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We obtained Hubble Space Telescope images of 2 Pallas in September 2007 that reveal distinct color and albedo variations across the surface of this large asteroid. Pallas's shape is an ellipsoid with radii of 291 ([+ or -] 9), 278 ([+ or -] 9), and 250 ([+ or -] 9) kilometers, implying a density of 2400 ([+ or -] 250) kilograms per cubic meter--a value consistent with a body that formed from water-rich material Our observations are consistent with the presence of an impact feature, 240 ([+ or -] 25) kilometers in diameter, within Pallas's ultraviolet-dark terrain. Our observations imply that Pallas is an intact protoplanet that has undergone impact excavation and probable internal alteration. 10.1126/science.1177734

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.08.022 Byline: A.S. Rivkin (a), E.L. Volquardsen (b), B.E. Clark (c) Keywords: Asteroid Ceres; Asteroids; composition; Infrared observations; Spectroscopy Abstract: We present observational evidence that carbonates and iron-rich clays are present on the surface of Ceres. These components are also present in CI chondrites and provide a means of explaining the unusual spectrum of this object as well as providing potential insight into its evolution. Author Affiliation: (a) JHU Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA (b) Institute for Astronomy, University of Hawai'i, 640 N. Aohaku Place, Hilo, HI 96720, USA (c) Ithaca College, Department of Physics, Ithaca, NY 14850, USA Article History: Received 21 June 2006; Revised 16 August 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.12.011 Byline: D. VokrouhlickA1/2 (a), M. BroA3/4 (a), A. Morbidelli (b), W.F. Bottke (c), D. NesvornA1/2 (c), D. Lazzaro (d), A.S. Rivkin (e) Keywords: Asteroids; dynamics; Thermal effects Abstract: The Eos asteroid family is the third most populous, after Themis and Koronis, and one of the largest non-random groups of asteroids in the main belt. It has been known and studied for decades, but its structure and history still presented difficulties to understand. We first revise the Eos family identification as a statistical cluster in the space of proper elements. Using the most to-date catalogue of proper elements we determine a nominal Eos family, defined by us using the hierarchical-clustering method with the cut-off velocity of 55 m/s, contains some 4400 members. This unforeseen increase in known Eos asteroids allows us to perform a much more detailed study than was possible so far. We show, in particular, that most of the previously thought peculiar features are explained within the following model: (i) collisional disruption of the parent body leads to formation of a compact family in the proper element space (with characteristic escape velocities of the observed asteroids of tens of meters per second, compatible with hydrocode simulations), and (ii) as time goes, the family dynamically evolves due to a combination of the thermal effects and planetary perturbations. This model allows us to explain sharp termination of the family at the J7/3 mean motion resonance with Jupiter, uneven distribution of family members about the J9/4 mean motion resonance with Jupiter, semimajor axis distribution of large vs small members in the family and anomalous residence of Eos members inside the high-order secular resonance z.sub.1. Our dynamical method also allows us to estimate Eos family age to 1.3.sub.-0.2.sup.+0.15Gyr. Several formal members of the Eos family are in conflict with our model and these are suspected interlopers. We use spectroscopic observations, whose results are also reported here, and results of 5-color wide-band Sloan Digital Sky Survey photometry to prove some of them are indeed spectrally incompatible with the family. Author Affiliation: (a) Institute of Astronomy, Charles University, V HoleA oviAkach 2, 18000 Prague 8, Czech Republic (b) Observatoire de la CA[acute accent]te d'Azur, BP 4229, 06304 Nice Cedex 4, France (c) Southwest Research Institute, 1050 Walnut St., Suite 400, Boulder, CO 80302, USA (d) Observatorio Nacional, Rua Gal. Jose Cristino 77, 20921-400 Rio de Janeiro, Brazil (e) Massachusetts Institute of Technology, 77 Massachusetts Av., Cambridge, MA 02139, USA Article History: Received 28 June 2005; Revised 16 November 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.09.012 Byline: A.S. Rivkin (a), L.A. McFadden (b), R.P. Binzel (a), M. Sykes (c) Keywords: Asteroids; Vesta; Asteroids; composition; Spectroscopy; Infrared observations; Surfaces; asteroids Abstract: We present new infrared (2-4 [mu]m) spectroscopic observations of Vesta obtained in 2001, 2003, and 2004. Together with previously published work, these present a picture of how Vesta's spectrum changes with sub-Earth latitude and longitude. Vesta's albedo and 2-[mu]m band vary regularly with its rotational phase. While establishing the continuum level for Vesta in the 3-[mu]m region is not straightforward, Vesta appears to have a spectrum consistent with the HED meteorites and not requiring a 3-[mu]m water of hydration band. We cannot formally rule out a shallow ([approximately equal to]1%) band, however. We place limits on the extent to which solar-wind implantation and contamination by CM-like impactors has changed the surface spectrum of Vesta. Author Affiliation: (a) Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA (b) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (c) Planetary Science Institute,1700 E. Fort Lowell Blvd. Suite 106, Tucson, AZ 85719, USA Article History: Received 22 June 2005; Revised 16 September 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2004.11.005 Byline: A.S. Rivkin (a), R.P. Binzel (a), S.J. Bus (b) Abstract: Low-albedo near-Earth objects (NEOs) are warm enough to emit detectable thermal flux at 2.5 [mu]m when near perihelion. Thermal radiation can account for 33% or more of the total flux for an object with an albedo a[c]1/20.04 at 1.0 AU. This is measurable using near-infrared spectroscopic instruments enabling albedos to be constrained for a larger sample of NEOs. Author Affiliation: (a) Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139, USA (b) Institute for Astronomy, 640 N. A'ohoku Place #209, Hilo, HI 96720, USA

We have obtained near-infrared (0.8-2.5 [micro]m) spectra of Hermes (1937 UB) using the NASA Infrared Telescope Facility on Mauna Kea. We find Hermes to have spectral properties consistent with L/LL chondrites, with a strong visual similarity to Gaffey (1976, J. Geophys. Res. 81, 905) average L6 chondrites. We define a ratio of band areas (RBA) using the Modified Gaussian Method (MGM: Sunshine and Pieters, 1990, in: Lunar and Planetary Institute Conference Abstracts, p. 1223, 1993, J. Geophys. Res. 98, 9075) to quantify near-infrared asteroidal data lacking a visible component. Hermes has a spectrum nearly indistinguishable from (19356) 1997 GH3. Together, these asteroids represent new endmembers on the continuum of spectra from ordinary chondrite meteorites to large main-belt S-class asteroids. We discuss regolith effects that may be occurring on Hermes and other possible ordinary chondrite parent bodies, and constrain the albedo of Hermes to 0.4 or higher (effective diameter 650 m or smaller) if it has a regolith. This value for albedo/diameter is consistent with radar results. Keywords: Asteroids; Asteroids near-Earth; Asteroids composition; Meteorites

Spectra of Asteroid 9969 Braille in the 1.25-2.6 [micro]m region returned by the Deep Space 1 (DS1) Mission show a ~ 10% absorption band centered at 2 [micro]m, and a reflectance peak at 1.6 [micro]m. Analysis of these features suggest that the composition of Braille is roughly equal parts pyroxene and olivine. Its spectrum between 0.4 and 2.5 [micro]m suggests that it is most closely related to the Q taxonomic type of asteroid. The spectrum also closely matches that of the ordinary chondrites, the most common type of terrestrial meteorite. The geometric albedo of Braille is unusually high ([p.sub.v] = 0.34), which is also consistent with its placement within the rarer classes of stony asteroids, and which suggests it has a relatively fresh, unweathered surface, perhaps due to a recent collision. Keywords: Asteroids NEOs; 9969 Braille; Deep Space 1

Mars is the only terrestrial planet known to have co-orbiting 'Trojan' asteroids. We have obtained visible and near-IR reflectance spectra of three of these objects: 5261 Eureka and 1998 VF31 in the L5 region and 1999 UJ7 in the L4 region. We also obtained JHK spectrophotometry and a visible lightcurve for 5261 Eureka. The asteroid 5261 Eureka has a visible spectrum that is classified as Sr in the Bus taxonomy, and has infrared colors consistent with the A-class asteroids. The data for 1998 VF31 have a restricted wavelength range, but are most consistent with the Sr or Sa class, though we note a marginal consistency with the D class. We can rule out a C-class classification. 1999 UJ7 has an X-class or T-class spectrum, which is unlike that of the other two Mars Trojans. The photometric data for Eureka are limited, but we can constrain the period to longer than 5 hours (likely 5.5-6 hours) and lightcurve amplitude of at least 0.15 magnitude at this viewing geometry. The spectral differences among the Mars Trojans suggests that either they did not all form at their present solar distances or that they have not always been at their present sizes. Keywords: Asteroids; Asteroids, composition; Spectroscopy; Satellites of Mars

We have observed the leading and trailing hemispheres of Phobos from 1.65 to 3.5 [micro]m and Deimos from 1.65 to 3.12 [micro]m near opposition. We find the trailing hemisphere of Phobos to be brighter than its leading hemisphere by 0.24 [+ or -] 0.06 magnitude at 1.65 [micro]m and brighter than Deimos by 0.98 [+ or -] 0.07 magnitude at 1.65 [micro]m. We see no difference larger than observational uncertainties in spectral slope between the leading and trailing hemispheres when the spectra are normalized to 1.65 [micro]m. We find no 3-[micro]m absorption feature due to hydrated minerals on either hemisphere to a level of ~5-10% on Phobos and ~20% on Deimos. When the infrared data are joined to visible and near-IR data obtained by previous workers, our data suggest the leading (Stickney-dominated) side of Phobos is best matched by T-class asteroids. The spectral slope of the trailing side of Phobos and leading side of Deimos are bracketed by the D-class asteroids. The best laboratory spectral matches to these Parts of Phobos are mature lunar soils and heated carbonaceous chondrites. The lack of 3-[micro]m absorption features on either side of Phobos argues against the presence of a large interior reservoir of water ice according to current models of Phobos' interior (F. P. Fanale and J. R. Salvail 1989, Geophys. Res. Lett. 16, 287-290; Icarus 88, 380-395). Key Words: satellites of Mars; asteroid composition; infrared observations; spectrophotometry.

It is possible that some E- and M-class asteroids may not have originated from igneous rocks. Spectrophotometric observations show a 3-micro m absorption band on E-class asteroid 44 Nysa, suggesting that they are hydrated, a state not compatible with igneous origins. There could be absorption on the E-class asteroid 64 Angelina. M-class asteroid 92 Undina and 201 Penelope also show similar features. These asteroids have properties similar to that of hydrated minerals, but their exact material composition is not known. It is possible that the asteroids may not have originated from igneous rocks.

Understanding the nature and origin of the asteroid population in Earth's vicinity (near-Earth asteroids, and its subset of potentially hazardous asteroids) is a matter of both scientific interest and practical [...]

We have performed spectrophotometry of the Near-Earth Asteroid Rendezvous flyby target 253 Mathilde using five filters from 1.25 to 3.35 [[micro]meter]. We present a synthesized spectrum of ground-based data for 253 Mathilde from 0.3 to 3.35 [[micro]meter] combining our data with those of R. P. Binzel et al. (1996, Icarus 119, 447-449). We find 253 Mathilde to have a spectrum consistent with C-class asteroids in the near-IR, though without the 3-[[micro]meter] water-of-hydration feature commonly (but not always) seen on asteroids of this class. We compare Mathilde with plausible meteorite analogs, and find that the surface of this asteroid is not consistent with a surface composition of common carbonaceous chondritic material. We suggest some alternative analogs on the basis of comparison of meteorite data with a synthesis of ground-based observations of Mathilde.