Galileo Encounter with 951 Gaspra: First Pictures of an Asteroid

Belton, M. J. S.; Veverka, J.; Thomas, P. C.; Helfenstein, P.; Simonelli, D. P.; Chapman, C. R.; Davies, Merton E.; Greeley, R.; Greenberg, R.; Head, J. W.; Murchie, S. L.; Klaasen, K. P.; Johnson, T. V.; McEwen, A. S.; Morrison, David; Neukum, G.; Fanale, F. P.; Anger, C. D.; Carr, M. H.; Pilcher, C. B.

doi:10.1126/science.257.5077.1647

Cited by 197 publications

(89 citation statements)

References 28 publications

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“…Where shadows inside craters are seen, their outlines are basically smooth curves, indicating a lack oflayer discontinuities in crater walls (METs = 143504831-143505761). some ridge crests also shared these same color properties; both units were interpreted to represent gravity-driven movement of regolith (Belton et al, 1992;Carr et al, 1994;Helfenstein et al, 1994). The Ida and Gaspra results all pointto the tentative interpretation that a substantial (~10 m) regolith affects much of the physical properties and morphology of both asteroids and that remobilization influenced by gravity is a key process on asteroids.…”

Section: Discussionmentioning

confidence: 96%

The geology of 433 Eros

Robinson

Thomas

Veverka

et al. 2002

Meteorit & Planetary Scien

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155

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Abstract-The global high-resolution imaging of asteroid 433 Eros by the Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small S-type asteroid. Eros displays a variety of surface features, and evidence of a substantial regolith. Large scale facets, grooves, and ridges indicate the presence of at least one global planar structure. Directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object. As with other small objects, impact craters dominate the overall shape as well as the small-scale topography of Eros. Depth/diameter ratios of craters on Eros average -0.13, but the freshest craters approach lunar values of -0.2. Ejecta block production from craters is highly variable; the majority oflarge blocks appear to have originated from one 7.6 km crater (Shoemaker). The interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of Phobos. This lack of mechanical boundaries, and the abundant evidence of regolith in nearly every high-resolution image, suggests a gradation in the porosity and fracturing with depth. The density of small craters is deficient at sizes below -200 m relative to predicted slopes of empirical saturation. This characteristic, which is also found on parts ofPhobos and lunar highland areas, probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith. Eros displays a variety ofregolith features, such as debris aprons, fine-grained "ponded" deposits, talus cones, and bright and dark streamers on steep slopes indicative ofefficient downslope movement ofregolith. These processes serve to mix materials in the upper loose fragmental portion ofthe asteroid (regolith). In the instance of "ponded" materials and crater wall deposits, there is evidence of processes that segregate finer materials into discrete deposits. The NEAR observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic. Future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the NEAR Shoemaker spacecraft.

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Section: Discussionmentioning

confidence: 96%

The geology of 433 Eros

Robinson

Thomas

Veverka

et al. 2002

Meteorit & Planetary Scien

Self Cite

155

140

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“…For radii r 1 km, perhaps the best constraints on the distribution come from the impact crater size distribution on asteroid Gaspra. There, craters from 0.4 km to 1.5 km in diameter (caused by projectiles perhaps 10-20 times smaller) are distributed as a power law with a differential size index −3.7 ± 0.5 (Belton et al 1992; note that Chapman et al (1996) report that fresh craters on Gaspra follow an even steeper distribution, with differential power-law index −4.3 ± 0.3). We assume that the total main-belt population is ∼1.4 × 10 6 asteroids with diameters >1 km.…”

Section: Discussionmentioning

confidence: 99%

The Active Asteroids

Jewitt

2012

The Astronomical Journal

286

244

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Some asteroids eject dust, unexpectedly producing transient, comet-like comae and tails. First ascribed to the sublimation of near-surface water ice, mass-losing asteroids (also called "main-belt comets") can in fact be driven by a surprising diversity of mechanisms. In this paper, we consider 11 dynamical asteroids losing mass, in nine of which the ejected material is spatially resolved. We address mechanisms for producing mass loss including rotational instability, impact ejection, electrostatic repulsion, radiation pressure sweeping, dehydration stresses, and thermal fracture, in addition to the sublimation of ice. In two objects (133P and 238P) the repetitive nature of the observed activity leaves ice sublimation as the only reasonable explanation, while in a third ((596) Scheila), a recent impact is the cause. Another impact may account for activity in P/2010 A2, but this tiny object can also be explained as having shed mass after reaching rotational instability. Mass loss from (3200) Phaethon is probably due to cracking or dehydration at extreme (∼1000 K) perihelion temperatures, perhaps aided by radiation pressure sweeping. For the other bodies, the mass-loss mechanisms remain unidentified, pending the acquisition of more and better data. While the active asteroid sample size remains small, the evidence for an astonishing diversity of mass-loss processes in these bodies is clear.

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“…This has been remarkably illustrated by fly-by and rendezvous space missions (Belton et al, 1992(Belton et al, , 1996Zuber et al, 2000;Fujiwara et al, 2006;Sierks et al, 2011;Russell et al, 2012Russell et al, , 2016, as well as observations from the Earth (e.g., Carry et al 2008Carry et al , 2010bMerline et al 2013). In the late nineties, observations of (4) Vesta with the Hubble Space Telescope (HST) led to the discovery of the now-called "Rheasilvia basin" and allowed for establishment of the origin of the Vestoids and HED meteorites found on Earth Binzel et al, 1997).…”

Section: Introductionmentioning

confidence: 99%

3D shape of asteroid (6) Hebe from VLT/SPHERE imaging: Implications for the origin of ordinary H chondrites

Marsset

Carry

Dumas

et al. 2017

A&A

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Context. The high-angular-resolution capability of the new-generation ground-based adaptive-optics camera SPHERE at ESO VLT allows us to assess, for the very first time, the cratering record of medium-sized (D∼100-200 km) asteroids from the ground, opening the prospect of a new era of investigation of the asteroid belt's collisional history. Aims. We investigate here the collisional history of asteroid (6) Hebe and challenge the idea that Hebe may be the parent body of ordinary H chondrites, the most common type of meteorites found on Earth (∼34% of the falls). Methods. We observed Hebe with SPHERE as part of the science verification of the instrument. Combined with earlier adaptiveoptics images and optical light curves, we model the spin and three-dimensional (3D) shape of Hebe and check the consistency of the derived model against available stellar occultations and thermal measurements. Results. Our 3D shape model fits the images with sub-pixel residuals and the light curves to 0.02 mag. The rotation period (7.274 47 h), spin (ECJ2000 λ,β of 343 • ,+47 • ), and volume-equivalent diameter (193 ± 6 km) are consistent with previous determinations and thermophysical modeling. Hebe's inferred density is 3.48 ± 0.64 g.cm −3 , in agreement with an intact interior based on its H-chondrite composition. Using the 3D shape model to derive the volume of the largest depression (likely impact crater), it appears that the latter is significantly smaller than the total volume of close-by S-type H-chondrite-like asteroid families. Conclusions. Our results imply that (6) Hebe is not the most likely source of H chondrites. Over the coming years, our team will collect similar high-precision shape measurements with VLT/SPHERE for ∼40 asteroids covering the main compositional classes, thus providing an unprecedented dataset to investigate the origin and collisional evolution of the asteroid belt.

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Galileo Encounter with 951 Gaspra: First Pictures of an Asteroid

Cited by 197 publications

References 28 publications

The geology of 433 Eros

The geology of 433 Eros

The Active Asteroids

3D shape of asteroid (6) Hebe from VLT/SPHERE imaging: Implications for the origin of ordinary H chondrites

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