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TOPIC: Asteroids
Blobrana


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Posts: 131433
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RE: Asteroids
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Title: Density of asteroids
Authors: Benoit Carry

A considerable amount of information regarding the processes that occurred during the accretion of the early planetesimals is still present among the small bodies of our solar system. A review of our current knowledge of the density of small bodies is presented here. Intrinsic physical properties of small bodies are sought by searching for relationships between the dynamical and taxonomic classes, size, and density. Mass and volume estimates for 287 small bodies are collected from the literature. The accuracy and biases affecting the methods used to estimate these quantities are discussed and best-estimates are strictly selected. Bulk densities are subsequently computed and compared with meteorite density, allowing to estimate the macroporosity within these bodies. Dwarf-planets apparently have no macroporosity, while smaller bodies can have large voids. This trend is apparently correlated with size: C and S-complex asteroids tends to have larger density with increasing diameter. The average density of each Bus-DeMeo taxonomic classes is computed. S-complex asteroids are more dense on average than those in the C-complex that in turn have a larger macroporosity, although both complexes partly overlap. Within the C-complex, B-types stand out in albedo, reflectance spectra, and density, indicating a unique composition. Asteroids in the X-complex span a wide range of densities, suggesting that many compositions are included in the complex. Comets and TNOs have high macroporosity and low density, supporting the current models of internal structures made of icy aggregates. The number of density estimates sky-rocketed during last decade from a handful to 287, but only a third of the estimates are more precise than 20%. Several lines of investigation to refine this are contemplated, including observations of multiple systems, 3-D shape modelling, and orbital analysis from Gaia astrometry.

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Blobrana


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Posts: 131433
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AVAST Survey
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Title: AVAST Survey 0.4-1.0 m Spectroscopy of Igneous Asteroids in the Inner and Middle Main Belt
Authors: Mark Hammergren, Michael Solontoi, Geza Gyuk, Andrew Puckett

We present the spectra of 60 asteroids, including 47 V-types observed during the first phase of the Adler V-Type Asteroid (AVAST) Survey. SDSS photometry was used to select candidate V-type asteroids for follow up by nature of their very blue i - z colour. 47 of the 61 observed candidates were positively classified as V-type asteroids, while an additional six show indications of a 0.9 {\mu}m feature consistent with V-type spectra, but not sufficient for formal classification. Four asteroids were found to be S-type, all of which had i - z values very near the adopted AVAST selection criteria of i - z \leq -0.2, including one candidate observed well outside the cut (at a mean i - z of -0.11). Three A-type asteroids were also identified. Six V-type asteroids were identified beyond the 3:1 mean motion resonance with Jupiter, and six more were found with low (< 5 deg) inclination, placing these asteroids outside of the normal dynamical range of classic Vestoids, and are suggestive of a non-Vesta origin for at least some of the population.

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Blobrana


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Posts: 131433
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RE: Asteroids
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Title: Identification of known objects in solar system surveys
Authors: Andrea Milani, Zoran Knezevic, Davide Farnocchia, Fabrizio Bernardi, Robert Jedicke, Larry Denneau, Richard J. Wainscoat, William Burgett, Tommy Grav, Nick Kaiser, Eugene Magnier, Paul A. Price

The discovery of new objects in modern wide-field asteroid and comet surveys can be enhanced by first identifying observations belonging to known solar system objects. The assignation of new observations to a known object is an attribution problem that occurs when a least squares orbit already exists for the object but a separate fit is not possible to just the set of new observations. In this work we explore the strongly asymmetric attribution problem in which the existing least squares orbit is very well constrained and the new data are sparse. We describe an attribution algorithm that introduces new quality control metrics in the presence of strong biases in the astrometric residuals. The main biases arise from the stellar catalogues used in the reduction of asteroid observations and we show that a simple debiasing with measured regional catalogue biases significantly improves the results. We tested the attribution algorithm using data from the PS1 survey that used the 2MASS star catalogue for the astrometric reduction. We found small but statistically significant biases in the data of up to 0.1 arcsec that are relevant only when the observations reach the level of accuracy made possible by instruments like PS1. The false attribution rate was measured to be < 1/1000 with a simple additional condition that can reduce it to zero while the attribution efficiency is consistent with 100%.

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Blobrana


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Title: NEOWISE Studies of Asteroids with Sloan Photometry: Preliminary Results
Authors: A. Mainzer, J. Masiero, T. Grav, J. Bauer, D. J. Tholen, R. S. McMillan, E. Wright, T. Spahr, R. M. Cutri, R. Walker, W. Mo, J. Watkins, E. Hand, C. Maleszewski

We have combined the NEOWISE and Sloan Digital Sky Survey data to study the albedos of 24,353 asteroids with candidate taxonomic classifications derived using Sloan photometry. We find a wide range of moderate to high albedos for candidate S-type asteroids that are analogous to the S-complex defined by previous spectrophotometrically-based taxonomic systems. The candidate C-type asteroids, while generally very dark, have a tail of higher albedos that overlaps the S types. The albedo distribution for asteroids with a photometrically derived Q classification is extremely similar to those of the S types. Asteroids with similar colours to (4) Vesta have higher albedos than the S types, and most have orbital elements similar to known Vesta family members. Finally, we show that the relative reflectance at 3.4 and 4.6 \mu m is higher for D-type asteroids and suggest that their red visible and near-infrared spectral slope extends out to these wavelengths. Understanding the relationship between size, albedo, and taxonomic classification is complicated by the fact that the objects with classifications were selected from the visible/near-infrared Sloan Moving Object Catalogue, which is biased against fainter asteroids, including those with lower albedos.

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Blobrana


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Posts: 131433
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Ceres and Vesta
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Title: A population of main belt asteroids co-orbiting with Ceres and Vesta
Authors: Apostolos A. Christou, Paul Wiegert

We have carried out a search for Main Belt Asteroids (MBAs) co-orbiting with the large MBA Vesta and the dwarf planet Ceres. Through improving the search criteria used in (Christou, 2000b) and numerical integrations of candidate coorbitals, we have identified approximately 51 (44) objects currently in co-orbital libration with Ceres (Vesta). We show that these form part of a larger population of transient coorbitals; 129 (94) MBAs undergo episodes of co-orbital libration with Ceres (Vesta) within a 2 Myr interval centred on the present. The lifetime in the resonance is typically a few times 10^5 yr butcan exceed 2*10^6 yr. The variational properties of the orbits of several co-orbitals were examined. It was found that their present states with respect to the secondary are well determined but knowledge of it is lost typically after ~2*10^5 years. Objects initially deeper into the coorbital region maintain their coorbital state for longer. Using the model of Namouni et al. (1999) we show that their dynamics are similar to those of temporary coorbital NEAs of the Earth and Venus. As in that case, the lifetime of resonant libration is dictated by planetary secular perturbations, the inherent chaoticity of the orbits and close encounters with massive objects other than the secondary. In particular we present evidence that, while in the coorbital state, close encounters with the secondary are generally avoided and that Ceres affects the stability of tadpole librators of Vesta. Finally we demonstrate the existence of Quasi-satellite orbiters of both Ceres and Vesta and conclude that decametre-sized objects detected in the vicinity of Vesta by the DAWN mission may, in fact, belong to this dynamical class rather than be bona-fide (i.e. keplerian) satellites of Vesta.

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Blobrana


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Posts: 131433
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RE: Asteroids
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Title: Mass Determination Studies of 104 Large Asteroids
Authors: Zielenbach, William

The techniques described in an earlier paper were used to determine masses of 104 asteroids by the method of asteroid-asteroid gravitational interaction. For each of the 104 perturbers, 4 large sets of test particles selected by different criteria were used to calculate 4 mass values from a weighted mean of individual results within each set. The sheer number of test particles and observations ameliorates the effects of random observational errors and the type of systematic errors known to have affected specific observatories at specific times. It also reduces the effect of mismodelled attractions by perturbers other than the one being estimated, because the various test particles are affected to different degrees and in different directions. For most of the perturbers that have been analysed by others, the results of this study agree reasonably well with values published in the past decade, giving credence to the approach. Thirty-eight of the results appear to be the first published masses for the respective asteroids, and 12 are the first determinations based on asteroid-asteroid interactions. Unrealistic and/or negative masses were obtained for some perturbers. Causes for this phenomenon are discussed and various means to obtain reasonable numbers are evaluated.

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Blobrana


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Title: Main Belt Asteroids with WISE/NEOWISE I: Preliminary Albedos and Diameters
Authors: Joseph R. Masiero, A. K. Mainzer, T. Grav, J. M. Bauer, R. M. Cutri, J. Dailey, P. R. M. Eisenhardt, R. S. McMillan, T. B. Spahr, M. F. Skrutskie, D. Tholen, R. G. Walker, E. L. Wright, E. DeBaun, D. Elsbury, T. Gautier IV, S. Gomillion, A. Wilkins

We present initial results from the Wide-field Infrared Survey Explorer (WISE), a four-band all-sky thermal infrared survey that produces data well suited to measuring the physical properties of asteroids, and the NEOWISE enhancement to the WISE mission allowing for detailed study of Solar system objects. Using a NEATM thermal model fitting routine we compute diameters for over 100,000 Main Belt asteroids from their IR thermal flux, with errors better than 10%. We then incorporate literature values of visible measurements (in the form of the H absolute magnitude) to determine albedos. Using these data we investigate the albedo and diameter distributions of the Main Belt. As observed previously, we find a change in the average albedo when comparing the inner, middle, and outer portions of the Main Belt. We also confirm that the albedo distribution of each region is strongly bimodal. We observe groupings of objects with similar albedos in regions of the Main Belt associated with dynamical breakup families. Asteroid families typically show a characteristic albedo for all members, but there are notable exceptions to this. This paper is the first look at the Main Belt asteroids in the WISE data, and only represents the preliminary, observed raw size and albedo distributions for the populations considered. These distributions are subject to survey biases inherent to the NEOWISE dataset and cannot yet be interpreted as describing the true populations; the debiased size and albedo distributions will be the subject of the next paper in this series.

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Blobrana


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Posts: 131433
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Asteroid families
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Title: Families among high-inclination asteroids
Authors: Bojan Novakovic, Alberto Cellino, Zoran Knezevic

We present a new classification of families identified among the population of high-inclination asteroids. We computed synthetic proper elements for a sample of 18,560 numbered and multi-opposition objects having sine of proper inclination greater than 0.295. We considered three zones at different heliocentric distances (inner, intermediate and outer region) and used the standard approach based on the Hierarchical Clustering Method (HCM) to identify families in each zone. In doing so, we used slightly different approach with respect to previously published methodologies, to achieve a more reliable and robust classification. We also used available SDSS color data to improve membership and identify likely family interlopers. We found a total of 38 families, as well as a significant number of clumps and clusters deserving further investigation.

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Blobrana


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Posts: 131433
Date:
Main-Belt Asteroids
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Title: Size Distribution of Main-Belt Asteroids with High Inclination
Authors: Tsuyoshi Terai, Yoichi Itoh

We investigated the size distribution of high-inclination main-belt asteroids (MBAs) to explore asteroid collisional evolution under hypervelocity collisions of around 10 km/s. We performed a wide-field survey for high-inclination sub-km MBAs using the 8.2-m Subaru Telescope with the Subaru Prime Focus Camera (Suprime-Cam). Suprime-Cam archival data were also used. A total of 616 MBA candidates were detected in an area of 9.0 deg˛ with a limiting magnitude of 24.0 mag in the SDSS r filter. Most of candidate diameters were estimated to be smaller than 1 km. We found a scarcity of sub-km MBAs with high inclination. Cumulative size distributions (CSDs) were constructed using Subaru data and published asteroid catalogues. The power-law indexes of the CSDs were 2.17 ±0.02 for low-inclination (< 15 deg) MBAs and 2.02 ±0.03 for high-inclination (> 15 deg) MBAs in the 0.7-50 km diameter range. The high-inclination MBAs had a shallower CSD. We also found that the CSD of S-like MBAs had a small slope with high inclination, whereas the slope did not vary with inclination in the C-like group. The most probable cause of the shallow CSD of the high-inclination S-like MBAs is the large power-law index in the diameter-impact strength curve in hypervelocity collisions. The collisional evolution of MBAs may have advanced with oligopolistic survival during the dynamical excitation phase in the final stage of planet formation.

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Blobrana


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Posts: 131433
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RE: Asteroids
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New thinking on asteroid belt

Scientists have come up with an alternative explanation for planetary formation which may shed light on how the solar system's asteroid belt was formed.
Sergei Nayakshin from the University of Leicester claims his new hypothesis neatly shows why the asteroid belt exists between the rocky inner planets and the giant gas planets of the outer solar system.
Nayakshin's giant planet embryo theory involves all the planets forming through the collapse of gas from protoplanetary clouds in what is now the outer solar system.
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