945 research outputs found
The Short Rotation Period of Nereid
We determine the period, p = 11.52 \pm 0.14 h, and a light curve peak-to-peak
amplitude, a = 0.029 \pm 0.003 magnitudes, of the Neptunian irregular satellite
Nereid. If the light curve variation is due to albedo variations across the
surface, rather than solely to the shape of Nereid variations, the rotation
period would be a factor of two shorter. In either case, such a rotation period
and light curve amplitude, together with Nereid's orbital period, p=360.14
days, imply that Nereid is almost certainly in a regular rotation state, rather
than the chaotic rotation state suggested by Schaefer and Schaefer (1988,2000)
and Dobrovolskis (1995).
Assuming that Nereid is perfectly spherical, the albedo variation is 3%
across the observed surface. Assuming a uniform geometric albedo, the observed
cross sectional area varies by 3%. We caution that the lightcurve found in this
paper only sets limits on the combination of albedo and physical irregularity
and that we cannot determine the orientation of Nereid's spin axis from our
data.Comment: Accepted by ApJ Letters, 11 pages (incl. 1 figure
Chiron: Evidence for historic cometary activity
The non-asteroidal brightening of (2060) Chiron, first noted by Tholen in 1988 is now ascribed to cometary activity. Photometry since 1988 has revealed a broad surge in brightness that peaked in 1989 about 1.0 mag above the brightness in the mid-1980s. The surge is evidently due to sporatic formation of dust coma, which is itself driven by the presence of extremely volatile ices at or near the surface. CN emission was recently reported. Since Chiron is now nearing perihelion, there is interest in determining whether it has exhibited anomalous brightening in the past, particularly at greater heliocentric distances. Photographic plates dating back to 1895 are known to contain images of Chiron. Using some of these archival material, the initial results are presented for a project to determine Chiron's brightness history over orbital timescales. A particularly homogeneous and high-quality set of plates taken prior to and around the time of Chiron's discovery in Oct. 1977 at the 1.2 m Oschin Schmidt telescope at Mt. Palomar Observatory were examined. Images of Chiron were identified and digitized using a PDS microdensitometer, and images of field stars around Chiron were both similarly digitized and photometrically calibrated using recently acquired B and V band CCD frames. As a result of the present work, eleven new data, including estimated errors, were added between 1969 and 1977. The implications that Chiron can be active at any heliocentric distance in its present orbit suggest that the active volatile is either N2, CH4, or CO, and that a substantial degree of mantling may have developed. Further historical data is presented, the error bars discussed, and possible mechanisms suggested for the observed activity
Spectral and Spin Measurement of Two Small and Fast-Rotating Near-Earth Asteroids
In May 2012 two asteroids made near-miss "grazing" passes at distances of a
few Earth-radii: 2012 KP24 passed at nine Earth-radii and 2012 KT42 at only
three Earth-radii. The latter passed inside the orbital distance of
geosynchronous satellites. From spectral and imaging measurements using NASA's
3-m Infrared Telescope Facility (IRTF), we deduce taxonomic, rotational, and
physical properties. Their spectral characteristics are somewhat atypical among
near-Earth asteroids: C-complex for 2012 KP24 and B-type for 2012 KT42, from
which we interpret the albedos of both asteroids to be between 0.10 and 0.15
and effective diameters of 20+-2 and 6+-1 meters, respectively. Among B-type
asteroids, the spectrum of 2012 KT42 is most similar to 3200 Phaethon and 4015
Wilson-Harrington. Not only are these among the smallest asteroids spectrally
measured, we also find they are among the fastest-spinning: 2012 KP24 completes
a rotation in 2.5008+-0.0006 minutes and 2012 KT42 rotates in 3.634+-0.001
minutes.Comment: 4 pages, 3 figures, accepted for publication in Icaru
The Composition of M-type asteroids II: Synthesis of spectroscopic and radar observations
This work updates and expands on results of our long-term radar-driven
observational campaign of main-belt asteroids (MBAs) focused on Bus-DeMeo Xc-
and Xk-type objects (Tholen X and M class asteroids) using the Arecibo radar
and NASA Infrared Telescope Facilities (Ockert-Bell et al. 2008; 2010; Shepard
et al. 2008; 2010). Eighteen of our targets were near-simultaneously observed
with radar and those observations are described in Shepard et al. (2010). We
combine our near-infrared data with available visible wavelength data for a
more complete compositional analysis of our targets. Compositional evidence is
derived from our target asteroid spectra using two different methods, a \c{hi}2
search for spectral matches in the RELAB database and parametric comparisons
with meteorites. We present four new methods of parametric comparison,
including discriminant analysis. Discriminant analysis identifies meteorite
type with 85% accuracy. This paper synthesizes the results of these two analog
search algorithms and reconciles those results with analogs suggested from
radar data (Shepard et al. 2010). We have observed 29 asteroids, 18 in
conjunction with radar observations. For eighteen out of twenty-nine objects
observed (62%) our compositional predictions are consistent over two or more
methods applied. We find that for our Xc and Xk targets the best fit is an iron
meteorite for 34% of the samples. Enstatite Chondrites were best fits for 6 of
our targets (21%). Stony-iron meteorites were best fits for 2 of our targets
(7%). A discriminant analysis suggests that asteroids with no absorption band
can be compared to iron meteorites and asteroids with both a 0.9 and 1.9 {\mu}m
absorption band can be compared to stony-iron meteorites.Comment: 30 pages, 5 figures, 10 table
NEOWISE Studies of Spectrophotometrically Classified Asteroids: Preliminary Results
The NEOWISE dataset offers the opportunity to study the variations in albedo
for asteroid classification schemes based on visible and near-infrared
observations for a large sample of minor planets. We have determined the
albedos for nearly 1900 asteroids classified by the Tholen, Bus and Bus-DeMeo
taxonomic classification schemes. We find that the S-complex spans a broad
range of bright albedos, partially overlapping the low albedo C-complex at
small sizes. As expected, the X-complex covers a wide range of albedos. The
multi-wavelength infrared coverage provided by NEOWISE allows determination of
the reflectivity at 3.4 and 4.6 m relative to the visible albedo. The
direct computation of the reflectivity at 3.4 and 4.6 m enables a new
means of comparing the various taxonomic classes. Although C, B, D and T
asteroids all have similarly low visible albedos, the D and T types can be
distinguished from the C and B types by examining their relative reflectance at
3.4 and 4.6 m. All of the albedo distributions are strongly affected by
selection biases against small, low albedo objects, as all objects selected for
taxonomic classification were chosen according to their visible light
brightness. Due to these strong selection biases, we are unable to determine
whether or not there are correlations between size, albedo and space
weathering. We argue that the current set of classified asteroids makes any
such correlations difficult to verify. A sample of taxonomically classified
asteroids drawn without significant albedo bias is needed in order to perform
such an analysis.Comment: Accepted to Ap
First results from the MIT optical rapid imaging system (MORIS) on the IRTF: A stellar occultation by Pluto and a transit by exoplanet XO-2b
We present a high-speed, visible-wavelength imaging instrument: MORIS (the MIT Optical Rapid Imaging System). MORIS is mounted on the 3 m Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Its primary component is an Andor iXon camera, a nearly 60" square field of view with high quantum efficiency, low read noise, low dark current, and full-frame readout rates ranging from as slow as desired to a maximum of between 3.5 Hz and 35 Hz (depending on the mode; read noise of 6 pixel and 49 pixel with electron-multiplying gain = 1 , respectively). User-selectable binning and subframing can increase the cadence to a few hundred hertz. An electron-multiplying mode can be employed for photon counting, effectively reducing the read noise to subelectron levels at the expense of dynamic range. Data cubes, or individual frames, can be triggered to several-nanosecond accuracy using the Global Positioning System. MORIS is mounted on the side-facing exit window of SpeX, allowing simultaneous near-infrared and visible observations. Here, we describe the components, setup, and measured characteristics of MORIS. We also report results from the first science observations: the 2008 June 24 stellar occultation by Pluto and an extrasolar planetary transit by XO-2b. The Pluto occultation of a 15.8 magnitude star has a signal-to-noise ratio of 35 per atmospheric scale height and a midtime error of 0.32 s. The XO-2b transit reaches photometric precision of 0.5 mmag in 2 minutes and has a midtime timing precision of 23 s.United States. National Aeronautics and Space Administration (grant NNX07AK95G
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