52 research outputs found
Near infrared reflectance spectra: Applications to problems in asteroid-meteorite relationships
An observing program designed to search for evidence of ordinary chondrite parent bodies near the 3:1 Kirkwood Gap was carried out in 1985 and 1986. Studies by Wisdom (1985), Wetherill (1985), and subsequent work by Milani et al. (1989) indicate that the 3:1 Kirkwood gap is the most probable source region for the majority of ordinary chondrite meteorites. The diversity of the reflectance spectra among this small data set is surprising. Early work by Gaffey and McCord (1978) showed that the inner region of the main asteroid belt is dominated by high albedo objects with mafic silicate surfaces. One would expect to see mostly spectra with 1- and 2-micron absorption bands based on this earlier work. Only 5 (of 12) spectra have these expected features. The distribution of taxonomic types presented by Gradie and Tedesco (1982) is in most cases a useful simplification of the compositional structure of the asteroid belt. The range of spectral characteristics seen with higher resolution in the near-IR has not been previously reported and is not represented in the standard asteroid taxonomy. Near-IR spectra contain valuable mineralogical information which enhances knowledge of the composition and structure of asteroids
Ceres' opposition effect observed by the Dawn framing camera
The surface reflectance of planetary regoliths may increase dramatically
towards zero phase angle, a phenomenon known as the opposition effect (OE). Two
physical processes that are thought to be the dominant contributors to the
brightness surge are shadow hiding (SH) and coherent backscatter (CB). The
occurrence of shadow hiding in planetary regoliths is self-evident, but it has
proved difficult to unambiguously demonstrate CB from remote sensing
observations. One prediction of CB theory is the wavelength dependence of the
OE angular width. The Dawn spacecraft observed the OE on the surface of dwarf
planet Ceres. We characterize the OE over the resolved surface, including the
bright Cerealia Facula, and to find evidence for SH and/or CB. We analyze
images of the Dawn framing camera by means of photometric modeling of the phase
curve. We find that the OE of most of the investigated surface has very similar
characteristics, with an enhancement factor of 1.4 and a FWHM of 3{\deg} (broad
OE). A notable exception are the fresh ejecta of the Azacca crater, which
display a very narrow brightness enhancement that is restricted to phase angles
{\deg} (narrow OE); suggestively, this is in the range in which CB is
thought to dominate. We do not find a wavelength dependence for the width of
the broad OE, and lack the data to investigate the dependence for the narrow
OE. The prediction of a wavelength-dependent CB width is rather ambiguous. The
zero-phase observations allow us to determine Ceres' visible geometric albedo
as . A comparison with other asteroids suggests that
Ceres' broad OE is typical for an asteroid of its spectral type, with
characteristics that are primarily linked to surface albedo. Our analysis
suggests that CB may occur on the dark surface of Ceres in a highly localized
fashion.Comment: Credit: Schr\"oder et al, A&A in press, 2018, reproduced with
permission, \copyright ES
Moon Search Algorithms for NASA's Dawn Mission to Asteroid Vesta
A moon or natural satellite is a celestial body that orbits a planetary body such as a planet, dwarf planet, or an asteroid. Scientists seek understanding the origin and evolution of our solar system by studying moons of these bodies. Additionally, searches for satellites of planetary bodies can be important to protect the safety of a spacecraft as it approaches or orbits a planetary body. If a satellite of a celestial body is found, the mass of that body can also be calculated once its orbit is determined. Ensuring the Dawn spacecraft's safety on its mission to the asteroid Vesta primarily motivated the work of Dawn's Satellite Working Group (SWG) in summer of 2011. Dawn mission scientists and engineers utilized various computational tools and techniques for Vesta's satellite search. The objectives of this paper are to 1) introduce the natural satellite search problem, 2) present the computational challenges, approaches, and tools used when addressing this problem, and 3) describe applications of various image processing and computational algorithms for performing satellite searches to the electronic imaging and computer science community. Furthermore, we hope that this communication would enable Dawn mission scientists to improve their satellite search algorithms and tools and be better prepared for performing the same investigation in 2015, when the spacecraft is scheduled to approach and orbit the dwarf planet Ceres
Ultraviolet Spectroscopy of Asteroid (4) Vesta
We report a comprehensive review of the UV-visible spectrum and rotational
lightcurve of Vesta combining new observations by Hubble Space Telescope and
Swift Gamma-ray Burst Observatory with archival International Ultraviolet
Explorer observations. The geometric albedos of Vesta from 220 nm to 953 nm are
derived by carefully comparing these observations from various instruments at
different times and observing geometries. Vesta has a rotationally averaged
geometric albedo of 0.09 at 250 nm, 0.14 at 300 nm, 0.26 at 373 nm, 0.38 at 673
nm, and 0.30 at 950 nm. The linear spectral slope as measured between 240 and
320 nm in the ultraviolet displays a sharp minimum near a sub-Earth longitude
of 20^{\circ}, and maximum in the eastern hemisphere. This is consistent with
the longitudinal distribution of the spectral slope in the visible wavelength.
The photometric uncertainty in the ultraviolet is ~20%, and in the visible
wavelengths it is better than 10%. The amplitude of Vesta's rotational
lightcurves is ~10% throughout the range of wavelengths we observed, but is
smaller at 950 nm (~6%) near the 1-\mum band center. Contrary to earlier
reports, we found no evidence for any difference between the phasing of the
ultraviolet and visible/near-infrared lightcurves with respect to sub-Earth
longitude. Vesta's average spectrum between 220 and 950 nm can well be
described by measured reflectance spectra of fine particle howardite-like
materials of basaltic achondrite meteorites. Combining this with the in-phase
behavior of the ultraviolet, visible, and near-infrared lightcurves, and the
spectral slopes with respect to the rotational phase, we conclude that there is
no global ultraviolet/visible reversal on Vesta. Consequently, this implies a
lack of global space weathering on Vesta, as previously inferred from
visible-near-infrared data.Comment: 44 pages, 5 figures, 1 tabl
Comparing Dawn, Hubble Space Telescope, and Ground-Based Interpretations of (4) Vesta
Observations of asteroid 4 Vesta by NASA's Dawn spacecraft are interesting
because its surface has the largest range of albedo, color and composition of
any other asteroid visited by spacecraft to date. These hemispherical and
rotational variations in surface brightness and composition have been
attributed to impact processes since Vesta's formation. Prior to Dawn's arrival
at Vesta, its surface properties were the focus of intense telescopic
investigations for nearly a hundred years. Ground-based photometric and
spectroscopic observations first revealed these variations followed later by
those using Hubble Space Telescope. Here we compare interpretations of Vesta's
rotation period, pole, albedo, topographic, color, and compositional properties
from ground-based telescopes and HST with those from Dawn. Rotational spectral
variations observed from ground-based studies are also consistent with those
observed by Dawn. While the interpretation of some of these features was
tenuous from past data, the interpretations were reasonable given the
limitations set by spatial resolution and our knowledge of Vesta and HED
meteorites at that time. Our analysis shows that ground-based and HST
observations are critical for our understanding of small bodies and provide
valuable support for ongoing and future spacecraft missions.Comment: Pages: 51, Figures: 9, Tables:
Evaluating Health Workers' Potential Resistance to New Interventions: A Role for Discrete Choice Experiments
BACKGROUND: The currently recommended approach for preventing malaria in pregnancy (MiP), intermittent preventive treatment with sulphadoxine-pyrimethamine (SP-IPT), has been questioned due to the spread of resistance to SP. Whilst trials are underway to test the efficacy of future alternative approaches, it is important to start exploring the feasibility of their implementation. METHODS AND FINDINGS: This study uses a discrete choice experiment (DCE) method to assess the potential resistance of health workers to changing strategies for control of MiP. In Ashanti region in Ghana, 133 antenatal clinic health workers were presented with 16 choice sets of two alternative policy options, each consisting of a bundle of six attributes representing certain clinical guidelines for controlling MiP (type of approach and drug used), possible associated maternal and neo-natal outcomes, workload and financial incentives. The data were analysed using a random effects logit model. Overall, staff showed a preference for a curative approach with pregnant women tested for malaria parasites and treated only if positive, compared to a preventive approach (OR 1.6; p = 0.001). Increasing the incidence of low birth weight or severe anaemia by 1% would reduce the odds of preferring an approach by 18% and 10% respectively. Midwives were more resistant to potential changes to current guidelines than lower-level cadres. CONCLUSIONS: In Ashanti Region, resistance to change by antenatal clinic workers from a policy of SP-IPT to IST would generally be low, and it would disappear amongst midwives if health outcomes for the mother and baby were improved by the new strategy. DCEs are a promising approach to identifying factors that will increase the likelihood of effective implementation of new interventions immediately after their efficacy has been proven
Shape, density, and geology of the nucleus of Comet 103P/Hartley 2
a b s t r a c t Data from the Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) mission show Comet 103P/Hartley 2 is a bi-lobed, elongated, nearly axially symmetric comet 2.33 km in length. Surface features are primarily small mounds <40 m across, irregularly-shaped smooth areas on the two lobes, and a smooth but variegated region forming a ''waist'' between the two lobes. Assuming parts of the comet body approach the shape of an equipotential surface, the mean density of Hartley 2 is modeled to be 200-400 kg m À3 . Such a mean density suggests mass loss per orbit of >1%. The shape may be the evolutionary product of insolation, sublimation, and temporary deposition of materials controlled by the object's complex rotation
Reflectance spectroscopy of diogenite meteorite types from Antarctica and their relationship to asteroids
Spectral reflectance measurements (0.4-2.5μm) of 3 diogenite meteorites found in Antarctica are examined and compared with previous measurements of diogenites to correlate mineralogy and petrography with the spectral characteristics that can also be measured on solar system objects of unknown composition. The Antarctic diogenites have the same spectral characteristics as other diogenites, however there are measurable differences among all diogenite spectra. Analysis of the spectra into sums of gaussian components reveals bands due to spin-forbidden transitions of (Fe)^ ions in pyroxene which are identified for the first time in reflectance spectra of powdered samples. From these laboratory measurements it is shown that spectral reflectance measurements of asteroids and other solar system objects made with the visible CVF spectrometer used for laboratory measurements is capable of providing useful information on their surface composition. The absolute calibration of the relationship between pyroxene chemistry and absorption band position of the 0.9 and 2.0μm bands (ADAMS, J.Geophys. Res., 79,4829,1974) should be recalibrated using new laboratory measurements and for numerically analyzed bands. The trend of the old calibration is not affected. The under-standing of the behavior of reflectance spectra of diogenites gained from this study will be used in a future report to analyze the surface composition of the near-earth asteroid 1915 Quetzalcoatl, which has a diogenite-like reflectance spectrum
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