Beth E. Clark

The landing of the NEAR-Shoemaker spacecraft on asteroid 433 Eros.

The NEAR-Shoemaker spacecraft was designed to provide a comprehensive characterization of the S-type asteroid 433 Eros (refs 1,2,3), an irregularly shaped body with approximate dimensions of 34 × 13 × 13 km. Following the completion of its year-long investigation, the mission was terminated with a controlled descent to its surface, in order to provide extremely high resolution images. Here we report the results of the descent on 12 February 2001, during which 70 images were obtained. The landing area is marked by a paucity of small craters and an abundance of ‘ejecta blocks’. The properties and distribution of ejecta blocks are discussed in a companion paper. The last sequence of images reveals a transition from the blocky surface to a smooth area, which we interpret as a ‘pond’. Properties of the ‘ponds’ are discussed in a second companion paper. The closest image, from an altitude of 129 m, shows the interior of a 100-m-diameter crater at 1-cm resolution.

Near-Infrared Spectral Results of Asteroid Itokawa from the Hayabusa Spacecraft

The near-infrared spectrometer on board the Japanese Hayabusa spacecraft found a variation of more than 10% in albedo and absorption band depth in the surface reflectance of asteroid 25143 Itokawa. Spectral shape over the 1-micrometer absorption band indicates that the surface of this body has an olivine-rich mineral assemblage potentially similar to that of LL5 or LL6 chondrites. Diversity in the physical condition of Itokawas surface appears to be larger than for other S-type asteroids previously explored by spacecraft, such as 433 Eros.

Evidence for non-synchronous rotation of Europa

Non-synchronous rotation of Europa was predicted on theoretical grounds, by considering the orbitally averaged torque exerted by Jupiter on the satellites tidal bulges. If Europas orbit were circular, or the satellite were comprised of a frictionless fluid without tidal dissipation, this torque would average to zero. However, Europa has a small forced eccentricity e ≈ 0.01 (ref. 2), generated by its dynamical interaction with Io and Ganymede, which should cause the equilibrium spin rate of the satellite to be slightly faster than synchronous. Recent gravity data suggest that there may be a permanent asymmetry in Europas interior mass distribution which is large enough to offset the tidal torque; hence, if non-synchronous rotation is observed, the surface is probably decoupled from the interior by a subsurface layer of liquid or ductile ice. Non-synchronous rotation was invoked to explain Europas global system of lineaments and an equatorial region of rifting seen in Voyager images,. Here we report an analysis of the orientation and distribution of these surface features, based on initial observations made by the Galileo spacecraft. We find evidence that Europa spins faster than the synchronous rate (or did so in the past), consistent with the possibility of a global subsurface ocean.

Developing space weathering on the asteroid 25143 Itokawa

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.

Spectroscopy of B-type asteroids: Subgroups and meteorite analogs

[1] B-type asteroids have a negative slope from -0.5 to ∼1.1 μm and beyond. What causes this? Visible to near-infrared reflectance spectra (0.4-2.5 μm) are assembled for 22 B-type asteroids. The spectra fall naturally into three groups: (1) those with negative (blue) spectral shapes like 2 Pallas (7 objects), (2) those with concave curve shapes like 24 Themis (11 objects), and (3) everything else (4 objects). The asteroid spectra are compared to mineral and meteorite spectra from the Reflectance Experiment Laboratory library of 15,000 samples, in a least squares search for particulate analogs, constrained by spectral brightness. The Pallas group objects show a trend of analogs from the CV, CO, and CK meteorite groups. Only three of the seven Pallas-like objects are determined to be dynamically related (2, 1508, and 6411). The Themis group objects show a trend of analogs from the CI, CM, CR, CI-Unusual, and CM-Unusual meteorites (as expected from the work of Hiroi et al. (1996)). Seven of the 11 Themis-like objects are dynamically related (24, 62, 222, 316, 379, 383, and 431). Allowing for reasonable uncertainties in the spectral matches, we find no need to invoke mineralogies that do not exist in the meteorite collection to explain B-type spectra or their negative slopes. Our Themis group results are as expected and are consistent with previous work, but our Pallas group results are new and, in some cases, in conflict with previous work.

Spectral mixing models of S‐type asteroids

We present the results of mixing model determinations of S-type asteroid mineralogies using the Hapke spectral theory. The goal is to quantify uncertainties resulting from poorly constrained model parameters and to produce sets of model compositions that we can analyze for trends that can be related to what is already known about S asteroids and the meteorites proposed to be derived from them. We present two different approaches to the selection of the albedo input parameter and show the resulting differences in model compositions. Similar grain sizes and composition patterns in terms of metal/silicate and olivine/pyroxene ratios were determined regardless of albedo approach. Encouraged by this agreement, we suggest that the two resulting sets of model compositions can be used to set limits on end-member abundances. Residual errors from model fits are comparable to those found by other compositional modeling attempts and, furthermore, tend to be located in the same spectral regions. These regions are (1) 0.4–0.7 μm, where the silicate and metal component UV drop-off in the asteroid spectra is too steep to be modeled with the current end-members, and (2) 1.5–2.5 μm, where the IR continuum turnover of the asteroid spectra is too flat to be fit with metallic spectrally red components included to match the IR continuum from 0.7 to 1.5 μm. We believe these systematic problems to be compositional information, as yet uninvestigated. More information is needed regarding the grain size conditions and the nature of opaque components present at asteroid surfaces before highly accurate compositional modeling is possible. The technique presented here is thus most useful in conjunction with other spectral deconvolution methods.

OSIRIS-REx: Sample Return from Asteroid (101955) Bennu

In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.

Evidence for the effects of space weathering spectral signatures on low albedo asteroids

Context. Space weathering changes asteroid surfaces. We know from observations of the Moon and high-albedo asteroids that interplanetary surface processes can alter the spectral properties of silicates. The next step is to extend the study to primitive asteroids. This work supports the OSIRIS-REx mission by providing predictions for what space weathering effects we can expect to find on the mission target: asteroid (101955) 1999 RQ36. Aims. To investigate the possible spectral signatures of surface processes on carbonaceous (low-albedo) asteroids, we study the reflected light spectra of carbonaceous meteorites (assumed to represent asteroid subsurface materials) and compare them with telescopic reflected light spectra of asteroids (assumed to represent asteroid surface materials). Methods. In this work, we assume that primitive C-complex asteroids are the parent bodies of carbonaceous chondrites. We reason that differences between spectra of particulate samples of the meteorites and spectra of the regolith of asteroids can be due to either differences in textural properties, or differences caused by surface processes on the asteroid. We use telescopic observations of Ch/Cgh-type asteroids (0.4 to 2.4 μm) and compare them statistically with 106 CM meteorite spectra from RELAB. Results. Our results indicate spectral blueing of asteroids, with little to no concurrent albedo change or band modification.

A spectral analysis of ordinary chondrites, S-type asteroids, and their component minerals: genetic implications

Three salient features of visible and infrared reflectance spectra of ordinary chondrites (OCs) and S-type asteroids are (1) albedo at 0.56 μm, (2) continuum slope, and (3) depth of the electronic absorption band due to octahedrally coordinated Fe2+ in olivine and pyroxene. These quantities were numerically extracted from the spectra of 23 OCs representing all metamorphic grades and 39 S-type asteroids to be plotted in a three-dimensional coordinate system. The spectral characteristics of three OCs which were comminuted, melted, recrystallized, and recomminuted are also presented in the same format. The results show that although laboratory simulation of melt alteration in an asteroidal regolith does alter OC spectra, the spectral parameters of these altered meteorites do not change enough to leave the parametric region defined by “unaltered” OC spectra. When the region containing the 39 S-asteroid spectra is compared with that of the altered and unaltered OCs, it is found that not one of the OCs falls within the S-asteroid region. The differences may be largely attributed to spectral differences between the respective metallic components. The range of S-asteroid parameters is then compared with potential pure “end-member” components most likely to result from magmatic differentiation of a chondritic protoasteroid: olivine, orthopyroxene, clinopyroxene, and Fe,Ni meteorite metal (alternatively represented by the M-asteroids). It is found that the S-asteroid array is consistent with random mixtures of the differentiated components except for a notable dominance of the spectral characteristics of the opaque (metallic) component. These results suggest that the M-asteroids may form a composition continuum with the S-asteroids. We discuss a scenario consistent with this analysis and with the Bell et al. theory of the geological structure of the asteroid belt. Ordinary chondritic protoasteroids of all sizes were probably the dominant primary condensates in the inner portion of the main asteroid belt. These were later heated by electromagnetic induction or by 26Al nuclide decay. As a result, the smaller ones were subjected to various degrees of metamorphism, while the larger ones were subjected to large-scale magmatic differentiation. Petrological domains of the S-asteroids (beneath a mixed regolith) may be large and supply achondrites, irons, and stony irons to Earth rather than well-mixed breccias of these components. The (smaller) OC protoasteroids may still be abundant in the asteroid belt but, if small enough to escape differentiation, may also be small enough to escape Earth-based identification.

Radar observations of Asteroids 64 Angelina and 69 Hesperia

Context. In the past decade, more than one hundred asteroid models wer e derived using the lightcurve inversion method. Measured by the number of derived models, lightcurve inversion has be come the leading method for asteroid shape determination. Aims. Tens of thousands of sparse-in-time lightcurves from astro metric projects are publicly available. We investigate the se data and use them in the lightcurve inversion method to derive new asteroid models. By having a greater number of models with kn own physical properties, we can gain a better insight into the na ture of individual objects and into the whole asteroid popul ation. Methods. We use sparse photometry from selected observatories from t he AstDyS database (Asteroids – Dynamic Site), either alone or in combination with dense lightcurves, to determine new a steroid models by the lightcurve inversion method. We inves tigate various correlations between several asteroid parameters and char acteristics such as the rotational state and diameter or fam ily membership. We focus on the distribution of ecliptic latitudes of pole di rections. We create a synthetic uniform distribution of lat itudes, compute the method bias, and compare the results with the distributi on of known models. We also construct a model for the long-ter m volution of spins. Results. We present 80 new asteroid models derived from combined data sets where sparse photometry is taken from the AstDyS database and dense lightcurves are from the Uppsala Asteroi d Ph tometric Catalogue (UAPC) and from several individual observers. For 18 asteroids, we present updated shape solutions based o n new photometric data. For another 30 asteroids we present t h ir partial models, i.e., an accurate period value and an estimate of the ecliptic latitude of the pole. The addition of new models inc reases the total number of models derived by the lightcurve inversion m ethod to∼200. We also present a simple statistical analysis of physic al properties of asteroids where we look for possible correlat ions between various physical parameters with an emphasis o n the spin vector. We present the observed and de-biased distribution s of ecliptic latitudes with respect to di fferent size ranges of asteroids as well as a simple theoretical model of the latitude distribut ion and then compare its predictions with the observed distr ibutions. From this analysis we find that the latitude distribution of small asteroids ( D < 30 km) is clustered towards ecliptic poles and can be explained by the YORP thermal e ff ct while the latitude distribution of larger asteroids ( D > 60 km) exhibits an evident excess of prograde rotators, probably of primordial origin.