Asteroid Itokawa Research Articles

Mechanical properties of rubble pile asteroids (Dimorphos, Itokawa, Ryugu, and Bennu) through surface boulder morphological analysis

Planetary defense efforts rely on estimates of the mechanical properties of asteroids, which are difficult to constrain accurately from Earth. The mechanical properties of asteroid material are also important in the interpretation of the Double Asteroid Redirection Test (DART) impact. Here we perform a detailed morphological analysis of the surface boulders on Dimorphos using images, the primary data set available from the DART mission. We estimate the bulk angle of internal friction of the boulders to be 32.7 ± 2. 5° from our measurements of the roundness of the 34 best-resolved boulders ranging in size from 1.67–6.64 m. The elongated nature of the boulders around the DART impact site implies that they were likely formed through impact processing. Finally, we find striking similarities in the morphology of the boulders on Dimorphos with those on other rubble pile asteroids (Itokawa, Ryugu and Bennu). This leads to very similar internal friction angles across the four bodies and suggests that a common formation mechanism has shaped the boulders. Our results provide key inputs for understanding the DART impact and for improving our knowledge about the physical properties, the formation and the evolution of both near-Earth rubble-pile and binary asteroids.

Asteroid-NeRF: A deep-learning method for 3D surface reconstruction of asteroids

Context. Asteroids preserve important information about the origin and evolution of the Solar System. Three-dimensional (3D) surface models of asteroids are essential for exploration missions and scientific research. Regular methods for 3D surface reconstruction of asteroids, such as stereo-photogrammetry (SPG), usually struggle to reconstruct textureless areas and can only generate sparse surface models. Stereo-photoclinometry (SPC) can reconstruct pixel-wise topographic details but its performance depends on the availability of images obtained under different illumination conditions and suffers from uncertainties related to surface reflectance and albedo. Aims. This paper presents Asteroid-NeRF, a novel deep-learning method for 3D surface reconstruction of asteroids that is based on the state-of-the-art neural radiance field (NeRF) method. Methods. Asteroid-NeRF uses a signed distance field (SDF) to reconstruct a 3D surface model from multi-view posed images of an asteroid. In addition, Asteroid-NeRF incorporates appearance embedding to adapt to different illumination conditions and to maintain the geometric consistency of a reconstructed surface, allowing it to deal with the different solar angles and exposure conditions commonly seen in asteroid images. Moreover, Asteroid-NeRF incorporates multi-view photometric consistency to constrain the SDF, enabling optimised reconstruction. Results. Experimental evaluations using actual images of asteroids Itokawa and Bennu demonstrate the promising performance of Asteroid-NeRF, complementing traditional methods such as SPG and SPC. Furthermore, due to the global consistency and pixel-wise training of Asteroid-NeRF, it produces highly detailed surface reconstructions. Asteroid-NeRF offers a new and effective solution for high-resolution 3D surface reconstruction of asteroids that will aid future exploratory missions and scientific research on asteroids.

Autonomous Maneuver Planning for Small-Body Reconnaissance via Reinforcement Learning

This paper presents a reinforcement learning (RL) based approach for autonomous maneuver planning of low-altitude flybys for site-specific reconnaissance of small bodies. Combined with Monte Carlo tree search and deep neural networks, the proposed method generates optimal maneuvers, even under complex dynamics and abstractly science goals. Formulating the mission objective as an observability function, the RL issue can be framed in terms of a Markov decision process. The neural network, trained by a novel policy gradient algorithm with a clipped surrogate objective, learns both policy and value functions that map the action and state spaces to the expected long-term return. An adaptive refinement search technique is applied to further enhance the trained policy network, finding optimal maneuvers from the policy distributions. Experiment results on a simulated reconnaissance mission around asteroid Itokawa illustrate the efficiency and robustness of the proposed approach in achieving multitarget observation.

Chemical compositions of Fe‐rich relict olivines from cosmic spherules, understanding their links with ordinary and carbonaceous chondrites

AbstractFe‐rich relict olivine grains are found in a small percentage of cosmic spherules, which are studied here to determine the nature of their precursors. We examined 128 Fe‐rich relict olivine grains with Fa >10 mol% from 53 cosmic spherules of different types collected from Antarctica (Antarctica micrometeorites [AMM]) and deep‐sea sediments (DSS) of the Indian Ocean. Fe‐rich olivines identified in cosmic spherules are close analogs of type II chondrule olivines formed in the early solar system. The olivine analysis shows well‐defined trends in molar Fe/Mn versus Fe/Mg with an affinity for ordinary and carbonaceous chondrites. The minor oxides in olivine are in ranges such as MnO ~0.1–0.8 wt%, Cr2O3 ~0–0.7 wt%, CaO ~0–0.6 wt%, and Al2O3 ~0–0.2 wt%, respectively. The chemical composition suggests that the precursors for these Fe‐rich olivine‐bearing cosmic spherules consist of ordinary chondrites (~21%–23%, AMM‐DSS), carbonaceous chondrites (~17%–36%, AMM‐DSS), and a large fraction overlapping both carbonaceous and ordinary chondrites (~41%–62% AMM‐DSS). The elemental ratios Fe/Si/CI and Mg/Si/CI for the Fe‐rich relict olivines ranging between the values 0.5–1.0 and 1.1–1.7 are compatible with IDPs, Comet 81P/Wild 2 as well as the Asteroid Itokawa and Ryugu, which are indistinguishable from carbonaceous and ordinary chondrites. In addition, pyroxene and olivine assemblages in their Fa versus Fs mol% show strong similarities to EOC chondrites. Our results on Fe‐rich relict olivines show that these grains in cosmic spherules are less common than Mg‐rich olivines, which show a narrow range of chemical compositions identical to those from ordinary chondrites and carbonaceous chondrites, indicating a supplementary contribution of an ordinary chondritic component to the micrometeorite source of dust.

The Iris CubeSat mission: Science payload description for a pathfinder geological space weathering investigation

The Iris mission is a student-led 3-U CubeSat comprised of a science payload of 20 geological samples placed in low-Earth orbit (LEO) to study the effects of space weathering (i.e., those aspects that can be investigated in LEO). This experiment is the first of its kind to take place in the vacuum of space. The science objective of the mission is to investigate how space weathering processes affect the surfaces of airless planetary bodies by monitoring the spectral reflectance properties of geological samples over time. Two three-color cameras acquire spectral reflectance data within the visible spectrum to measure any spectral and/or textural changes exhibited throughout the ∼1-year orbital mission. This mission will provide valuable data to help understand how the space environment may affect the surfaces of the Moon, Mercury, and main-belt and near-Earth asteroids. The acquired data will help constrain meteorite-asteroid links and provide insight into the spectroscopic analysis of samples returned from the Moon, and near-Earth asteroids Itokawa, Bennu, and Ryugu. The results of this mission will also complement the results of laboratory simulations of space weathering. All samples selected for flight on the CubeSat have undergone several tests to ensure suitability for launch and operations. This paper describes the payload subsystem, sample selection and preparation.

Deflecting asteroid Itokawa by subsurface explosion: Effect of explosion position on asteroid angular velocity change

Explosion deflection has recently become an important topic in hazardous asteroid deflection. Changes in asteroid velocity and angular velocity, as well as the degree of structural damage, should be fully considered in the explosion deflection scheme. This study investigates the effect of explosion position on the deflection of the asteroid 25143 Itokawa. First, we propose a mathematical model to quickly determine the changes in the angular velocity of Itokawa with explosions at different positions, and verified its reliability through numerical simulations. Mathematical model indicated a maximum 349-fold difference in Itokawa's angular velocity change under explosions at different positions, and revealed that the decisive factor is the force arm (maximum 850-fold difference), not the moment of inertia (maximum 4.01-fold difference). Then, to demonstrate the effect of porosity on shock wave propagation, we established a mesoscopic model based on the porosities of Itokawa's head and body. We used the single-material arbitrary Lagrangian-Eulerian (ALE) and finite element-smoothed particle hydrodynamics (FE-SPH) adaptive methods to simulate the deflection of the asteroid with a 300-kt TNT-equivalent explosion at different positions 10 m underground. The simulated changes in asteroid velocity and angular velocity were consistent with the predictions of the mathematical model. Finally, we investigated the effects of the asteroid geometry and material composition on the shock wave propagation and asteroid damage evolution (material response and structural response).

Mixed Fidelity Shape Models for Efficient Small Body Gravity Modeling

The polyhedral gravity model is the de facto standard for computing the gravity field near the surface of small bodies due to their irregular shapes. However, the computational load to compute the gravitational acceleration can increase significantly for high-resolution shape models. In this work, an algorithm for creating mixed-resolution shapes is developed with the goal of getting similar accuracy in the gravitational acceleration while reducing the computational cost. This method is tested for full integration into low-altitude trajectory simulations. The results show that blended models can provide comparable accuracy while improving the computation performance. Additionally, this approach does not require any fitting or estimation process as it only involves creating shape model connections, which is done in a deterministic manner. Test cases with asteroids Bennu and Itokawa demonstrate that the errors in the gravitational accelerations computed with the hybrid models are comparable to expected uncertainties from unknown heterogeneous mass distributions. For precomputed hybrid shape model strategies, we see savings in computational time on the order of 50% compared to using the high-resolution shape model, while getting much lower propagation errors than using only a low-resolution model alone.

Space weathering acts strongly on the uppermost surface of Ryugu

Returned samples from Cb-type asteroid (162173) Ryugu exhibit very dark spectra in visible and near-infrared ranges, generally consistent with the Hayabusa2 observations. A critical difference is that a structural water absorption of hydrous silicates is around twice as deep in the returned samples compared with those of Ryugu’s surface, suggesting Ryugu surface is more dehydrated. Here we use laboratory experiments data to indicate the spectral differences between returned samples and asteroid surface are best explained if Ryugu surface has (1) higher porosity, (2) larger particle size, and (3) more space-weathered condition, with the last being the most effective. On Ryugu, space weathering by micrometeoroid bombardments promoting dehydration seem to be more effective than that by solar-wind implantation. Extremely homogeneous spectra of the Ryugu’s global surface is in contrast with the heterogeneous S-type asteroid (25143) Itokawa’s spectra, which suggests space weathering has proceeded more rapidly on Cb-type asteroids than S-type asteroids.

Hydrothermal fluid activity on asteroid Itokawa

Hydrothermal fluid activity on asteroid Itokawa

Hydrogen in acapulcoites and lodranites: A unique source of water for planetesimals in the inner Solar System

Acapulcoites and lodranites are primitive achondrites, sampling a common planetesimal formed a few million years after calcium-aluminium rich inclusions in the inner Solar System, that provides information into melting and differentiation processes in the early inner Solar System. The chemistry and mineralogy of their chondritic parent body lies in between enstatite and ordinary chondrites. As they record a range in planetary differentiation degree, from 1% up to 20% partial melting, with lodranites experiencing the most melt extraction, we investigate (i) the behaviour of hydrogen in terms of abundance and isotopic composition during the early stages of planetary differentiation and (ii) the source(s) of hydrogen for the acapulcoite-lodranite parent body in order to place it in our current understanding of the source(s) of water in the inner Solar System. In this study, we analysed water content and hydrogen isotopic composition of phosphates and nominally anhydrous minerals in a range of acapulcoite and lodranite meteorite samples. While apatites seemed to have recorded a degassing signature, no such variations were observed in the H2O–δD systematics of the nominally anhydrous minerals suggesting that subsequent to their crystallisation, acapulcoites and lodranites experienced minimal modifications to their volatile composition during thermal metamorphism and partial melting. The low abundance of water in acapulcoite and lodranite nominally anhydrous minerals (i.e., average 5.2±1.6 μg/g H2O) suggests that their parent body was much drier than what has been estimated for enstatite and ordinary chondrite parent bodies. We estimated a bulk water content for the acapulcoite-lodranite parent body of 3 to 19 μg/g H2O, similar to the ureilite parent body. The hydrogen isotopic composition of nominally anhydrous minerals in acapulcoites and lodranites (–211±145‰), and in particular for the two falls Acapulco and Lodran (–239±149‰), matches with the hydrogen isotopic composition recorded by nominally anhydrous minerals in ordinary chondrite falls, eucrites, S–type asteroid Itokawa, consistent with a common source of water for the inner Solar System planetesimals, isotopically distinctive to bulk carbonaceous chondrites.

Relativistic Effects in the Rotation of Dwarf Planets and Asteroids

Abstract The effect of the geodetic rotation (which includes two relativistic effects: geodetic precession and geodetic nutation) is the most significant relativistic effect in the rotation of the celestial bodies. For the first time in this research, this relativistic effect is determined in the rotation of dwarf planets (Ceres, Pluto, and Charon) and asteroids (Pallas, Vesta, Lutetia, Europa, Ida, Eros, Davida, Gaspra, Steins, and Itokawa) in the Solar System with known values of their rotation parameters. Calculations of the values of their geodetic rotation are made by a method for studying any bodies in the Solar System with a long-term ephemeris. Values of geodetic precession and geodetic nutation for all these celestial bodies were calculated in ecliptic Euler angles relative to their proper coordinate systems and in their rotational elements relative to the fixed equator of the Earth and the vernal equinox (at the epoch J2000.0). The obtained analytical values of the geodetic rotation for the celestial bodies can be used to numerically investigate their rotation in the relativistic approximation, and also used to estimate the influence of relativistic effects on the orbital–rotational dynamics for the bodies of exoplanetary systems.

The influence of interparticle cohesion on rebounding slow impacts on rubble pile asteroids

The ballistic sorting effect has been proposed to be a driver behind the observed size sorting on the rubble pile asteroid Itokawa. This effect depends on the inelasticity of slow collisions with granular materials. The inelasticity of a collision with a granular material, in turn, depends on grain size. Here we argue that determining the inelasticity of such collisions in an asteroid-like environment is a nontrivial task. We show non-monotonic dependency of the coefficient of restitution (COR) on target particle size using experiments in microgravity. Employing numerical simulations, we explain these results with the growing influence of adhesion for smaller-sized particles. We conclude that there exists an optimum impactor to target particle size ratio for ballistic sorting.

Boulder exhumation and segregation by impacts on rubble-pile asteroids

Small asteroids are often considered to be rubble-pile objects, and such asteroids may be the most likely type of Near Earth Objects (NEOs) to pose a threat to Earth. However, impact cratering on such bodies is complex and not yet understood. We perform three low-velocity (≈ 400 m/s) impact experiments in granular targets with and without projectile-size boulders. We conducted SPH simulations that closely reproduced the impact experiments.Our results suggest that cratering on heterogeneous targets displaces and ejects boulders, rather than fragmenting them, unless directly hit. We also see indications that as long as the energy required to disrupt the boulder is small compared to the kinetic energy of the impact, the disruption of boulders directly hit by the projectile may have minimal effect on the crater size.The presence of boulders within the target causes ejecta curtains with higher ejection angles compared to homogeneous targets. At the same time, there is a segregation of the fine ejecta from the boulders, resulting in boulders landing at larger distances than the surrounding fine grained material. However, boulders located in the target near the maximum extent of the expanding excavation cavity are merely exhumed and distributed radially around the crater rim, forming ring patterns similar to the ones observed on asteroids Itokawa, Ryugu and Bennu. Altogether, on rubble-pile asteroids this process will redistribute boulders and finer-grained material heterogeneously, both areally around the crater and vertically in the regolith. In the context of a kinetic impactor on a rubble-pile asteroid and the DART mission, our results indicate that the presence of boulders will reduce the momentum transfer compared to a homogeneous, fine-grained target.

Pulse Broadening Effects on Ranging Performance of a Laser Altimeter with Return-to-Zero Pseudorandom Noise Code Modulation.

A laser altimeter using code modulation techniques receives a backscattered pulse wider than the transmitted rectangular pulse when scanning a rough or sloped target surface. This leads to degrading the ranging performance in terms of signal-to-noise ratio (SNR) and detection probability. Unlike the pulsed techniques, little work has focused on the pulse broadening effect of the code modulation techniques. In this study, mathematical models were derived to investigate the pulse broadening effect on the ranging performance of a return-to-zero pseudorandom noise (RZPN) laser altimeter. Considering that the impulse response can be approximated by a Gaussian function, the analytical waveform was derived using a new flat-topped multi-Gaussian beam (FMGB) model. The closed-form expressions were also analytically derived for a peak cross-correlation, SNR, and detection probability in terms of the pulse broadening effect. With the use of a three-dimensional model of asteroid Itokawa for practical surface profiles, the analytical expressions were validated by comparing to the results obtained from numerical simulations. It was also demonstrated that the pulse broadening effect dropped down the peak cross-correlation and then deteriorated the ranging performance. These analytical expressions will play an important role in not only designing a laser altimeter using the RZPN code modulation technique but also analyzing its ranging performance.

Measuring the shock stage of Itokawa and asteroid regolith grains by electron backscattered diffraction, optical petrography, and synchrotron X‐ray diffraction

AbstractOur goal was to devise a bridge between shock determinations of asteroid regolith grains by standard light optical petrography, synchrotron X‐ray diffraction (SXRD), and electron backscattered diffraction (EBSD). We determined the optimal conditions under which to measure the shock stage of olivine crystals in astromaterial grains by EBSD. We applied this EBSD procedure to the shock stage determination of four regolith grains from asteroid Itokawa, returned to earth by the Hayabusa spacecraft. Interpretation of these data required a parallel examination of three ordinary chondrite standards that exhibited shock histories ranging from stage 2 to stage 4, using all three techniques. Standard light optical petrography indicated shock stage of S2/3 for the 24 Itokawa grains analyzed. SXRD results for seven Itokawa grains indicate a shock stage of S2. EBSD maps of four Itokawa grains indicate shock stage S3. Thus, the different techniques indicate slightly different shock stages, probably due to small sampling populations for EBSD and SXRD. We therefore recommend that significantly more than seven regolith grains should be separately analyzed by any shock determination technique, probably between 10 and 20. In any case, Itokawa regolith grains have been shocked to stage S2/3, or approximately 5–10 GPa. Finally, we investigated the crystallinity of one Itokawa olivine by SXRD, determining that the 5–10 GPa shock it had experienced did not appreciably alter the size of the unit cell, contrary to some previous suggestions.

Non‐protein amino acids identified in carbon‐rich Hayabusa particles

AbstractAmino acid abundances in acid‐hydrolyzed hot water extracts of gold foils containing five Category 3 (carbon‐rich) Hayabusa particles were studied using liquid chromatography with tandem fluorescence and accurate mass detection. Initial particle analyses using field emission scanning electron microscopy with energy‐dispersive X‐ray spectrometry indicated that the particles were composed mainly of carbon. Prior to amino acid analysis, infrared and Raman microspectroscopy showed some grains possessed primitive organic carbon. Although trace terrestrial contamination, namely l‐protein amino acids, was observed in all Hayabusa extracts, several terrestrially uncommon non‐protein amino acids were also identified. Some Hayabusa particles contained racemic (d≈l) mixtures of the non‐protein amino acids β‐aminoisobutyric acid (β‐AIB) and β‐amino‐n‐butyric acid (β‐ABA) at low abundances ranging from 0.09 to 0.31 nmol g−1. Larger abundances of the non‐protein amino acid β‐alanine (9.2 nmol g−1, ≈4.5 times greater than background levels) were measured in an extract of three Hayabusa particles. This β‐alanine abundance was ≈6 times higher than that measured in an extract of a CM2 Murchison grain processed in parallel. The comparatively high β‐alanine abundance is surprising as asteroid Itokawa is similar to amino acid‐poor LL ordinary chondrites. Elevated β‐alanine abundances and racemic β‐AIB and β‐ABA in Hayabusa grains suggested these compounds have non‐biological and plausibly non‐terrestrial origins. These results are the first evidence of plausibly extraterrestrial amino acids in asteroid material from a sample‐return mission and demonstrate the capabilities of the analytical protocols used to study asteroid Ryugu and Bennu samples returned by the JAXA Hayabusa2 and NASA OSIRIS‐REx missions, respectively.

Space weathering signatures in sulfide and silicate minerals from asteroid Itokawa

Transmission electron microscopy analyses of the polymineralic regolith particle RC-MD01-0025 show microstructural and microchemical characteristics indicative of space weathering on the surface of asteroid Itokawa. The depletion of sulfur and nickel was identified in space weathered rims on troilite and pentlandite minerals. This corresponds to the first report of nickel depletion in samples returned from asteroid Itokawa by the Hayabusa mission. Microstructurally, the sulfide minerals present crystalline rims and the olivine presents both crystalline and amorphous zones in the rim. These results suggest that sulfides might be more resistant to amorphization caused by solar wind irradiation. The space weathering features identified in the regolith particle analyzed here are likely formed via solar wind irradiation. Additionally, the differences in the space weathering features in olivine, pentlandite, and troilite suggest that silicates and sulfides respond differently to the same space weathering conditions in interplanetary space.Graphical

Solar wind contributions to Earth’s oceans

The isotopic composition of water in Earth’s oceans is challenging to recreate using a plausible mixture of known extraterrestrial sources such as asteroids—an additional isotopically light reservoir is required. The Sun’s solar wind could provide an answer to balance Earth’s water budget. We used atom probe tomography to directly observe an average ~1 mol% enrichment in water and hydroxyls in the solar-wind-irradiated rim of an olivine grain from the S-type asteroid Itokawa. We also experimentally confirm that H+ irradiation of silicate mineral surfaces produces water molecules. These results suggest that the Itokawa regolith could contain ~20 l m−3 of solar-wind-derived water and that such water reservoirs are probably ubiquitous on airless worlds throughout our Galaxy. The production of this isotopically light water reservoir by solar wind implantation into fine-grained silicates may have been a particularly important process in the early Solar System, potentially providing a means to recreate Earth’s current water isotope ratios. Water and hydroxyl enrichment in the solar-wind-irradiated rim of an olivine grain from asteroid Itokawa suggests that its regolith could contain ~20 l m−3 of water from solar wind—a potential water source for airless planetary bodies.

Coordinated analyses on space weathering signatures on a Fe-sulfide grain from asteroid Itokawa

Coordinated analyses on space weathering signatures on a Fe-sulfide grain from asteroid Itokawa

Geologic History and Crater Morphology of Asteroid (162173) Ryugu

AbstractCrater morphology and surface age of asteroid (162173) Ryugu are characterized using the high‐resolution images obtained by the Hayabusa2 spacecraft. Our observations reveal that the abundant boulders on and under the surface of the rubble‐pile asteroid affect crater morphology. Most of the craters on Ryugu exhibit well‐defined circular depressions, unlike those observed on asteroid Itokawa. The craters are typically outlined by boulders remaining on the rim. Large craters (diameter > 100 m) host abundant and sometimes unproportionally large boulders on their floors. Small craters (<20 m) are characterized by smooth circular floors distinguishable from the boulder‐rich exterior. Such small craters tend to have dark centers of unclear origin. The correlation between crater size and boulder number density suggests that some processes sort the size of boulders in the shallow (<30 m) subsurface. Furthermore, the crater size‐frequency distributions (CSFDs) of different regions on Ryugu record multiple geologic events, revealing the diverse geologic history on this 1‐km asteroid. Our crater‐counting analyses indicate that the equatorial ridge is the oldest structure of Ryugu and was formed 23–30 Myr ago. Then, Ryugu was partially resurfaced, possibly by the impact that formed the Urashima crater 5–12 Myr ago. Subsequently, a large‐scale resurfacing event formed the western bulge and the fossae 2–9 Myr ago. Following this process, the spin of Ryugu slowed down plausibly due to the YORP effect. The transition of isochrons in a CSFD suggests that Ryugu was decoupled from the main belt and transferred to a near‐Earth orbit 0.2–7 Myr ago.