Search

We use a statistical-thermodynamic model to investigate the formation and composition of noble-gas-rich clathrates on Pluto's surface. By considering an atmospheric composition close to that of today's Pluto and a broad range of surface pressures, we find that Ar, Kr and Xe can be efficiently trapped in clathrates if they formed at the surface, in a way similar to what has been proposed for Titan. The formation on Pluto of clathrates rich in noble gases could then induce a strong decrease in their atmospheric abundances relative to their initial values. A clathrate thickness of order of a few centimeters globally averaged on the planet is enough to trap all Ar, Kr and Xe if these noble gases were in protosolar proportions in Pluto's early atmosphere. Because atmospheric escape over an extended period of time (millions of years) should lead to a noble gas abundance that either remains constant or increases with time, we find that a potential depletion of Ar, Kr and Xe in the atmosphere would best be explained by their trapping in clathrates. A key observational test is the measurement of Ar since the Alice UV spectrometer aboard the New Horizons spacecraft will be sensitive enough to detect its abundance $\sim$10 times smaller than in the case considered here., Comment: Accepted for publication in Icarus

In early 2007, the New Horizons spacecraft flew through the Jovian magnetosphere on the dusk side. Here, we present results from a novel means of detecting energetic electrons along New Horizons' trajectory: the background count rate of the Alice ultraviolet spectrograph. Electrons with energies >1 MeV can penetrate the thin aluminum housing of Alice, interact with the microchannel plate detector, and produce a count that is indistinguishable from an FUV photon. We present Alice data, proportional to the MeV electron flux, from an 11-day period centered on the spacecraft's closest approach to Jupiter, and compare it to electron data from the PEPSSI instrument. We find that a solar wind compression event passed over the spacecraft just prior to it entering the Jovian magnetosphere. Subsequently, the magnetopause boundary was detected at a distance of 67 R_J suggesting a compressed magnetospheric configuration. Three days later, when the spacecraft was 35-90 R_J downstream of Jupiter, New Horizons observed a series of 15 current sheet crossings, all of which occurred significantly northward of model predictions implying solar wind influence over the middle and outer Jovian magnetosphere, even to radial distances as small as ~35 R_J. In addition, we find the Jovian current sheet, which had a half-thickness of at least 7.4 R_J between 1930 and 2100 LT abruptly thinned to a thickness of ~3.4 R_J around 2200 LT., Comment: 39 pages, 8 figures

Interior to the orbit of Mercury, between 0.07 and 0.21 AU, is a dynamically stable region where a population of asteroids, known as Vulcanoids, may reside. We present the results from our search for Vulcanoids using archival data from the Heliospheric Imager-1 (HI-1) instrument on NASA's two STEREO spacecraft. Four separate observers independently searched through images obtained from 2008-12-10 to 2009-02-28. Roughly, all Vulcanoids with e<=0.15 and i<=15deg will pass through the HI-1 field of view at least twice during this period. No Vulcanoids were detected. Based on the number of synthetic Vulcanoids added to the data that were detected, we derive a 3 sigma upper limit (i.e. a confidence level >0.997) that there are presently no Vulcanoids larger than 5.7 km in diameter, assuming an R-band albedo of p_R=0.05 and a Mercury-like phase function. The present-day Vulcanoid population, if it exists at all, is likely a small remnant of the hypothetical primordial Vulcanoid population due to the combined effects of collisional evolution and subsequent radiative transport of collisional fragments. If we assume an extant Vulcanoid population with a collisional equilibrium differential size distribution with a power law index of -3.5, our limit implies that there are no more than 76 Vulcanoids larger than 1 km., Comment: 27 pages, 6 figures

We present the discovery of a long-term stable L5 (trailing) Neptune Trojan in data acquired to search for candidate Trans-Neptunian objects for the New Horizons spacecraft to fly by during an extended post-Pluto mission. This Neptune Trojan, 2011 HM102, has the highest inclination (29.4 degrees) of any known member of this population. It is intrinsically brighter than any single L5 Jupiter Trojan at H~8.18. We have determined its gri colors (a first for any L5 Neptune Trojan), which we find to be similar to the moderately red colors of the L4 Neptune Trojans, suggesting similar surface properties for members of both Trojan clouds. We also present colors derived from archival data for two L4 Neptune Trojans (2006 RJ103 and 2007 VL305), better refining the overall color distribution of the population. In this document we describe the discovery circumstances, our physical characterization of 2011 HM102, and this object's implications for the Neptune Trojan population overall. Finally, we discuss the prospects for detecting 2011 HM102 from the New Horizons spacecraft during their close approach in mid- to late-2013., Comment: Accepted for publication in the Astronomical Journal, January 16, 2013

We report observations of the lunar helium exosphere made between December 29, 2011, and January 26, 2012, with the Lyman Alpha Mapping Project (LAMP) ultraviolet spectrograph on NASA's Lunar Reconnaissance Orbiter Mission (LRO). The observations were made of resonantly scattered He I 584 from illuminated atmosphere against the dark lunar surface on the dawn side of the terminator. We find no or little variation of the derived surface He density with latitude but day-to-day variations that likely reflect variations in the solar wind alpha flux. The 5-day passage of the Moon through the Earth's magnetotail results in a factor of two decrease in surface density, which is well explained by model simulations., Comment: 21 pages, 5 figures, 3 supplementary figures, accepted for publication in Icarus

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in May 2009, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F_lambda ~ 1.0E10-14 ergs/s/cm2/Angstrom, COS can achieve comparable signal to noise (when compared to STIS echelle modes) in 1-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (September 2009 - June 2011) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is 9 times that sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of June 2011. COS has measured, for the first time with high reliability, broad Lya absorbers and Ne VIII in the intergalactic medium, and observed the HeII reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy., Comment: 17 pages, 15 figures

The European Space Agency's Rosetta spacecraft, en route to a 2014 encounter with comet 67P/Churyumov-Gerasimenko, made a gravity assist swing-by of Mars on 25 February 2007, closest approach being at 01:54UT. The Alice instrument on board Rosetta, a lightweight far-ultraviolet imaging spectrograph optimized for in situ cometary spectroscopy in the 750-2000 A spectral band, was used to study the daytime Mars upper atmosphere including emissions from exospheric hydrogen and oxygen. Offset pointing, obtained five hours before closest approach, enabled us to detect and map the HI Lyman-alpha and Lyman-beta emissions from exospheric hydrogen out beyond 30,000 km from the planet's center. These data are fit with a Chamberlain exospheric model from which we derive the hydrogen density at the 200 km exobase and the H escape flux. The results are comparable to those found from the the Ultraviolet Spectrometer experiment on the Mariner 6 and 7 fly-bys of Mars in 1969. Atomic oxygen emission at 1304 A is detected at altitudes of 400 to 1000 km above the limb during limb scans shortly after closest approach. However, the derived oxygen scale height is not consistent with recent models of oxygen escape based on the production of suprathermal oxygen atoms by the dissociative recombination of O2+., Comment: 17 pages, 8 figures, accepted for publication in Icarus

We report the detection of several emission bands in the CO Fourth Positive Group from comet 103P/Hartley during ultraviolet spectroscopic observations from the Hubble Space Telescope (HST) on 2010 November 4 near the time of closest approach by NASA's EPOXI spacecraft. The derived CO/H2O ratio is 0.15-0.45%, which places 103P among the most CO-depleted comets. Apparently this highly volatile species, whose abundance varies by a factor of ~50 among the comets observed to date, does not play a major role in producing the strong and temporally variable activity in 103P/Hartley. The CO emissions varied by ~30% between our two sets of observations, apparently in phase with the temporal variability measured for several gases and dust by other observers. The low absolute abundance of CO in 103P suggests several possibilities: the nucleus formed in a region of the solar nebula that was depleted in CO or too warm to retain much CO ice, repeated passages through the inner solar system have substantially depleted the comet's primordial CO reservoir, or any CO still in the nucleus is buried below the regions that contribute significantly to the coma., Comment: 14 pages, 3 figures, 2 tables Accepted for publication in Astrophysical Journal Letters

We conducted a satellite search around the dwarf planet 1 Ceres using Hubble Space Telescope and ground-based Palomar data. No candidate objects were found orbiting Ceres in its entire stability region down to ~500km from the surface of Ceres. Assuming a satellite would have the same albedo as Ceres, which has a visual geometric albedo of 0.07-0.10, our detection limit is sensitive to satellites larger than 1-2 km in diameter., Comment: Accepted for publication in AJ

The asteroid (21) Lutetia is the target of a planned close encounter by the Rosetta spacecraft in July 2010. To prepare for that flyby, Lutetia has been extensively observed by a variety of astronomical facilities. We used the Hubble Space Telescope (HST) to determine the albedo of Lutetia over a wide wavelength range, extending from ~150 nm to ~700 nm. Using data from a variety of HST filters and a ground-based visible light spectrum, we employed synthetic photometry techniques to derive absolute fluxes for Lutetia. New results from ground-based measurements of Lutetia's size and shape were used to convert the absolute fluxes into albedos. We present our best model for the spectral energy distribution of Lutetia over the wavelength range 120-800 nm. There appears to be a steep drop in the albedo (by a factor of ~2) for wavelengths shorter than ~300 nm. Nevertheless, the far ultraviolet albedo of Lutetia (~10%) is considerably larger than that of typical C-chondrite material (~4%). The geometric albedo at 550 nm is 16.5 +/- 1%. Lutetia's reflectivity is not consistent with a metal-dominated surface at infrared or radar wavelengths, and its albedo at all wavelengths (UV-visibile-IR-radar) is larger than observed for typical primitive, chondritic material. We derive a relatively high FUV albedo of ~10%, a result that will be tested by observations with the Alice spectrograph during the Rosetta flyby of Lutetia in July 2010., Comment: 14 pages, 2 tables, 8 figures

We examine the ability of impacts by Kuiper Belt debris to cause regolith exchange between objects in the Pluto system. We find that ejecta velocities from KB impacts are too low to escape from Pluto and Charon. However, ejecta can escape Nix and Hydra, and is capable of covering one another to depths as high as 10s of meters, and Charon and Pluto, perhaps to depths up to several 10s of cm. Although Pluto's annual atmospheric frost deposition cycle will cover such imported debris on timescales faster than it is emplaced, no such masking mechanism is available on Hydra, Nix, and Charon. As a result, ejecta exchange between these bodies is expected to evolve their colors, albedos, and other photometric properties to be similar. We examined the ability of ejecta exchange to work for other Kuiper Belt binaries and found the process can be effective in many cases. This process may also operate in asteroid binary systems., Comment: 8 pages, 3 tables, 0 figures

NASA's New Horizons (NH) Pluto-Kuiper belt (PKB) mission was launched on 19 January 2006 on a Jupiter Gravity Assist (JGA) trajectory toward the Pluto system for a 14 July 2015 closest approach; Jupiter closest approach occurred on 28 February 2007. It was competitively selected by NASA for development on 29 November 2001. New Horizons is the first mission to the Pluto system and the Kuiper belt; and will complete the reconnaissance of the classical planets. The ~400 kg spacecraft carries seven scientific instruments, including imagers, spectrometers, radio science, a plasma and particles suite, and a dust counter built by university students. NH will study the Pluto system over a 5-month period beginning in early 2015. Following Pluto, NH will go on to reconnoiter one or two 30-50 kilometer diameter Kuiper belt Objects (KBOs), if NASA approves an extended mission. If successful, NH will represent a watershed development in the scientific exploration of a new class of bodies in the solar system - dwarf planets, of worlds with exotic volatiles on their surfaces, of rapidly (possibly hydrodynamically) escaping atmospheres, and of giant impact derived satellite systems. It will also provide the first dust density measurements beyond 18 AU, cratering records that shed light on both the ancient and present-day KB impactor population down to tens of meters, and a key comparator to the puzzlingly active, former dwarf planet (now satellite of Neptune) called Triton, which is as large as Eris and Pluto., Comment: 18 pages, 4 figures, 2 tables; To appear in a special volume of Space Science Reviews on the New Horizons mission

The New Horizons spacecraft was launched on 19 January 2006. The spacecraft was designed to provide a platform for seven instruments that will collect and return data from Pluto in 2015. The design drew on heritage from previous missions developed at The Johns Hopkins University Applied Physics Laboratory (APL) and other missions such as Ulysses. The trajectory design imposed constraints on mass and structural strength to meet the high launch acceleration needed to reach the Pluto system prior to the year 2020. The spacecraft subsystems were designed to meet tight mass and power allocations, yet provide the necessary control and data handling finesse to support data collection and return when the one-way light time during the Pluto flyby is 4.5 hours. Missions to the outer solar system require a radioisotope thermoelectric generator (RTG) to supply electrical power, and a single RTG is used by New Horizons. To accommodate this constraint, the spacecraft electronics were designed to operate on less than 200 W. The spacecraft system architecture provides sufficient redundancy to provide a probability of mission success of greater than 0.85, even with a mission duration of over 10 years. The spacecraft is now on its way to Pluto, with an arrival date of 14 July 2015. Initial inflight tests have verified that the spacecraft will meet the design requirements., Comment: 33 pages, 13 figures, 4 tables; To appear in a special volume of Space Science Reviews on the New Horizons mission

The New Horizons instrument named Ralph is a visible/near infrared multi-spectral imager and a short wavelength infrared spectral imager. It is one of the core instruments on New Horizons, NASA's first mission to the Pluto/Charon system and the Kuiper Belt. Ralph combines panchromatic and color imaging capabilities with IR imaging spectroscopy. Its primary purpose is to map the surface geology and composition of these objects, but it will also be used for atmospheric studies and to map the surface temperature. It is a compact, low-mass (10.5 kg), power efficient (7.1 W peak), and robust instrument with good sensitivity and excellent imaging characteristics. Other than a door opened once in flight, it has no moving parts. These characteristics and its high degree of redundancy make Ralph ideally suited to this long-duration flyby reconnaissance mission., Comment: 18 pages, 15 figures, 4 tables; To appear in a special volume of Space Science Reviews on the New Horizons mission

The New Horizons ALICE instrument is a lightweight (4.4 kg), low-power (4.4 Watt) imaging spectrograph aboard the New Horizons mission to Pluto/Charon and the Kuiper Belt. Its primary job is to determine the relative abundances of various species in Pluto's atmosphere. ALICE will also be used to search for an atmosphere around Pluto's moon, Charon, as well as the Kuiper Belt Objects (KBOs) that New Horizons hopes to fly by after Pluto-Charon, and it will make UV surface reflectivity measurements of all of these bodies as well. The instrument incorporates an off-axis telescope feeding a Rowland-circle spectrograph with a 520-1870 angstroms spectral passband, a spectral point spread function of 3-6 angstroms FWHM, and an instantaneous spatial field-of-view that is 6 degrees long. Different input apertures that feed the telescope allow for both airglow and solar occultation observations during the mission. The focal plane detector is an imaging microchannel plate (MCP) double delay-line detector with dual solar-blind opaque photocathodes (KBr and CsI) and a focal surface that matches the instrument's 15-cm diameter Rowland-circle. In what follows, we describe the instrument in greater detail, including descriptions of its ground calibration and initial in flight performance., Comment: 24 pages, 29 figures, 2 tables; To appear in a special volume of Space Science Reviews on the New Horizons mission

The New Horizons mission was launched on 2006 January 19, and the spacecraft is heading for a flyby encounter with the Pluto system in the summer of 2015. The challenges associated with sending a spacecraft to Pluto in less than 10 years and performing an ambitious suite of scientific investigations at such large heliocentric distances (> 32 AU) are formidable and required the development of lightweight, low power, and highly sensitive instruments. This paper provides an overview of the New Horizons science payload, which is comprised of seven instruments. Alice provides spatially resolved ultraviolet spectroscopy. The Ralph instrument has two components: the Multicolor Visible Imaging Camera (MVIC), which performs panchromatic and color imaging, and the Linear Etalon Imaging Spectral Array (LEISA), which provides near-infrared spectroscopic mapping capabilities. The Radio Experiment (REX) is a component of the New Horizons telecommunications system that provides both occultation and radiometry capabilities. The Long Range Reconnaissance Imager (LORRI) provides high sensitivity, high spatial resolution optical imaging capabilities. The Solar Wind at Pluto (SWAP) instrument measures the density and speed of solar wind particles. The Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) measures energetic protons and CNO ions. The Venetia Burney Student Dust Counter (VB-SDC) is used to record dust particle impacts during the cruise phases of the mission., Comment: 17 pages, 4 figures, 1 table; To appear in a special volume of Space Science Reviews on the New Horizons mission

The New Horizons spacecraft will achieve a wide range of measurement objectives at the Pluto system, including color and panchromatic maps, 1.25-2.50 micron spectral images for studying surface compositions, and measurements of Pluto's atmosphere (temperatures, composition, hazes, and the escape rate). Additional measurement objectives include topography, surface temperatures, and the solar wind interaction. The fulfillment of these measurement objectives will broaden our understanding of the Pluto system, such as the origin of the Pluto system, the processes operating on the surface, the volatile transport cycle, and the energetics and chemistry of the atmosphere. The mission, payload, and strawman observing sequences have been designed to acheive the NASA-specified measurement objectives and maximize the science return. The planned observations at the Pluto system will extend our knowledge of other objects formed by giant impact (such as the Earth-moon), other objects formed in the outer solar system (such as comets and other icy dwarf planets), other bodies with surfaces in vapor-pressure equilibrium (such as Triton and Mars), and other bodies with N2:CH4 atmospheres (such as Titan, Triton, and the early Earth)., Comment: 40 pages, 9 figures, 7 tables; To appear in a special volume of Space Science Reviews on the New Horizons mission

We report on spectroscopic observations of periodic comet 9P/Tempel 1 by the Alice ultraviolet spectrograph on the Rosetta spacecraft in conjunction with NASA's Deep Impact mission. Our objectives were to measure an increase in atomic and molecular emissions produced by the excavation of volatile sub-surface material. We unambiguously detected atomic oxygen emission from the quiescent coma but no enhancement at the 10% (1-sigma) level following the impact. We derive a quiescent water production rate of 9 x 10^27 molecules per second with an estimated uncertainty of 30%. Our upper limits to the volatiles produced by the impact are consistent with other estimates., Comment: 11 pages, 4 postscript figures. Accepted for publication in Icarus special issue on Deep Impact

Simple theoretical calculations have suggested that small body impacts onto Pluto's newly discovered small satellites, Nix and Hydra, are capable of generating time-variable rings or dust sheets in the Pluto system. Using HST/ACS data obtained on 2006 February 15 and 2006 March 2, we find no observational evidence for such a ring system and present the first constraints on the present-day I/F and optical depth of a putative ring system. At the 1500-km radial resolution of our search, we place a 3-sigma upper limit on the azimuthally-averaged normal I/F of ring particles of 5.1x10^-7 at a distance of 42,000 km from the Pluto-Charon barycenter, the minimum distance for a dynamically stable ring (Stern et al., 1994; Nagy et al., 2006); 4.4x10^-7 at the orbit of Nix; and 2.5x10^-7 at the orbit of Hydra. For an assumed ring particle albedo of 0.04 (0.38), these I/F limits translate into 3-sigma upper limits on the normal optical depth of macroscopic ring particles of 1.3x10^-5 (1.4x10^-6), 1.1x10^-5 (1.2x10^-6), 6.4x10^-6 (6.7x10^-7), respectively. Were the New Horizons spacecraft to fly through a ring system with optical depth of 1.3x10^-5, it would collide with a significant number of potentially damaging ring particles. We therefore recommend that unless tighter constraints can be obtained, New Horizons cross the putative ring plane within 42,000 km of the Pluto-Charon barycenter, where rings are dynamically unstable. We derive a crude estimate of the lifetime of putative ring paritcles of 900 years., Comment: 14 pages, including 3 figures and 2 tables

Pluto's two small satellites, temporarily designated S/2005 P 1 and S/2005 P 2, were observed on four dates (15.1 and 18.1 May 2005, 15.7 February 2006, and 2.8 March 2006) using the Hubble Space Telescope's (HST) Advanced Camera for Surveys (ACS). Here we collect together the astrometric positions of these two satellites (henceforth P1 and P2), as well as a single color measurement for each satellite and initial constraints on their photometric variability obtained during these observations. We find that both satellites have essentially neutral (grey) reflectivities, like Charon. We also find that neither satellite exhibited strong photometric variation, which might suggest that P1 and P2 are toward the large end of their allowable size range, and therefore may have far lower reflectivities than Charon., Comment: 9 pages, including 3 tables and 1 figure

We describe the design, performance and scientific objectives of the NASA-funded ALICE instrument aboard the ESA Rosetta asteroid flyby/comet rendezvous mission. ALICE is a lightweight, low-power, and low-cost imaging spectrograph optimized for cometary far-ultraviolet (FUV) spectroscopy. It will be the first UV spectrograph to study a comet at close range. It is designed to obtain spatially-resolved spectra of Rosetta mission targets in the 700-2050 A spectral band with a spectral resolution between 8 A and 12 A for extended sources that fill its ~0.05 deg x 6.0 deg field-of-view. ALICE employs an off-axis telescope feeding a 0.15-m normal incidence Rowland circle spectrograph with a concave holographic reflection grating. The imaging microchannel plate detector utilizes dual solar-blind opaque photocathodes (KBr and CsI) and employs a 2 D delay-line readout array. The instrument is controlled by an internal microprocessor. During the prime Rosetta mission, ALICE will characterize comet 67P/Churyumov-Gerasimenko's coma, its nucleus, and the nucleus/coma coupling; during cruise to the comet, ALICE will make observations of the mission's two asteroid flyby targets and of Mars, its moons, and of Earth's moon. ALICE has already successfully completed the in-flight commissioning phase and is operating normally in flight. It has been characterized in flight with stellar flux calibrations, observations of the Moon during the first Earth fly-by, and observations of comet Linear T7 in 2004 and comet 9P/Tempel 1 during the 2005 Deep Impact comet-collision observing campaign, Comment: 11 pages, 7 figures

Pluto's first known moon, Charon, was discovered in 1978 (Christy 1978) and has a diameter about half that of Pluto (Buie 1992,Young 1994, Sicardy 2005), which makes it larger relative to its primary than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful (Stern 1991, Stern 1994, Stern 2003), but they were not sensitive to objects <=150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites (Stern 1994). Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P1 (hereafter P1) and S/2005 P2 (hereafter P2), which makes Pluto the first Kuiper belt object (KBO) known to have multiple satellites. These new satellites are much smaller than Charon (diameter~1200 km), with P1 ranging in diameter from 60-165 km depending on the surface reflectivity, and P2 about 20% smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of ~38 days (P1) and ~25 days (P2). The implications of the discovery of P1 and P2 for the origin and evolution of the Pluto system, and for the satellite formation process in the Kuiper belt, are discussed in a companion paper (Stern 2006)., Comment: Preprint of a paper accepted for publication in the journal Nature

Our discovery of two new satellites of Pluto, designated S/2005 P 1 and S/2005 P 2 (henceforth, P1 and P2), combined with the constraints on the absence of more distant satellites of Pluto, reveal that Pluto and its moons comprise an unusual, highly compact, quadruple system. The two newly discovered satellites of Pluto have masses that are very small compared to both Pluto and Charon, creating a striking planet-satellite system architecture. These facts naturally raise the question of how this puzzling satellite system came to be. Here we show that P1 and P2's proximity to Pluto and Charon, along with their apparent locations in high-order mean-motion resonances, likely result from their being constructed from Plutonian collisional ejecta. We argue that variable optical depth dust-ice rings form sporadically in the Pluto system, and that rich satellite systems may be found, perhaps frequently, around other large Kuiper Belt objects., Comment: 15 pages, 1 figure

We present new astrometry of Pluto's three satellites from images taken of the Pluto system during 2002-3 with the High Resolution Camera (HRC) mode of the Advanced Camera for Surveys (ACS) instrument on the Hubble Space Telescope. The observations were designed to produce an albedo map of Pluto but they also contain images of Charon and the two recently discovered satellites, S/2005 P1 and S/2005 P2. Orbits fitted to all three satellites are nearly co-planar and, for Charon and P2, have eccentricities consistent with zero. The orbit of the outermost satellite, P1, has a significant eccentricity of 0.0052 +/- 0.0011. Orbital periods of P1, P2, and Charon are 38.2065 +/- 0.0014, 24.8562 +/- 00013, and 6.3872304 +/- 0.0000011 days, respectively. The total system mass based on Charon's orbit is 1.4570 +/- 0.0009 x 10^22 kg. We confirm previous results that orbital periods are close to the ratio of 6:4:1 (P1:P2:Charon) indicative of mean-motion resonances, but our results formally preclude precise integer period ratios. The orbits of P1 and P2, being about the barycenter rather than Pluto, enable us to measure the Charon/Pluto mass ratio as 0.1165 +/- 0.0055. This new mass ratio implies a density of 1.65 +/- 0.06 g cm^-3 for Charon (603.6 km radius) and 2.03 +/- 0.06 g cm^-3 for Pluto (1153 km radius) thus adding confirmation that Charon is significantly less dense than Pluto. Finally, by stacking all images, we can extract globally averaged photometry. P1 has a mean opposition magnitude of V=24.39 +/- 0.09 and color of (B-V) = 0.64 +/- 0.12. P2 has a mean opposition magnitude of V=24.55 +/- 0.10 and color of (B-V) = 0.91 +/- 0.15., Comment: 24 pages, 5 figures, 4 tables submitted to Astronomical Journal 2005/12/19 revised 2006/3/18

Observations of Pluto and its solar-tidal stability zone were made using the Advanced Camera for Surveys' (ACS) Wide Field Channel (WFC) on the Hubble Space Telescope on UT 2005 May 15 and UT 2005 May 18. Two small satellites of Pluto, provisionally designated S/2005 P 1 and S/2005 P 2, were discovered, as discussed by Weaver et al. (2006) and Stern et al. (2006a). Confirming observations of the newly discovered moons were obtained using the ACS in the High Resolution Channel (HRC) mode on 2006 Feb 15 (Mutchler et al. 2006). Both sets of observations provide strong constraints on the existence of any additional satellites in the Pluto system. Based on the May 2005 observations using the ACS/WFC, we place a 90%-confidence lower limit of m_V = 26.8 (m_V = 27.4 for a 50%-confidence lower limit) on the magnitude of undiscovered satellites greater than 5" (1.1x10^5 km) from Pluto. Using the 2005 Feb 15 ACS/HRC observations we place 90%-confidence lower limits on the apparent magnitude of any additional satellites of m_V = 26.4 between 3"-5" (6.9x10^4-1.1x10^5 km) from Pluto, m_V = 25.7 between 1"-3" (2.3x10^4-6.9x10^4 km) from Pluto, and m_V = 24. between 0.3"-1" (6.9x10^3-2.3x10^4 km) from Pluto. The 90%-confidence magnitude limits translate into upper limits on the diameters of undiscovered satellites of 29 km outside of 5" from Pluto, 36 km between 3"-5" from Pluto, 49 km between 1"-3" from Pluto, and 115 km between 0.3"-1" for a comet-like albedo of p_V = 0.04. If potential satellites are assumed to have a Charon-like albedo of p_V = 0.38, the diameter limits are 9 km, 12 km, 16 km, and 37 km, respectively., Comment: Accepted by the Astronomical Journal 17 pages including 4 figures

Recently, Brown et al. (2004) reported the exciting discovery of an ~800 km radius object, (90377) Sedna, on a distant, eccentric orbit centered at ~490 AU from the Sun. Here we undertake a first look exploring the feasibility of accreting this object and its possible cohorts between 75 AU (Sedna's perihelion distance) and 500 AU (Sedna's semi-major axis distance) from the Sun. We find such accretion possible in a small fraction of the age of the solar system, if such objects were initially on nearly circular orbits in this region, and if the solar nebula extended outward to distances far beyond the Kuiper Belt. If Sedna did form in situ, it is likely to be accompanied by a cohort of other large bodies in this region of the solar system., Comment: 06 pages, plus 2 tables and 2 figures

The Kuiper Belt (KB) region of the solar system, stretching from 30 to at least 50 AU, contains Pluto-Charon, some ~105 Kuiper Belt Objects (KBOs) larger than 100 km in diameter, and myriad smaller bodies produced by collisions among KBOs (Farinella, Davis, & Stern 2000). The most notable physical attribute of the KBOs discovered to date is their reflectance color diversity, ranging from gray (i.e., neutral) to very red (Tegler & Romanishin 2000). We report here evidence for a statistical correlation between the colors of KBOs and the mean random impact speed that these objects experience, lending evidence to suggestions some years ago by modelers (Jewitt & Luu 1996) that a competition between collisional resurfacing and radiation reddening may contribute to the color diversity of KBOs.

Recently, it has been discovered that at least 1% of Kuiper Belt Objects (KBOs) are accompanied by large satellites. Here we examine the energetics of KBO satellite formation via collisions, finding collisions require a dynamically excited Kuiper Belt. Further, even under optimistic assumptions (including perfect accretion of the satellites), we find that collisional processes cannot make KBO satellites at the observed frequency of occurrence unless KBO projectiles large enough to generate the observed satellites were \~40 times more numerous in the ancient past, resulting in an increased collision rate with sufficiently large projectiles, or unless the fraction of impact energy estimated to be imparted to ejecta is of order unity. Neither alternative is very palatable. However, an easier to accept alternative also exists: KBO primary and/or KBO satellite surface albedos presently be underestimated by making the canonical assumption of 4% surface albedos; specifically, surface albedos of KBO primaries and/or their satellites could be in the neighborhood of 15% (or higher). This would reduce KBO primary and/or satellite sizes and masses, thereby in turn reducing the required size of the impacting projectiles required to generate KBO satellites, which in turn would increase the impact rate with sufficiently large projectiles to values in line with the observed fraction of KBOs with large satellites. This proposition is expected to be easily testable with by SIRTF and by other means in the next 2 to 3 years.

We present HST Faint Object Camera observations of the asteroid 1 Ceres at near-, mid-, and far-UV wavelengths (lambda = 3636, 2795, and 1621 A, respectively) obtained on 1995 June 25. The disk of Ceres is well-resolved for the first time, at a scale of ~50km. We report the detection of a large, ~250km diameter surface feature for which we propose the name ``Piazzi''; however it is presently uncertain if this feature is due to a crater, albedo variegation, or other effect. From limb fits to the images, we obtain semi-major and semi-minor axes of R_1=484.8+/-5.1km and R_2=466.4+/-5.9km, respectively, for the illumination-corrected projected ellipsoid. Although albedo features are seen, they do not allow for a definitive determination of the rotation or pole positions of Ceres, particularly because of the sparse sampling (two epochs) of the 9 hour rotation period. From full-disk integrated albedo measurements, we find that Ceres has a red spectral slope from the mid- to near-UV, and a significant blue slope shortward of the mid-UV. In spite of the presence of Piazzi, we detect no significant global differences in the integrated albedo as a function of rotational phase for the two epochs of data we obtained. From Minnaert surface fits to the near- and mid-UV images, we find an unusually large Minnaert parameter of k~0.9, suggesting a more Lambertian than lunar-like surface., Comment: 9 pages, 4 figures, in AASTeX v5.02 document class with "emulateapj5" and "onecolfloat" styles (onecolfloat.sty is included). To appear in the Astronomical Journal (January 2002). Also available at http://www.boulder.swri.edu/~joel/papers.html

We present a new analysis of the currently available orbital elements for the known Kuiper belt objects. In the non-resonant, main Kuiper belt we find a statistically significant relationship between an object's absolute magnitude (H) and its inclination (i). Objects with H<6.5 (i.e. radii >~170 km for a 4% albedo) have higher inclinations than those with H>6.5 (radii >~ 170 km). We have shown that this relationship is not caused by any obvious observational bias. We argue that the main Kuiper belt consists of the superposition of two distinct distributions. One is dynamically hot with inclinations as large as \~35 deg and absolute magnitudes as bright as 4.5; the other is dynamically cold with i<~5 deg and H>6.5. The dynamically cold population is most likely dynamically primordial. We speculate on the potential causes of this relationship., Comment: 14 pages, 6 postscript figures