175 research outputs found
Data base on physical observations of near-Earth asteroids and establishment of a network to coordinate observations of newly discovered near-Earth asteroids
This program consists of two tasks: (1) development of a data base of physical observations of near-earth asteroids and establishment of a network to coordinate observations of newly discovered earth-approaching asteroids; and (2) a simulation of the surface of low-activity comets. Significant progress was made on task one and, and task two was completed during the period covered by this progress report
Spectral variability on primitive asteroids of the Themis and Beagle families: space weathering effects or parent body heterogeneity?
Themis is an old and statistically robust asteroid family populating the
outer main belt, and resulting from a catastrophic collision that took place
2.51.0 Gyr ago. Within the old Themis family a young sub-family, Beagle,
formed less than 10 Myr ago, has been identified. We present the results of a
spectroscopic survey in the visible and near infrared range of 22 Themis and 8
Beagle families members. The Themis members investigated exhibit a wide range
of spectral behaviors, while the younger Beagle family members look spectrally
bluer with a smaller spectral slope variability. The best meteorite spectral
analogues found for both Themis and Beagle families members are carbonaceous
chondrites having experienced different degrees of aqueous alteration,
prevalently CM2 but also CV3 and CI, and some of them are chondrite samples
being unusual or heated. We extended the spectral analysis including the data
available in the literature on Themis and Beagle families members, and we
looked for correlations between spectral behavior and physical parameters using
the albedo and size values derived from the WISE data. The analysis of this
larger sample confirm the spectral diversity within the Themis family and that
Beagle members tend to be bluer and to have an higher albedo. The differences
between the two family may be partially explained by space weathering
processes, which act on these primitive surfaces in a similar way than on
S-type asteroids, i.e. producing reddening and darkening. However we see
several Themis members having albedos and spectral slopes similar to the young
Beagle members. Alternative scenarios are proposed including heterogeneity in
the parent body having a compositional gradient with depth, and/or the survival
of projectile fragments having a different composition than the parent body.Comment: Manuscript pages: 40; Figures: 15 ; Tables: 4 Icarus (2016),in pres
Airborne 20-65 micron spectrophotometry of Comet Halley
Observations of Comet Halley with a grating spectrometer on board the Kuiper Airborne Observatory on four nights in Dec. 1985 to Apr. 1986 are reported. Low resolution 20 to 65 micrometer spectra of the nucleus with a 40 arcsec FWHM beam was obtained on 17 Dec. 1985, and on 15 and 17 Apr. 1986. On 20 Dec. 1985, only a 20 to 35 micrometer spectrum was obtained. Most of the data have been discussed in a paper where the continuum was dealt with. In that paper, models were fit to the continuum that showed that more micron sized particles of grain similar to amorphous carbon were needed to fit the spectrum than were allowed by the Vega SP-2 mass distribution, or that a fraction of the grains had to be made out of a material whose absorption efficiency fell steeper than lambda sup -1 for lambda greater than 20 micrometers. Spectra was also presented taken at several points on the coma on 15 Apr. which showed that the overall shape to the spectrum is the same in the coma. Tabulated values of the data and calibration curves are available. The spectral features are discussed
Spitzer observations of the asteroid-comet transition object and potential spacecraft target 107P (4015) Wilson-Harrington
Context. Near-Earth asteroid-comet transition object 107P/ (4015)
Wilson-Harrington is a possible target of the joint European Space Agency (ESA)
and Japanese Aerospace Exploration Agency (JAXA) Marco Polo sample return
mission. Physical studies of this object are relevant to this mission, and also
to understanding its asteroidal or cometary nature. Aims. Our aim is to obtain
significant new constraints on the surface thermal properties of this object.
Methods. We present mid-infrared photometry in two filters (16 and 22 microns)
obtained with NASA's Spitzer Space Telescope on February 12, 2007, and results
from the application of the Near Earth Asteroid Thermal Model (NEATM).We
obtained high S/N in two mid-IR bands allowing accurate measurements of its
thermal emission. Results. We obtain a well constrained beaming parameter (eta
= 1.39 +/- 0.26) and obtain a diameter and geometric albedo of D = 3.46 +/-
0.32 km, and pV = 0.059 +/- 0.011. We also obtain similar results when we apply
this best-fitting thermal model to single-band mid-IR photometry reported by
Campins et al. (1995), Kraemer et al. (2005) and Reach et al. (2007).
Conclusions. The albedo of 4015 Wilson-Harrington is low, consistent with those
of comet nuclei and primitive C-, P-, D-type asteorids. We establish a rough
lower limit for the thermal inertia of W-H of 60 Jm^-2s^(-0.5)K^-1 when it is
at r=1AU, which is slightly over the limit of 30 Jm^-2s^(-0.5)K-1 derived by
Groussin et al. (2009) for the thermal inertia of the nucleus of comet
22P/Kopff.Comment: 4 pages, 1 figure and 3 tables. Paper accepted for publicatio
Overcoming the Challenges Associated with Image-based Mapping of Small Bodies in Preparation for the OSIRIS-REx Mission to (101955) Bennu
The OSIRIS-REx Asteroid Sample Return Mission is the third mission in NASA's
New Frontiers Program and is the first U.S. mission to return samples from an
asteroid to Earth. The most important decision ahead of the OSIRIS-REx team is
the selection of a prime sample-site on the surface of asteroid (101955) Bennu.
Mission success hinges on identifying a site that is safe and has regolith that
can readily be ingested by the spacecraft's sampling mechanism. To inform this
mission-critical decision, the surface of Bennu is mapped using the OSIRIS-REx
Camera Suite and the images are used to develop several foundational data
products. Acquiring the necessary inputs to these data products requires
observational strategies that are defined specifically to overcome the
challenges associated with mapping a small irregular body. We present these
strategies in the context of assessing candidate sample-sites at Bennu
according to a framework of decisions regarding the relative safety,
sampleability, and scientific value across the asteroid's surface. To create
data products that aid these assessments, we describe the best practices
developed by the OSIRIS-REx team for image-based mapping of irregular small
bodies. We emphasize the importance of using 3D shape models and the ability to
work in body-fixed rectangular coordinates when dealing with planetary surfaces
that cannot be uniquely addressed by body-fixed latitude and longitude.Comment: 31 pages, 10 figures, 2 table
Comet 162P/Siding Spring: A Surprisingly Large Nucleus
We present an analysis of thermal emission from comet 162P/Siding Spring
(P/2004 TU12) measured during its discovery apparition in 2004 December. The
comet showed no dust coma at this time, so we have sampled emission from the
comet's nucleus. Observations using the Mid-Infrared Spectrometer and Imager
(MIRSI) were performed at NASA's Infrared Telescope Facility, where the peak of
the comet's spectral energy distribution was observed between 8 and 25 microns.
In combination with the three near-IR spectra presented by Campins et al.
(2006, Astron. J. 132, 1346) that show the Wien-law tail of the thermal
emission, the data provide powerful constraints on surface properties of the
nucleus. We find that the nucleus's effective radius is 6.0+/-0.8 km. This is
one of the largest radii known among Jupiter-family comets, which is unusual
considering that the comet was discovered only recently. Its geometric albedo
is 0.059+/-0.023 in the H band, 0.037+/-0.014 in the R band, and 0.034+/-0.013
in the V band. We also find that the nucleus of 162P has little IR beaming, and
this implies that the nucleus has low thermal inertia. Including all near-IR
spectra yields a beaming parameter of 1.01+/-0.20. This result is in agreement
with others showing that cometary nuclei have low thermal inertia and little IR
beaming. If confirmed for many nuclei, the interpretation of radiometry may not
be as problematic as feared.Comment: 17 pages, 4 figures, appearing in The Astronomical Journal, September
2006 issu
Risk Factors for Hepatitis C Virus Transmission to Health Care Workers after Occupational Exposure: A European Case-Control Study
Background. Additional studies are required to identify risk factors for hepatitis C virus (HCV) transmission to health care workers after occupational exposure to HCV. Methods. We conducted a matched case-control study in 5 European countries from 1 January 1991 through 31 December 2002. Case patients were health care workers who experienced seroconversion after percutaneous or mucocutaneous exposure to HCV. Control subjects were HCV-exposed health care workers who did not experience seroconversion and were matched with case patients for center and period of exposure. Results. Sixty case patients and 204 control subjects were included in the study. All case patients were exposed to HCV-infected fluids through percutaneous injuries. The 37 case patients for whom information was available were exposed to viremic source patients. As risk factors for HCV infection, multivariate analysis identified needle placement in a source patient's vein or artery (odds ratio [OR], 100.1; 95% confidence interval [CI], 7.3-1365.7), deep injury (OR, 155.2; 95% CI, 7.1-3417.2), and sex of the health care worker (OR for male vs. female, 3.1; 95% CI, 1.0-10.0). Source patient HCV load was not introduced in the multivariate model. In unmatched univariate analysis, the risk of HCV transmission increased 11-fold for health care workers exposed to source patients with a viral load >6 log10 copies/mL (95% CI, 1.1-114.1), compared with exposures to source patients with a viral load ⩽4 log10 copies/mL. Conclusion. In this study, HCV occupational transmission was found to occur after percutaneous exposures. The risk of HCV transmission after percutaneous exposure increased with deep injuries and procedures involving hollow-bore needle placement in the source patient's vein or artery. These results highlight the need for widespread adoption of needlestick-prevention devices in health care settings, together with other preventive measure
Circumstellar material in the Vega inner system revealed by CHARA/FLUOR
Only a handful of debris disks have been imaged up to now. Due to the need
for high dynamic range and high angular resolution, very little is known about
the inner planetary region, where small amounts of warm dust are expected to be
found. We investigate the close neighbourhood of Vega with the help of infrared
stellar interferometry and estimate the integrated K-band flux originating from
the central 8 AU of the debris disk. We performed precise visibility
measurements at both short (~30 m) and long (~150 m) baselines with the FLUOR
beam-combiner installed at the CHARA Array (Mt Wilson, California) in order to
separately resolve the emissions from the extended debris disk (short
baselines) and from the stellar photosphere (long baselines). After revising
Vega's K-band angular diameter (3.202+/-0.005 mas), we show that a significant
deficit in squared visibility (1.88+/-0.34%) is detected at short baselines
with respect to the best-fit uniform disk stellar model. This deficit can be
either attributed to the presence of a low-mass stellar companion around Vega,
or as the signature of the thermal and scattered emissions from the debris
disk. We show that the presence of a close companion is highly unlikely, as
well as other possible perturbations (stellar morphology, calibration), and
deduce that we have most probably detected the presence of dust in the close
neighbourhood of Vega. The resulting flux ratio between the stellar photosphere
and the debris disk amounts to 1.29+/-0.19% within the FLUOR field-of-view
(~7.8 AU). Finally, we complement our K-band study with archival photometric
and interferometric data in order to evaluate the main physical properties of
the inner dust disk. The inferred properties suggest that the Vega system could
be currently undergoing major dynamical perturbations.Comment: A&A, accepted -- Press release available at
http://www2.cnrs.fr/presse/communique/848.ht
Recommended from our members
Widespread carbon-bearing materials on near-Earth asteroid (101955) Bennu
(101955) Bennu is a dark asteroid on an Earth-crossing orbit, thought to have assembled from the fragments of an ancient collision. We use spatially-resolved visible and near-infrared spectra of Bennu to investigate its surface properties and composition. In addition to a hydrated phyllosilicate band, we detect a ubiquitous 3.4-micron absorption feature, which we attribute to a mix of organic and carbonate materials. The shape and depth of this absorption feature vary across Bennu’s surface, spanning the range seen among similar main-belt asteroids. Its distribution does not correlate with temperature, reflectance, spectral slope, or hydrated minerals, although some of those characteristics correlate with each other. The deepest 3.4-micron absorptions occur on individual boulders. The variations may be due to differences in abundance, recent exposure, or space weathering
- …