period, differed from the direction estimated in this study, which fell on another part of the candidate direction; this was not selected in a previous study. Assuming that the uncertainty of alignment determination of the laser altimeter boresight was 4.6 pixels in the camera image, the trajectory error of the spacecraft in the cross- and/or along-track directions was determined to be 0.4, 2.1, or 8.6 m for altitudes of 1, 5, or 20 km, respectively.

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.

A PZT type telescope for observations of gravity gradient and lunar rotation was developed, and a Bread Board Model (BBM) for ground experiments was completed. Some developments were made for the BBM such as a tripod with attitude control system, a stable mercury pool and a method for collecting the effects of vibrations. Laboratory experiments and field observations were performed from August to September of 2014, in order to check the entire system of the telescope and the software, and the results were compared to the centroid experiments which pursue the best accuracy of determination of the center of star images with a simple optical system. It was also investigated how the vibrations of mercury surface affect the centroid position on Charge Coupled Device (CCD). The results of the experiments showed that the effects of vibrations are almost common to stars in the same view, and they can be corrected by removing mean variation of the stars; and that the vibration of mercury surface can cause errors in centroid as large as 0.2 arcsec; and that there is a strong correlation between the Standard Deviation (SD) of variation of the centroid position and signal to noise ratio (SNR) of star images. It is likely that the accuracy of one (1) milli arcsecond is possible if SNR is high enough and the effects of vibrations are corrected.

Accepted: 2016-02-05, 資料番号: SA1150344000

International audience; 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 bodies1

The Ganymede Laser Altimeter (GALA) is one of the ten scientific instruments selected for the Jupiter Icy Moons Explorer (JUICE) mission currently implemented under responsibility of the European Space Agency (ESA). JUICE is scheduled for launch in mid 2022; arrival at Jupiter will be by end of 2029 with the nominal science mission—including close flybys at Ganymede, Europa, and Callisto and a Ganymede orbit phase—ending by mid 2033. GALA’s main objective is to obtain topographic data of the icy satellites of Jupiter: Europa, Ganymede, and Callisto. By measuring the diurnal tidal deformation of Ganymede, which crucially depends on the decoupling of the surface ice layer from the deep interior by a liquid water ocean, GALA will obtain evidence for (or against) a subsurface ocean in a 500 km orbit around the satellite and will provide constraints on Ganymede’s ice shell thickness. In combination with other instruments, it will characterize the morphology of surface units on Ganymede, Europa, and Callisto providing not only topography but also surface roughness and albedo (at 1064 nm) measurements. GALA is a single-beam laser altimeter operating with up to 50 Hz (nominal 30 Hz) shot frequency at a wavelength of 1064 nm and pulse lengths of 5.5 ± 2.5 ns using a Nd:YAG laser. The return pulse is detected by an Avalanche Photo Diode (APD) with 100 MHz bandwidth and is digitized at a sampling rate of 200 MHz providing range measurements with a subsample resolution of 0.1 m and surface roughness measurements from pulse-shape analysis on the scale of the footprint size of about 50 m at 500 km altitude. The instrument is developed in collaboration of institutes and industry from Germany, Japan, Switzerland, and Spain. © 2019, CEAS., Financial support was provided under grant 50 QJ 1401 on behalf of the DLR Space Administration by the German Bundesministerium fur Wirtschaft und Energie. This research has been supported by the Spanish Ministerio de Economia y Competitividad under Contract ESP 2016-76076-R.

著者人数: 120名(所属. 宇宙航空研究開発機構宇宙科学研究所 (JAXA)(ISAS): 澤田, 弘崇; 横田, 康弘; 坂谷, 尚哉; 岡田, 達明; 田中, 智; 飯島, 祐一; 早川, 雅彦; 松岡, 萌; R.-L. Ballouz; 嶌生, 有理; 水野, 貴秀; 石原, 吉明; 長谷川, 直; 藤本, 正樹; 吉川, 真; 山本, 幸生; 白井, 慶; R. Noguchi; 尾川, 順子; 照井, 冬人; 菊地, 翔太; 山口, 智宏; 竹内, 央, 三桝, 裕也; 高橋, 忠輝; 竹井, 洋; 藤井, 淳; 中澤, 暁; 細田, 聡史; 森, 治; 嶋田, 貴信; Soldini, Stefania; 岩田, 隆浩; 安部, 正真; 矢野, 創; 月崎, 竜童; 尾崎, 正伸; 西山, 和孝; 佐伯, 孝尚; 渡邊, 誠一郎; 津田, 雄一), Accepted: 2019-03-12, 資料番号: SA1180387000

著者人数: 12名, Accepted: 2016-02-11, 資料番号: SA1150345000

著者人数: 12名, Accepted: 2015-12-23, 資料番号: SA1150343000

Asteroid 162173 Ryugu is a rubble-pile asteroid, whose top-shape is compatible with models of deformation by spin up. Rims of major craters on Ryugu have an east-west asymmetric profile; their western crater rims are sharp and tall, while their eastern crater rims are rounded and low. Although there are various possible explanations, we theoretically assess the effect of asteroid rotation as the possible reason for this east-west asymmetry. It is known that the trajectories and fates of ejecta are affected by the rotation. The Coriolis force and the inertial speed of the rotating surface are the factors altering the ejecta trajectories. Consequently, we found that the east-west asymmetric crater rims might be formed as a result of rotation, when the inertial speed of the rotating surface is nearly equal to the first cosmic velocity of the body. In other words, it is possible that the observed east-west asymmetric rims of the Urashima, Cendrillon, and Kolobok craters were formed when Ryugu's rotation period was ~3.6 h., Comment: 30 pages 13 figures

Precise information of spacecraft position with respect to target body is of importance in terms of scientific interpretation of remote sensing data. In case of Hayabusa2, a sample return mission from asteroid Ryugu, such information is also necessary for landing site selection activity. We propose a quick method to improve the spacecraft trajectory when laser altimeter range measurements and a shape model are provided together with crude initial trajectory, spacecraft attitude information, and asteroid spin information. We compared topographic features contained in the altimeter data with those expressed by the reference shape model, and estimated long-period trajectory correction so that discrepancy between the two topographic profiles was minimized. The improved spacecraft positions are consistent with those determined by image-based stereophotoclinometry method within a few tens of meters. With such improved trajectory, the altimeter ranges can be converted to Ryugu's topographic profiles that are appropriate for geophysical interpretation. We present a geophysical application that invokes possibility of impact-induced formation of the Ryugu's western bulge.

The linear polarization degree (referred to the scattering plane, P_r) as a function of the solar phase angle, {\alpha}, of solar system objects is a good diagnostic to understand the scattering properties of their surface materials. We report Pr of Phaethon over a wide range of {\alpha} from 19.1 deg to 114.3 deg in order to better understanding properties of its surface materials. The derived phase-polarization curve shows that the maximum of P_r, P_max, is >42.4% at {\alpha} >114.3 deg, a value significantly larger than those of the moderate albedo asteroids (P_max ~9%). The phase-polarization curve classifies Phaethon as B-type in the polarimetric taxonomy, being compatible with the spectral property. We compute the geometric albedo, p_v, of 0.14 +/- 0.04 independently by using an empirical slope-albedo relation, and the derived p_v is consistent with previous results determined from mid-infrared spectra and thermophysical modeling. We could not find a fit to the period in our polarimetric data in the range from 0 up to 7.208 hr (e.g., less than twice the rotational period) and found significant differences between our P_r during the 2017 approach to the Earth and that of the 2016. These results imply that Phaethon has a region with different properties for light scattering near its orbital pole., Comment: 30 pages, 3 figures, 3 tables (includes long data table)

We report the surface roughness analysis of the lunar highlands for the baseline range 0.15–100 km. We use the Median Differential Slope αm to investigate the scale dependency of the roughness and derive the global αm distribution from SELENE Laser Altimeter and Terrain Camera data. While αm(l) versus baseline l (km) plots vary among different highland types, all highlands commonly show a peak at 3–30 km. The Pre-Nectarian surface shows a relatively large αm(20–30 km). Our analysis is supported by the simulation of synthetic surface cratering models and crater statistics. In our simulation, a peak of αm(30 km) is successfully reproduced. The actual crater density shows good correlation with an empirical roughness indicator. However, a large part of the Nectarian surface shows a peak at 6–9 km baseline. This peak may be caused by secondary craters and ejecta deposit textures from the Nectarian system basins.

In this paper we review the lunar laser ranging conducted by the laser altimeter (LALT) on board the KAGUYA lunar explorer (2007–2009). Five aspects of LALT measurements are described: (1) General operational history, (2) Laser shot and data statistics, (3) Revisions to LALT topographic data, (4) Variations in laser output energy, and (5) Peak height analysis of laser echo pulses. LALT was able to range to the lunar surface despite some troubles with respect to laser output energy in the middle of the KAGUYA mission. The time series topographic data set was revised (Ver. 2) by incorporating new lunar gravity model based on KAGUYA and other historical lunar satellite’s orbit data, along with other improvements, for example by incorporating the accurate position of the laser collimator on board the KAGUYA; however, more than half of the acquired range data could not be converted properly due to problems with orbit accuracy during the extended phase of the mission. The spherical harmonic coefficients and the basic lunar figure parameters derived from LALT_LGT_TS agree very well with LRO-LOLA and the Chang’E-1 LAM model. It is possible that partial failure to the laser diode was responsible for the gradual degradation of laser power (0.835 mJ per million shots) and the rapid decrease that occurred over April 9–14, 2008. The laser power also proved to be extremely sensitive to the temperature of the laser oscillator. The peak height ratio – that is peak height telemetry data divided by calculated ratio – is about 19% on average using the mean slope and albedo data from LALT and Spectral Profiler on KAGUYA space craft, respectively, which suggests the performance of peak height measurement is more than 1/5 for more than 70 km altitude, if compared with calculated one. The peak height ratio may be better if we take the effect of small scale topography within a footprint into account.

A single aperture and hollow retroreflector [corner-cube mirror (CCM)] that in principle has no internal optical path difference is a key instrument for achieving lunar laser ranging one order or more accurate than the current level (~2 cm). We are developing CCM whose aperture is 20 cm with optimized dihedral angles. The 20-cm CCM yields two times peak height for returned laser pulse compared with Apollo 15’s retroreflector. Two investigations were conducted to confirm the feasibility of the 20-cm aperture CCM. The first is thermo-optical simulation and evaluation of the 20-cm CCM in the lunar thermal environment. Through this simulation, it has turned out for the first time that 20-cm aperture CCM made of single-crystal Si or “ultra-low expansion glass–ceramics” such as CCZ-EX® (OHARA Inc.) can be used for CCM with no thermal control, if the perfectly fixed point of CCM is limited to one. The second is annealing and shear loading experiments of single-crystal silicon (Si) samples. Through these experiments, high-temperature annealing from 100 to 1000 °C is confirmed to be effective for the enhancement of the adhesive strength between optically contacted surfaces with no optical damage in roughness and accuracy, indicating that this annealing process would enhance the rigidity of CCM fabricated by the optically contacted plates.

著者人数: 87名, Accepted: 2011-02-08, 資料番号: SA1004099000

The distribution of two-dimensional velocity aberration is off-centered by 5 to 6 microradians in lunar laser ranging, due to the stable measurement geometry in the motion of the Earth and the Moon. The optical responses of hollow-type retroreflectors are investigated through numerical simulations, especially focusing on large-size, single-reflector targets that can ultimately minimize the systematic error in future lunar laser ranging. An asymmetric dihedral angle offset, i.e. setting unequal angles between the three back faces, is found to be effective for retroreflectors that are larger than 100 mm in diameter. Our numerical simulation results reveal that the optimized return energy increases approximately 3.5 times more than symmetric dihedral angle cases, and the optimized dihedral angle offsets are 0.65–0.8 arcseconds for one angle, and zeroes for the other two angles.

Both the Chang’E-1 and SELenological and ENgineering Explorer (SELENE) lunar missions, launched in 2007, provide an opportunity for significant advances in lunar science. In particular, both orbiters provide refined lunar topography models with unprecedented finer resolution and improved accuracy, especially for the far side and the polar areas of the Moon where fewer valid measurements have been available to date. An evaluation of the derived topography models is essential for the improved interpretation of selenophysics, including a knowledge of the interior of the Moon. This study provides both external and internal accuracy assessments for the derived topographic models (note that different data sets are used, albeit independent). The external comparison, which consists of comparing the topographic models with landmarks established with lunar laser ranging and radio tracking, yields an accuracy estimate of 157 m for the Change’E-1 model and 58 m for the SELENE model (150 m and 55 m if the newly recovered Lunakhod-1 site is included). The internal comparison, consisting of crossover analysis of the altimeter data after the removal of once-per-orbital-revolution errors, yields an accuracy estimate of 206 m and 68 m, respectively. These comparisons allow the establishment of conservative estimates of accuracy of 200 m and 70 m for the Chang’E-1 and SELENE models, respectively. Given the conservative estimates on the accuracy, both models yield significant improvement by factors of 2.5 and 8, respectively, when compared with a contemporary lunar topography model, i.e., the Unified Lunar Control Network 2005 (ULCN2005).

We have used the Kaguya laser altimeter-derived topography to conduct a comprehensive study of the illumination conditions at the Moon’s south pole. We have determined, by comparing simulated and actual Clementine images, that the Kaguya topography can be used to generate realistic illumination conditions. We generated an average illumination map for the year 2020 for the lunar south pole region. From this we identified the areas that receive the most illumination. The place receiving the most illumination (86% of the year) is located close to the rim of Shackleton crater at 88.74°S 124.5°E. However two other areas, less than 10 km apart from each other, are collectively lit for 94% of the year. We found that sites exist near the south pole that are continuously lit for several months during summer. We were also able to map the locations and durations of eclipse periods for these areas. Finally we analyzed the seasonal variations in lighting conditions, from summer to winter, for key areas near the south pole. We conclude that areas exist near the south pole that have illumination conditions that make them ideal candidates as future outpost sites.

We numerically examined various retroreflectors as laser ranging targets for future missions to the Moon. The geometric conditions, such as the angle of incidence and velocity aberration, with lunar targets are much more restricted than those with most of the earth-orbiting artificial satellites. The numerical optical response simulation carried out in this study indicates that a single retroreflector with a diameter of 150–250 mm performs similar to the existing Apollo retroreflector arrays. Further, no dihedral angle is required for small retroreflectors with diameters below 150 mm for uncoated ones and below 100 mm for coated and hollow ones. Retroreflectors with larger diameters require dihedral angles of 0.20, 0.25, and 0.35 arcsec for coated, uncoated and hollow types, respectively. The objective of this fundamental study is to underlie the development of future laser ranging targets that are to be placed on the Moon.

The farside gravity field of the Moon is improved from the tracking data of the Selenological and Engineering Explorer (SELENE) via a relay subsatellite. The new gravity field model reveals that the farside has negative anomaly rings unlike positive anomalies on the nearside. Several basins have large central gravity highs, likely due to super-isostatic, dynamic uplift of the mantle. Other basins with highs are associated with mare fill, implying basalt eruption facilitated by developed faults. Basin topography and mantle uplift on the farside are supported by a rigid lithosphere, whereas basins on the nearside deformed substantially with eruption. Variable styles of compensation on the near- and farsides suggest that reheating and weakening of the lithosphere on the nearside was more extensive than previously considered.

A global lunar topographic map with a spatial resolution of finer than 0.5 degree has been derived using data from the laser altimeter (LALT) on board the Japanese lunar explorer Selenological and Engineering Explorer (SELENE or Kaguya). In comparison with the previous Unified Lunar Control Network (ULCN 2005) model, the new map reveals unbiased lunar topography for scales finer than a few hundred kilometers. Spherical harmonic analysis of global topographic data for the Moon, Earth, Mars, and Venus suggests that isostatic compensation is the prevailing lithospheric support mechanism at large scales. However, simple rigid support is suggested to dominate for the Moon, Venus, and Mars for smaller scales, which may indicate a drier lithosphere than on Earth, especially for the Moon and Venus.

著者人数：20名, 資料番号: SA1000216000

The measurement of the rotation of the Moon is one of the key techniques to get the information of the internal structure. For this purpose, we proposed a small telescope experiment on the surface of the Moon in which motion of stars are utilized for the estimation of the rotation parameter. This paper describes results of simulation of observation, in which star trajectories observed are decomposed to librations, polar motion, and the precession and the amplitude and phase of each component are estimated. The standard deviation of the parameter estimation becomes nearly 1 ms of arc, which will be better than the Lunar Laser Ranging observation. From the viewpoints of accuracy of observation, thermal condition, and electric power generation, the instrument should be placed where the much sunshine is achieved on the lunar polar region.

The SELENE Laser Altimeter (LALT) is designed to map the Moon’s topography and will be launched in summer 2007. LALT incorporates Q-switched Cr doped Nd:YAG laser (1064 nm) with an output energy of 100 mJ and 1 Hz repetition frequency for about one year mission period. The laser pulse travels to the Moon’s surface and reflections from the surface are detected by a silicon avalanche photo-diode. The ranging distance is 50–150 km with about 5 m accuracy. Several corrections for accurate ranging data are investigated. The flight hardware has been qualified and passed all the integration tests. A principal goal of the LALT instrument is to obtain a much more detailed lunar topographic map which is superior in global coverage, measurement accuracy and number of data points to previous observations and models. The overall science objectives of LALT are (1) determination of lunar global figure, (2) internal structure and surface processes, (3) exploration of the lunar pole regions, and (4) reduction of lunar occultation data.