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One of the methods used for tritium gas analysis is beta-ray induced X-ray spectrometry (BIXS). Gas cell design is important in this method. The structure of the gas cell for BIXS was optimized by Monte Carlo simulation of beta-ray induced X-ray spectra in various window geometries using the Geant4 tool kit (version 10.01.p02). The simulated spectrum from tritium decay fitted the observed one, and the simulation model was used to obtain the cell parameters for BIXS. The optimum thickness of the gold layer on a beryllium window was around 150 nm. This simulation model also considered the relationship between self-absorption by hydrogen gas and the cell length. Self-absorption increased with increasing cell length and the relationship between the sample pressure and cell length was formulated. [ABSTRACT FROM AUTHOR]

Retention and release behavior of tritium charged into stainless steel type 316 by two different methods were tracked by β-ray-induced X-ray spectrometry (BIXS). Exposure to tritium gas and irradiation of tritium ions were employed. Effects of pre-irradiation of helium ions on tritium retention were also investigated. After charging tritium by these methods and tracking by BIXS, thermal release behavior of tritium was examined by BIXS. The observed X-ray spectrum showed clearly that thermally charged tritium at a high temperature diffused into the bulk of SS316, but tritium ions irradiated at room temperature were trapped in surface layers of the sample irrespective of tritium ion energy. Thermally charged tritium was not able to remove completely even heating at high temperature of 973 K. On the contrary, it was seen that the ion-irradiated tritium decreased almost linearly with temperature irrespective of ion energy. Furthermore, rapid decrease of tritium appeared in the helium pre-irradiated samples. [ABSTRACT FROM AUTHOR]

A new β-ray-induced X-ray measurement system equipped with a silicon drift detector, which was named “Advanced-BIXS”, was constructed to study in detail retention behavior of surface tritium by measurements of low energy X-rays below 1 keV such as C( K α ) and O( K α ) as well as high energy X-rays induced by β-rays from tritium. In this study, basic performance of the present system has been examined using various tritium-containing samples. It was seen that energy linearity, energy resolution and sensitivity were quite enough for measurements of low energy X-rays induced by β-rays. Intensity of characteristic X-rays emitted from the surface and/or bulk of a tritium-containing sample was lowered by argon used as a working gas of the Advanced-BIXS. Pressure dependence of transmittance of C( K α ) and Fe( K α ) was examined as examples of low and high energy X-rays, and it was able to represent by using the mass absorption coefficient in argon. It was concluded, therefore, that the present system has high potentiality for nondestructive measurements of tritium retained in surface layers and/or bulk of fusion related materials. [ABSTRACT FROM AUTHOR]

We report the electron density in a plasma tail of Comet ISON (C/2012 S1) derived from interplanetary scintillation (IPS) observations during November 1–28, 2013. Comet ISON showed a well-developed plasma tail (longer than 2.98 × 10 7 km ) before its perihelion passage on November 28. We identified a radio source whose line-of-sight approached the ISON’s plasma tail in the above period and obtained its IPS data using the Solar Wind Imaging Facility at 327 MHz. We used the Heliospheric Imager onboard the Solar-Terrestrial Relation Observatory to distinguish between the cometary tail and solar eruption origins of their enhanced scintillation. From our examinations, we confirmed three IPS enhancements of a radio source 1148–00 on November 13, 16, and 17, which could be attributed to the disturbance in the cometary tail. Power spectra of 1148–00 had the steeper slope than normal ones during its occultation by the plasma tail. We estimated the electron density in the ISON’s plasma tail and found 84 cm - 3 around the tail axis at a distance of 3.74 × 10 7 km from the cometary nucleus and an unexpected variation of the electron density in the vicinity of the tail boundary. [ABSTRACT FROM AUTHOR]

Abstract: The Japanese Space Agency’s Hayabusa II mission is scheduled to rendezvous with and return a sample from the near-Earth Asteroid (162173) 1999 JU3. Previous visible-wavelength spectra of this object show significant variability across multiple epochs which has been attributed to a compositionally heterogeneous surface. We present new visible and near-infrared spectra to demonstrate that thermally altered carbonaceous chondrites are plausible compositional analogs, however this is a tentative association due to a lack of prominent absorption features in our data. We have also conducted a series of high signal-to-noise visible-wavelength observations to investigate the reported surface heterogeneity. Our time series of visible spectra do not show variability at a precision level of a few percent. This result suggests two most likely possibilities. One, that the surface of 1999 JU3 is homogenous and that unaccounted for systematic effects are causing spectral variation across epochs. Or two, that the surface of 1999 JU3 is regionally heterogenous, in which case existing shape models suggest that any heterogeneity must be limited to terrains smaller than approximately 5% of the total surface area. These new observations represent the last opportunity before both the launch and return of the Hayabusa II spacecraft to perform ground-based characterization of this asteroid. Ultimately, these predictions for composition and surface properties will be tested upon completion of the mission. [Copyright &y& Elsevier]

Abstract: An ultraviolet–visible spectrum between 300 and 450nm of a cometary meteoroid originated from 55P/Tempel-Tuttle was investigated. The spectroscopy was carried out an intensified high definition TV camera with a slit-less reflection grating during the 2001 Leonid meteor shower over Japan. A best fit calculation mixed with atoms and molecules confirmed the first discovery of bands in the UV meteor spectrum. temperature was estimated to 10,000K with a low number density of 1.55×105 cm−3. We also discuss the possibility that enhanced emissions in a meteor and a train around 310nm are caused by the band head of OH A 2Σ+ → X 2Π. Since cometary dust may have contributed organics and water to the Earth from its early period until now, OH A–X (0,0) must be investigated. [Copyright &y& Elsevier]

The ranging instrument aboard the Hayabusa spacecraft measured the surface topography of asteroid 25143 Itokawa and its mass. A typical rough area is similar in roughness to debris located on the interior wall of a large crater on asteroid 433 Eros, which suggests a surface structure on Itokawa similar to crater ejecta on Eros. The mass of Itokawa was estimated as (3.58 ± 0.18) × 1010 kilograms, implying a bulk density of (1.95 ± 0.14) grams per cubic centimeter for a volume of (1.84 ± 0.09) × 107 cubic meters and a bulk porosity of ∼40%, which is similar to that of angular sands, when assuming an LI (low iron chondritic) meteorite composition. Combined with surface observations, these data indicate that Itokawa is the first subkilometer-sized small asteroid showing a rubble-pile body rather than a solid monolithic asteroid. [ABSTRACT FROM AUTHOR]

Sodium is a perfect indicator to investigate the volatility within meteoroid composition. Spectroscopic observations of the Geminid meteors were carried out using modern high sensitive large format CMOS cameras. 149 visible spectra of the Geminid meteors observed in 2017 and 2018 were analyzed by a newly developed procedure. The variation of Na, Mg and Fe during meteor ablation in the atmosphere has been studied. Although sodium is depleted in ~ 80% of the Geminids, statistically significant variations of sodium in the Geminid meteors were found. It should be noted that Na depletion in the Geminid meteors increased with decreasing meteoroid size. A yearly variation of Na in the Geminid meteor streams was also found. Solar-heating age across the different parts of the meteoroids stream ejected from a parent active asteroid (3200) Phaethon having 0.14 AU small perihelion distance seems to the most likely explanation for the sodium variation and depletion. Surprisingly, three iron Geminid meteors were detected across the maximum of 2018 Geminids. • 149 visible spectra of the Geminid meteors in 2017 and 2018 were observed by using modern high sensitive large format CMOS cameras with a newly developed procedure. • Although sodium is depleted in ~80 % of the Geminids, tatistically significant variations of sodium in the Geminid meteors were found. • Solar-heating age across the different parts of the meteoroids stream ejected from a parent active asteroid (3200) Phaethon having 0.14 AU small perihelion distance seems to the most likely explanation for the sodium variation and depletion. • Three iron Geminid meteors were also detected across the maximum of 2018 Geminids. [ABSTRACT FROM AUTHOR]

When a meteoroid impacts on the Moon, a brief flash phenomenon—called a "lunar impact flash"—can be observed by ground-based telescopes. The statistics of these events can efficiently determine the meteoroid size-frequency distribution in cis-lunar space. The luminous efficiency—that is, the ratio of luminous energy emitted to the impactor's kinetic energy—is the most important parameter for precise meteoroid-size estimation. Thus, it is necessary to understand the impact-flash mechanism and determine the luminous efficiency under various conditions. We have, therefore, performed hypervelocity-impact experiments at JAXA, and measured the relation between luminous efficiency and ambient pressure at various vacuum levels. Based on a high-speed spectroscopy, we find that the flash consists of blackbody radiation from hot ejecta and blackbody radiation from a vapor cloud together with emission lines and molecular bands. Spectrum analysis reveals that the initial luminous efficiency within the first few μs does depend on the vacuum level, whereas the luminous efficiency over several ms does not depend on the vacuum level. • Hypervelocity Impact flash consists of blackbody radiation from ejecta and vapor cloud, emission lines, and molecular bands. • Luminous efficiency within a few μs is affected by vacuum level due to ablation between impact fragments and ambient atmosphere. • Luminous efficiency over several ms does not depend on vacuum level. • For lunar impact flash study, we can perform rough vacuum level (e.g., several tens of Pa) and frequent impact experiments. [ABSTRACT FROM AUTHOR]

Japanese asteroid explorer Hayabusa2 arrived at C-type asteroid 162,173 Ryugu in June 2018. The laser altimeter (LIDAR) onboard Hayabusa2 measured its own transmitted laser and returned pulse intensities from a Ryugu surface until November 2019. Because the Ryugu surface is extremely rough, topography dominates over the material properties in the conventional derivation of normal albedo. Thus, we developed a method to retrieve the normal albedo from the rough surface of a C-type asteroid at a LIDAR laser wavelength of 1.064 μm. The albedo map covering an equatorial band between – 40° and + 20° in latitude was created with 3˚-by-3˚ resolution using the intensity data obtained before the conjunction of the spacecraft with the Sun. The average of the normal albedo is 0.0405 ± 0.0027, whereas approximately half of the 3°-by-3° grids are between 0.04 and 0.045. The low and uniform normal albedo feature is common to other remote-sensing observations of Ryugu by visible and near-infrared cameras onboard Hayabusa2. [ABSTRACT FROM AUTHOR]

This study proposes a method for the observation of the upper atmosphere using an artificial meteor injected by a mass driver installed on a microsatellite. The mass driver injects a pill at a velocity of 200 m/s and deorbits it into the atmosphere. The emission of the pill can then be observed from the ground at the necessary time and location. This approach could contribute to a better understanding of the global environment as well as different aspects of astronomy and planetary science. To realize the proposed method, the required size and emission of the pill have to be determined. Therefore, we conducted flow-field simulations, spectroscopic estimations, and an experiment on an artificial meteor in the arc heater wind tunnel at the Institute of Space and Astronautical Science in the Japan Aerospace Exploration Agency (ISAS/JAXA). From the results, we confirmed that the light emission could be observed as a shooting star by the naked eye and thus verified the feasibility of the method. [ABSTRACT FROM AUTHOR]

On 2020 December 5 at 17:28 UTC, the Japan Aerospace Exploration Agency's Hayabusa-2 sample return capsule came back to the Earth. It re-entered the atmosphere over South Australia, visible for 53 seconds as a fireball from near the Northern Territory border toward Woomera where it landed in the the Woomera military test range. A scientific observation campaign was planned to observe the optical, seismo-acoustic, radio, and high energy particle phenomena associated with the entry of an interplanetary object. A multi-institutional collaboration between Australian and Japanese universities resulted in the deployment of 49 instruments, with a further 13 permanent observation sites. The campaign successfully recorded optical, seismo-acoustic, and spectral data for this event which will allow an in-depth analysis of the effects produced by interplanetary objects impacting the Earth's atmosphere. This will allow future comparison and insights to be made with natural meteoroid objects. [ABSTRACT FROM AUTHOR]

• Ray tracing analysis was conducted to reduce stray light of OROCHI onboard the MMX. • OROCHI's optical system is simulated and its CCD reflectance was measured. • Our techniques are incorporated into the preliminary design of OROCHI. The Martian Moons eXploration (MMX) spacecraft is equipped with two cameras, i.e., the TElescopic Nadir imager for GeOmOrphology (TENGOO) and the Optical Radiometer composed Of CHromatic Imagers (OROCHI), for the scientific observation of the Martian moon Phobos. OROCHI is a wide-angle multiband camera system comprising seven cameras with different bandpass filters and one monochromatic camera in the visible and near-infrared range. Thus, OROCHI can simultaneously obtain multiband images. Previous space observations have revealed that the reflection spectra of Phobos exhibit regional differences, including a red unit and a blue unit with weak redness. Moreover, Phobos shows an absorption band near 650 nm, attributed to the red unit of Phobos. Global observations using OROCHI require the determination of such regional variations with high precision; therefore, we must reduce noise in the optical system design. In this study, to meet this requirement, we focus on stray light reduction. Stray light is a source of noise generated by reflections at surfaces of optical components (e.g., sensors, filters, lenses, and lens barrels). We design simulation models of the OROCHI optical system, measure the reflectance of its charge-coupled device (CCD) image sensor, and conduct stray light analysis via ray tracing. We estimate the ratio of the stray light intensity to the target signal intensity by deriving both the reflected (constituting stray light) and unreflected rays. We show that placing the filter away from the sensor and reducing the reflectances of the filter and lenses effectively reduce stray light. Such approaches are incorporated into the preliminary design of OROCHI. [ABSTRACT FROM AUTHOR]

The JAXA's Martian Moons Exploration (MMX) mission is planned to reveal the origin of Phobos and Deimos. It will remotely observe both moons and return a sample from Phobos. The nominal instruments include the TElescopic Nadir imager for GeOmOrphology (TENGOO) and Optical RadiOmeter composed of CHromatic Imagers (OROCHI). The scientific objective of TENGOO is to obtain the geomorphological features of Phobos and Deimos. The spatial resolution of TENGOO is 0.3 m at an altitude of 25 km in the quasi-satellite orbit. The scientific objective of OROCHI is to obtain material distribution using spectral mapping. OROCHI possesses seven wide-angle bandpass imagers without a filter wheel and one monochromatic imager dedicated to the observation during the landing phase. Using these two instruments, we plan to select landing sites and obtain information that supports the analysis of return samples. [ABSTRACT FROM AUTHOR]

The Hayabusa spacecraft was intentionally destroyed in the atmosphere at superorbital velocity at the conclusion of its asteroid sample return mission in June 2010. This study uses single-station ground-based video observations of the reentry to analyze the breakup of the spacecraft and estimate the trajectory of 80 individual spacecraft fragments. An extended Kaiman filter with batch initialization is used to estimate the position, velocity, and aerodynamic ballistic coefficients of the fragments. The breakup is characterized and compared with preflight predictions. A high area-to-mass object is seen early during the reentry, which matches closely with the predicted solar panel separation. Nearly all fragments have decreasing freestream dynamic pressure during their observed trajectories. Fragments with high drag ballistic coefficients are more likely to be observed early in the reentry. Assuming simple aluminum spheres, the estimated ballistic coefficients show that the fragments would have radius and mass on the order of centimeters and grams, respectively. The ablation coefficient is also calculated for several fragments using the estimated velocities and ballistic coefficients; results resemble natural meteors and agree with other studies of the Hayabusa reentry using different methods. [ABSTRACT FROM AUTHOR]

Abstract: The selective adsorption amounts of 2,6- and 2,7-dimethylnaphtalene (DMN) isomers dissolved in supercritical carbon dioxide (SC-CO2) on barium (Ba) ion-exchanged Y-type zeolite were measured by a constant volume method at 308.2K. The total concentrations of 2,6- and 2,7-DMN in SC-CO2 were measured with an ultraviolet detector. The concentration ratio of 2,6- and 2,7-DMN in SC-CO2 was analyzed by gas chromatography. The separation coefficients of 2,6-DMN (2) and 2,7-DMN (3) are defined by K 32 =(Q 3 /C 3)/(Q 2 /C 2), where Q 2 and Q 3 (mol/g-zeolite) are the amounts of 2,6- and 2,7-DMN adsorbed on Y-type zeolite and C 2 and C 3 (mol/cm3) are the concentrations of 2,6- and 2,7-DMN in SC-CO2, respectively. The separation coefficients in SC-CO2 on Ba ion-exchanged Y-type zeolite are much higher than those in liquid octane. The separation coefficients in SC-CO2 using Ba ion-exchanged Y-type zeolite depend on ion exchange rate and amount of chloride remained on the zeolite. The separation coefficients at high pressure using Ba ion-exchanged Y-type zeolite which is higher ion exchange rate and contains lower chloride in SC-CO2 system are much higher than those using NaY-type zeolite. Therefore, the use of ion-exchanged zeolite will be useful in applications such as the design of pressure swing adsorption and supercritical chromatography processes. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Japanese asteroid explorer Hayabusa2 was launched at the end of 2014. Hayabusa2 is supposed to observe the near-Earth C-type asteroid 162173 Ryugu (1999 JU3) and bring surface material samples back to Earth in 2020. It is equipped with Light Detection and Ranging (LIDAR) instrument for laser ranging which can be used to measure the intensities of transmitted and received pulses. The intensity data can be used to estimate the normal albedo of Ryugu at a laser wavelength of 1.064 μm. To perform this estimation, we determined the transfer functions of the laser module and receiver to convert the intensity data into pulse energies, along with the utilization ratio of the returned pulse energy, through verification tests of the LIDAR flight model. Then, we evaluated the error of the normal albedo. This error is affected not only by the performance of the LIDAR but also by the slope and roughness of the asteroid's surface. In this paper, we focus on the error in the normal albedo due only to the instrument error, which will be 18.0 % in an observation at a nominal altitude of 20 km. [ABSTRACT FROM AUTHOR]

In this study, 1 keV DT + ion irradiation was performed on tungsten pre-irradiated with helium. The thermal desorption behavior and the reduction of tritium retention during vacuum preservation at room temperature, as well as isochronal annealing were investigated using an IP technique, taking advantage of the fact that tritium detection is nondestructive and is highly sensitive. At a pre-irradiated helium fluence of 1 × 10 17 He/cm 2 , retained tritium tended to be desorbed at higher temperatures when compared to no helium irradiation case. Tritium retention during preservation in vacuum and during isochronal annealing became smaller with increasing helium fluence up to 1 × 10 17 He/cm 2 . At a helium fluence of 1 × 10 18 He/cm 2 , the reduction of tritium retention was found to be greater compared to 1 × 10 17 He/cm 2 . The results indicate that helium irradiation has a significant influence not only on the thermal desorption temperature of tritium but on longtime tritium reduction at room and elevated temperatures. [ABSTRACT FROM AUTHOR]

Abstract: We report on our search for genetically related asteroids amongst the near-Earth object (NEO) population—families of NEOs akin to the well known main belt asteroid families. We used the technique proposed by Fu et al. (Fu, H., Jedicke, R., Durda, D.D., Fevig, R. Binzel, R.P. [2005]. Icarus 178(2), 434–449) supplemented with a detailed analysis of the statistical significance of the detected clusters. Their significance was assessed by comparison to identical searches performed on 1000 ‘fuzzy-real’ NEO orbit distribution models that we developed for this purpose. The family-free ‘fuzzy-real’ NEO models maintain both the micro and macro distribution of five orbital elements (ignoring the mean anomaly). Three clusters were identified that contain four or more NEOs but none of them are statistically significant at . The most statistically significant cluster at the level contains four objects with and all members have long observational arcs and concomitant good orbital elements. Despite the low statistical significance we performed several other tests on the cluster to determine if it is likely a genetic family. The tests included examining the cluster’s taxonomy, size–frequency distribution, consistency with a family-forming event during tidal disruption in a close approach to Mars, and whether it is detectable in a proper element cluster search. None of these tests exclude the possibility that the cluster is a family but neither do they confirm the hypothesis. We conclude that we have not identified any NEO families. [Copyright &y& Elsevier]

Abstract: We present lightcurve observations and multiband photometry for 107P/Wilson–Harrington using five small- and medium-sized telescopes. The lightcurve has shown a periodicity of 0.2979day (7.15h) and 0.0993day (2.38h), which has a commensurability of 3:1. The physical properties of the lightcurve indicate two models: (1) 107P/Wilson–Harrington is a tumbling object with a sidereal rotation period of 0.2979day and a precession period of 0.0993day. The shape has a long axis mode (LAM) of L 1:L 2:L 3 =1.0:1.0:1.6. The direction of the total rotational angular momentum is around λ =310°, β =−10°, or λ =132°, β =−17°. The nutation angle is approximately constant at 65°. (2) 107P/Wilson–Harrington is not a tumbler. The sidereal rotation period is 0.2979day. The shape is nearly spherical but slightly hexagonal with a short axis mode (SAM) of L 1:L 2:L 3 =1.5:1.5:1.0. The pole orientation is around λ =330°, β =−27°. In addition, the model includes the possibility of binary hosting. For both models, the sense of rotation is retrograde. Furthermore, multiband photometry indicates that the taxonomy class of 107P/Wilson–Harrington is C-type. No clear rotational color variations are confirmed on the surface. [Copyright &y& Elsevier]

Abstract: We have performed a simulation of a next generation sky survey’s (Pan-STARRS 1) efficiency for detecting Earth-impacting asteroids. The steady-state sky-plane distribution of the impactors long before impact is concentrated towards small solar elongations (Chesley, S.R., Spahr T.B., 2004. In: Belton, M.J.S., Morgan, T.H., Samarashinha, N.H., Yeomans, D.K. (Eds.), Mitigation of Hazardous Comets and Asteroids. Cambridge University Press, Cambridge, pp. 22–37) but we find that there is interesting and potentially exploitable behavior in the sky-plane distribution in the months leading up to impact. The next generation surveys will find most of the dangerous impactors (>140m diameter) during their decade-long survey missions though there is the potential to miss difficult objects with long synodic periods appearing in the direction of the Sun, as well as objects with long orbital periods that spend much of their time far from the Sun and Earth. A space-based platform that can observe close to the Sun may be needed to identify many of the potential impactors that spend much of their time interior to the Earth’s orbit. The next generation surveys have a good chance of imaging a bolide like 2008 TC3 before it enters the atmosphere but the difficulty will lie in obtaining enough images in advance of impact to allow an accurate pre-impact orbit to be computed. [Copyright &y& Elsevier]

Abstract: The surface topography of Asteroid 25143 Itokawa is explored using the LIght Detection And Ranging instrument (LIDAR). The data confirm the presence of a rough highland and a smooth lowland. The highland is dominated by boulders, but also possesses topography associated with surface lineaments and broad surface facets. The boulders ensure that the roughness of the highlands over short distances is typically greater relative to most surfaces on 433 Eros. Over larger distances, Itokawa is always smoother than Eros possibly because of its smaller size and weak rubble-pile structure. The lowlands of Itokawa are very smooth, and are typically devoid of boulders. Some transitional regions midway between the highlands and lowlands also exist. In these areas, craters that retain their regolith fill possess flat floors and resemble “ponds” seen on 433 Eros. Analyses of surface elevation, imagery and a quantitative measure of surface roughness are consistent with regolith flowing downhill from the highlands to fill in the low areas of Itokawa, probably covering up any pre-existing rough terrain. Using this interpretation, we find a minimum thick layer of regolith in the lowlands, which, if spread evenly across the entire asteroid, corresponds to a thick layer. It is very difficult to generate this amount of regolith with the population of craters seen on Itokawa. However, an Itokawa composed of several large masses may have retained this regolith during its formation. The presence of such large masses could account for the observed lineaments and what appear to be exposures of bedrock on the largest steep slope observed. [Copyright &y& Elsevier]

Abstract: A photometric analysis of the S-type Asteroid 25143 Itokawa is performed over multiple wavelengths ranging from 0.85 to 2.10 μm based on disk-resolved reflectance spectra obtained with the Hayabusa near-infrared spectrometer (NIRS). We derive the global photometric properties of Itokawa in terms of Hapke''s photometric model. We find that Itokawa has a single-scatter albedo that is 35–40% less than that of Asteroid 433 Eros. Itokawa also has a single-particle phase function that is more strongly back-scattering than that of Eros. Despite its hummocky surface strewn with large boulders, Itokawa exhibits an opposition effect. However, the total amplitude of the opposition surge for Itokawa was estimated to be less than unity while Eros and other S-type asteroids have been found to have model values exceeding unity. The wavelength dependence of the opposition surge width reveals that coherent backscatter contributes to the opposition effect on Itokawa''s surface. The photometric roughness of Itokawa is well constrained to a value of 26° ± 1° which is similar to Eros, suggesting that photometric roughness models the smallest surface roughness scale for which shadows exist. [Copyright &y& Elsevier]

High-resolution images of the surface of asteroid Itokawa from the Hayabusa mission reveal it to be covered with unconsolidated millimeter-sized and larger gravels. Locations and morphologic characteristics of this gravel indicate that Itokawa has experienced considerable vibrations, which have triggered global-scale granular processes in its dry, vacuum, microgravity environment. These processes likely include granular convection, landslide-like granular migrations, and particle sorting, resulting in the segregation of the fine gravers into areas of potential lows. Granular processes become major resurfacing processes because of Itokawa's small size, implying that they can occur on other small asteroids should those have regolith. [ABSTRACT FROM AUTHOR]

Asteroid 162,173 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. • Major craters on Ryugu have east–west asymmetric rims. • Effect of rotation assessed as the possible cause of the east–west asymmetry. • The effect creates the E -W asymmetry only if rotation period was less than 3.6 h. [ABSTRACT FROM AUTHOR]

Lunar impact flashes have been observed at collisions of meteoroids against the non-sunlit lunar surface at speeds exceeding 10 ​km ​s−1. We detected 13 flash candidates between 6.2 and 9.9 in R-magnitude on December 15, 2018 during the Geminids meteor activity. Two or three observatories confirmed eleven of them. We obtained their spectra in the wavelength range between 400 and 870 ​nm. They are continuous and red, with best-fitted single blackbody spectra indicating the temperatures of about 2000–4000 ​K. The temperatures for a few successive movie frames at 16 ​ms or 25 ​ms intervals decrease with time. Incandescent ejecta, consisting of melt droplets or dust, and the radiant floor of an impact crater could be the source of these flashes, except for the initial stages. At the beginning of some flashes, we found an excess of fluxes at short wavelengths of less than about 600 ​nm. The composites of two blackbody spectra may fit the spectra better where their temperatures are about 2000 K and 6000 ​K. The contribution of a high-temperature vapor plume, generated at the very beginnings of the impact phenomena, could be important. Image 1 • Visible and near-infrared spectra of lunar impact flashes were obtained for the first time. • The spectra of the lunar flashes due to the impacts of the Geminids' meteoroids are continuous and red. • Best-fitted single blackbody spectra show flash temperatures of about 2000–4000 ​K. • The composite of two blackbody spectra, where its high-temperature component would represent radiation from an impact-generated vapor plume, could explain the observed spectra better in the initial stage of the flashes. [ABSTRACT FROM AUTHOR]

Radar and optical simultaneous observations of meteors are important to understand the size distribution of the interplanetary dust. However, faint meteors detected by high power large aperture radar observations, which are typically as faint as 10 mag. In optical, have not been detected until recently in optical observations, mainly due to insufficient sensitivity of the optical observations. In this paper, two radar and optical simultaneous observations were organized. The first observation was carried out in 2009–2010 using Middle and Upper Atmosphere Radar (MU radar) and an image-intensified CCD camera. The second observation was carried out in 2018 using the MU radar and a mosaic CMOS camera, Tomo-e Gozen, mounted on the 1.05-m Kiso Schmidt Telescope. In total, 331 simultaneous meteors were detected. The relationship between radar cross sections and optical V -band magnitudes was well approximated by a linear function. A transformation function from the radar cross section to the V -band magnitude was derived for sporadic meteors. The transformation function was applied to about 150,000 meteors detected by the MU radar in 2009–2015, large part of which are sporadic, and a luminosity function was derived in the magnitude range of −1.5–9.5 mag. The luminosity function was well approximated by a single power-law function with the population index of r = 3.52 ± 0.12. The present observation indicates that the MU radar has capability to detect interplanetary dust of 10 − 5 – 10 0 g in mass as meteors. • In total, 331 meteors were detected simultaneously by radar and optically. • A correlation between the radar cross section and the optical magnitude is firmly confirmed. • The mass range of the meteor detected by MU radar is constrained to about 105−100 ​g. [ABSTRACT FROM AUTHOR]

We present time-series optical spectroscopic observations of 3200 Phaethon during its 2007 return, complemented with R -band photometry. The spectral gradient centered around 0.55 μm varies between − 5.0 % / 0.1 μm and 0.6 % / 0.1 μm, indicating a B- or C-type color. Moreover, rotational modulation of the spectral slope of this asteroid suggests its surface compositional inhomogeneity, particularly the existence of a C-type surface at a certain longitude on this asteroid. [ABSTRACT FROM AUTHOR]

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. • Hayabusa2 LIDAR data contribute to quick improvement of the spacecraft trajectory • With this trajectory, topography of Ryugu is delineated by scan observations. • Formation of Ryugu's western bulge may have been triggered by an oblique impact. [ABSTRACT FROM AUTHOR]

Puzzlingly, the parent bodies of ordinary chondrites (the most abundant type of meteorites) do not seem to be abundant among asteroids. One possible explanation is that surfaces of the parent bodies become optically altered, to become the S-type asteroids which are abundant in the main asteroid belt. The process is called ‘space weathering’—it makes the visible and near-infrared reflectance spectrum of a body darker and redder. A recent survey of small, near-Earth asteroids suggests that the surfaces of small S asteroids may have developing stages of space weathering. Here we report that a dark region on a small (550-metre) asteroid—25143 Itokawa—is significantly more space-weathered than a nearby bright region. Spectra of both regions are consistent with those of LL5-6 chondrites after continuum removal. A simple calculation suggests that the dark area has a shorter mean optical path length and about 0.04 per cent by volume more nanophase metallic iron particles than the bright area. This clearly shows that space-weathered materials accumulate on small asteroids, which are likely to be the parent bodies of LL chondrites. We conclude that, because LL meteorites are the least abundant of ordinary (H, L, and LL) chondrites, there must be many asteroids with ordinary-chondrite compositions in near-Earth orbits. [ABSTRACT FROM AUTHOR]

We investigated the magnitude-phase relation of (162173) 1999 JU3, a target asteroid for the JAXA Hayabusa 2 sample return mission. We initially employed the International Astronomical Union's H-G formalism but found that it fits less well using a single set of parameters. To improve the inadequate fit, we employed two photometric functions: the Shevchenko and Hapke functions. With the Shevchenko function, we found that the magnitude-phase relation exhibits linear behavior in a wide phase angle range (α = 5°-75°) and shows weak nonlinear opposition brightening at α < 5°, providing a more reliable absolute magnitude of HV = 19.25 ± 0.03. The phase slope (0.039 ± 0.001 mag deg–1) and opposition effect amplitude (parameterized by the ratio of intensity at α = 0.°3 to that at α = 5°, I(0.°3)/I(5°) = 1.31 ± 0.05) are consistent with those of typical C-type asteroids. We also attempted to determine the parameters for the Hapke model, which are applicable for constructing the surface reflectance map with the Hayabusa 2 onboard cameras. Although we could not constrain the full set of Hapke parameters, we obtained possible values, w = 0.041, g = –0.38, B0 = 1.43, and h = 0.050, assuming a surface roughness parameter = 20°. By combining our photometric study with a thermal model of the asteroid, we obtained a geometric albedo of pv = 0.047 ± 0.003, phase integral q = 0.32 ± 0.03, and Bond albedo AB = 0.014 ± 0.002, which are commensurate with the values for common C-type asteroids. [ABSTRACT FROM AUTHOR]