We have surveyed the Hill sphere of Mars for irregular satellites. Our search covered nearly the entire Hill sphere, but scattered light from Mars excluded the inner few arcminutes where the satellites Phobos and Deimos reside. No new satellites were found to an apparent limiting red magnitude of 23.5, which corresponds to radii of about 0.09 km using an albedo of 0.07.

We present a deep optical survey of Uranus's Hill sphere for small satellites. The 8 m Subaru Telescope was used to survey about 3.5 square degrees with a 50% detection efficiency at limiting red magnitude m(R) = 26.1. This magnitude corresponds to objects that are about 7 km in radius (assuming an albedo of 0.04). We detected ( without prior knowledge of their positions) all previously known outer satellites and discovered two new irregular satellites (S/2001 U2 and S/2003 U3). The two inner satellites Titania and Oberon were also detected. One of the newly discovered bodies (S/2003 U3) is the first known irregular prograde satellite of the planet. The population, size distribution, and orbital parameters of Uranus's irregular satellites are remarkably similar to those of the irregular satellites of gas giant Jupiter. Both have shallow size distributions (power-law indices q similar to 2 for radii larger than 7 km) with no correlation between the sizes of the satellites and their orbital parameters. However, unlike those of Jupiter, Uranus's irregular satellites do not appear to occupy tight, distinct dynamical groups in semimajor-axis versus inclination phase space. Two groupings in semimajor-axis versus eccentricity phase space appear to be statistically significant.

A detailed description of the Halley-type Comet C/2001 OG(108) (LONEOS) has been derived from visible, near-infrared, and mid-infrared observations obtained in October and November 2001. These data represent the first high-quality ground-based observations of a bare Halley-type comet nucleus and provide the best characterization of a Halley-type comet other than 1P/Halley itself. Analysis of time series photometry suggests that the nucleus has a rotation period of 57.2 +/- 0.5 h with a minimum nuclear axial ratio of 1.3, a phase-darkening slope parameter G of -0.01 +/- 0.10, and an estimated H = 13.05 +/- 0.10. The rotation period of C/2001 OG(108) is one of the longest observed among comet nuclei. The V-R color index for this object is measured to be 0.461 +/- 0.02, which is virtually identical to that of other cometary nuclei and other possible extinct comet candidates. Measurements of the comet's thermal emission constrain the projected elliptical nuclear radii to be 9.6 +/- 1.0 km and 7.4 +/- 1.0 km, which makes C/2001 OG(108) one of the larger cometary nuclei known. The derived geometric albedo in V-band of 0.040 +/- 0.010 is typical for comet nuclei. Visible-wavelength spectrophotometry and near-infrared spectroscopy were combined to derive the nucleus's reflectance spectrum over a 0.4 to 2.5 mu m wavelength range. These measurements represent one of the few nuclear spectra ever observed and the only known spectrum of a Halley-type comet. The spectrum of this comet nucleus is very nearly linear and shows no discernable absorption features at a 5% detection limit. The lack of any features, especially in the 0.8 to 1.0 mu m range such as are seen in the spectra of carbonaceous chondrite meteorites and many low-albedo asteroids, is consistent with the presence of anhydrous rather than hydrous silicates on the surface of this comet. None of the currently recognized meteorites in the terrestrial collections have reflectance spectra that match C/2001 OG(108). The near-infrared spectrum, the geometric albedo, and the visible spectrophotometry all indicate that C/2001 OG(108) has spectral properties analogous to the D-type, and possibly P-type asteroids. Comparison of the measured albedo and diameter of C/2001 OG(108) with those of Damocloid asteroids reveals similarities between these asteroids and this comet nucleus, a finding which supports previous dynamical arguments that Damocloid asteroids could be composed of cometary-like materials. These observations are also consistent with findings that two Jupiter-family comets may have spectral signatures indicative of D-type asteroids. C/2001 OG(108) probably represents the transition from a typical active comet to an extinct cometary nucleus, and, as a Halley-type comet, suggests that some comets originating in the Oort cloud can become extinct without disintegrating. As a near-Earth object, C/2001 OG(108) supports the suggestion that some fraction of the near-Earth asteroid population consists of extinct cometary nuclei. (c) 2005 Elsevier Inc. All rights reserved.

The dynamical and physical properties of asteroids offer one of the few constraints on the formation, evolution, and migration of the giant planets. Trojan asteroids share a planet's semimajor axis but lead or follow it by about 60 degrees near the two triangular Lagrangian points of gravitational equilibrium. Here we report the discovery of a high-inclination Neptune Trojan, 2005 TN53. This discovery demonstrates that the Neptune Trojan population occupies a thick disk, which is indicative of "freeze-in'' capture instead of in situ or collisional formation. The Neptune Trojans appear to have a population that is several times larger than the Jupiter Trojans. Our color measurements show that Neptune Trojans have statistically indistinguishable slightly red colors, which suggests that they had a common formation and evolutionary history and are distinct from the classical Kuiper Belt objects.

The redshift distribution of the short-duration gamma-ray bursts (GRBs) is a crucial, but currently fragmentary, clue to the nature of their progenitors. Her ewe present optical observations of nine short GRBs obtained with Gemini, Magellan, and the Hubble Space Telescope. We detect the afterglows and host galaxies of two short bursts, and host galaxies for two additional bursts with known optical afterglow positions, and five with X-ray positions (less than or similar to 6 '' radius). In eight of the nine cases we find that the most probable host galaxies are faint, R approximate to 23-26.5 mag, and are therefore starkly different from the first few short GRB hosts with R approximate to 17-22 mag and z less than or similar to 0.5. Indeed, we measure spectroscopic redshifts of z approximate to 0.4-1.1 for the four brightest hosts. A comparison to large field galaxy samples, as well as the hosts of long GRBs and previous short GRBs, indicates that the fainter hosts likely reside at z greater than or similar to 1. Our most conservative limit is that at least half of the five hosts without a known redshift reside at z > 0.7 (97% confidence level), suggesting that about 1/3 to 2/3 of all short GRBs originate at higher redshifts than previously determined. This has two important implications: (1) we constrain the acceptable age distributions to a wide lognormal (sigma greater than or similar to 1) with tau(*)similar to 4-8 Gyr, or to a power law, P(tau) proportional to tau(n), with -1 less than or similar to n less than or similar to 0; and ( 2) the inferred isotropic energies, E-gamma,E-iso similar to 10(50)-10(52) ergs, are significantly larger than similar to 10(48)-10(49) ergs for the low-redshift, short GRBs, indicating a large spread in energy release or jet opening angles. Finally, we reiterate the importance of short GRBs as potential gravitational-wave sources and find a conservative detection rate with the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) of similar to 2-6 yr(-1).