Nuclear Spectra of Comet 162P/Siding Spring (2004 TU12)

We present visible and near-IR spectra of the nucleus of comet 162P/Siding Spring (also known as 2004 TU12) obtained in 2004 December, while it had no detectable coma. This is the third object observed to have intermittent cometary activity even when relatively close to the Sun. The spectra show no strong features in this wavelength range. This paucity of deep absorptions is common among low-albedo asteroids and the few comet nuclei observed in this spectral region. Marginal spectral structure is observed in the visible spectrum, and beyond 2 μm the flux from the nucleus is dominated by thermal emission. We compare the spectrum of 162P with published spectra of other comet nuclei, primitive asteroids, and meteorites. Comet nuclei display a range of spectral shapes and slopes not unlike those observed among outer main-belt asteroids but closest to Trojan asteroids. No suitable spectral matches to comet 162P were found among primitive (chondritic) meteorites. We modeled our visible and near-IR spectra using the scattering theory described by Shkuratov et al. (1999), and our approach is similar to that used by Emery and Brown for modeling Trojan asteroids. Our best fits to the spectral shape and albedo include mixtures containing amorphous carbons, organics, and silicates. The absence of strong spectral features prevents the identification of specific minerals, and the resulting model compositions are not unique. The observations beyond 2 μm are interpreted in a companion publication by Fernández and coworkers

Nuclear Spectra of Comet 28P Neujmin 1

We present visible and near-IR spectra of the nucleus of comet 162P/Siding Spring (also known as 2004 TU12) obtained in 2004 December, while it had no detectable coma. This is the third object observed to have intermittent cometary activity even when relatively close to the Sun. The spectra show no strong features in this wavelength range. This paucity of deep absorptions is common among low-albedo asteroids and the few comet nuclei observed in this spectral region. Marginal spectral structure is observed in the visible spectrum, and beyond 2 μm the flux from the nucleus is dominated by thermal emission. We compare the spectrum of 162P with published spectra of other comet nuclei, primitive asteroids, and meteorites. Comet nuclei display a range of spectral shapes and slopes not unlike those observed among outer main-belt asteroids but closest to Trojan asteroids. No suitable spectral matches to comet 162P were found among primitive (chondritic) meteorites. We modeled our visible and near-IR spectra using the scattering theory described by Shkuratov et al. (1999), and our approach is similar to that used by Emery and Brown for modeling Trojan asteroids. Our best fits to the spectral shape and albedo include mixtures containing amorphous carbons, organics, and silicates. The absence of strong spectral features prevents the identification of specific minerals, and the resulting model compositions are not unique. The observations beyond 2 μm are interpreted in a companion publication by Fernández and coworkers

Thermophysical Characterization of Potential Spacecraft Target (101955) 1999 RQ36

We report on thermal emission measurements of 1999 RQ36 from Spitzer. The derived size is in agreement with radar measurements, and we find a moderately high thermal inertia and homogeneous surface properties

A Comparative Study of the Themis and Veritas Asteroid Families

Our primary goal is to characterize the surface composition (and other properties such as radius, albedo and thermal inertia) of our sample of Themis-family and Veritas-family asteroids based on their 5 to 14 micron spectra. We chose these two families for several reasons. First, they are compositionally primitive (non-igneous) so they can yield information about their physical and chemical conditions of their formation environment. Second, their parent bodies formed in the same region, yet their disruption ages are sharply different: 2.5 Gy and 8.3 My, respectively. This gives us a remarkable opportunity to understand the evolutionary processes that have affected the asteroids fragments. For example, Nesvorny et al. (2005) found clear evidence of color variations between young and old asteroids families. They identified a well defined trend among primitive asteroids, with the Themis and Veritas families at opposite ends of this color variation, which they attribute to space weathering. Finally, both families formed beyond the \u27frost line\u27 and some fragments appear to have retained water-ice reservoirs for the age of the solar system; more specifically Rivkin (2007) reported the first (preliminary) detection of water ice on 24 Themis. If confirmed, this detection of water-ice opens up interesting possibilities that could transform of our understanding of these asteroids. For example, since water ice is not stable on the surface of 24 Themis over the age of the solar system what is its source? What does this imply about the interior of this asteroid and of the other members of these two families. Why does 24 Themis not show cometary activity? These are some of the questions this proposed study will address. Understanding the abundance of water-ice and hydrated minerals in this area of the solar system is particularly important, as it may be linked to the origin of Earth\u27s water. Our total time request is 6.4 hours

Rotational Variability of Asteroid 490 Veritas in the Near-Infrared

We present rotationally resolved spectra of asteroid 490 Veritas in the near-infrared (NIR) that show interesting differences. Dynamical arguments indicate that 490 Veritas is the main fragment of a recent (8.3 My) asteroidal break-up. We obtained NIR spectra (0.8-2.4 microns) on UT May 11, 2008, using the SpeX instrument on NASA\u27s Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Since published visible spectra of 490 Veritas show some variability, we decided to observe its NIR spectrum at four distinct rotational phases. Veritas\u27s rotational light curve has a period of 7.93 hours with amplitudes ranging from 0.3 to 0.5 magnitudes in the visible. We define the time of our first observation as zero rotational phase and our subsequent observations are at 0.29, 0.52, and 0.70. Our reflectance spectra show a range of slopes. To characterize these slope differences, we normalized each spectrum to 1.0 reflectance at 1.25 microns and measured the reflectance at 2.2 microns. The values obtained are reflectances of 0.99, 1.02, 0.98, and 0.93 at rotational phase 0.00, 0.29, 0.52, and 0.70, respectively. The uncertainty in each reflectance value is ± 3%. In this work, we consider possible causes of this variability, including heterogeneity of the parent body and/or space weathering i.e., from different surfaces having experienced various exposure ages

Principal Component Analysis of Dynamically Distinct D-Type Asteroids

Principal Component Analysis (PCA), a common statistically based classification technique, has been used to classify asteroids into broad spectral categories. In some cases, a spectral superclass considered in isolation may undergo sub-classification (e.g. S-type subclasses). Since D-type asteroids populate at least three distinct dynamical regions in the asteroid belt -- namely Hilda, L4 Trojans and L5 Trojans, and since the recently-developed Nice” model (Morbidelli et al. 2005. Nature 435, 462; Levison et al. 2008, ACM 2008 abstract #8156) hypothesizes that these regions may share a common origin, examining the appropriateness of a D-type sub-classification scheme is warranted. Toward this end, we performed PCA on the D-type L4, L5, and Hilda asteroids. Our PCA was based on the Sloan Digital Sky Survey broadband colors (u - g, g - r, r - i, and i - z) of 31 L4, 24 L5, and 32 Hilda asteroids with radii ranging from approximately 5 to 45 km. PCA showed 90.2% of the variance in the spectra could be condensed into the first two principal components, PC1 and PC2, with the first and second component accounting for 50.7% and 39.4% respectively. No significant clustering is observed on a PC1 vs. PC2 plot suggesting the D-type L4, L5, and Hilda asteroids do not form three independent groups, but rather are spectrally indistinguishable. We performed several statistical analyses of the means and variances of the principal components to test the validity of this conclusion. No statistically significant difference in the means among the three groups was found, nor was there any such difference in the variances, although the statistic comparing the L4 Trojans and Hildas was close to the critical value. Further measurements of colors of both large and small Trojans and Hildas will let us continue to investigate the spectral diversity of these objects

The Potential of AutoClass as an Asteroidal Data Mining Tool

AutoClass-C, an artificial intelligence program designed to classify large data sets, was developed by NASA to classify stars based upon their infrared colors. Wanting to investigate its ability to classify asteroidal data, we conducted a preliminary test to determine if it could accurately reproduce the Tholen taxonomy using the data from the Eight Color Asteroid Survey (ECAS). For our initial test, we limited ourselves to those asteroids belonging to S, C, or X classes, and to asteroids with a color difference error of less than +/- 0.05 magnitudes. Of those 406 asteroids, AutoClass was able to confidently classify 85%: identifying the remaining asteroids as belonging to more than one class. Of the 346 asteroids that AutoClass classified, all but 3 (\u3c1%) were classified as they had been in the Tholen classification scheme. Inspired by our initial success, we reran AutoClass, this time including IRAS albedos and limiting the asteroids to those that had also been observed and classified in the Bus taxonomy. Of those 258 objects, AutoClass was able to classify 248 with greater than 75% certainty, and ranked albedo, not color, as the most influential factor. Interestingly, AutoClass consistently put P type objects in with the C class (there were 19 P types and 7 X types mixed in with the other 154 C types), and omitted P types from the group associated with the other X types (which had only one rogue B type in with its other 49 X-types). Autoclass classified the remaining classes with a high accuracy: placing one A and one CU type in with an otherwise perfect S group; placing three P type and one T type in an otherwise perfect D group; and placing the four remaining asteroids (V, A, R, and Q) into a class together

Taxonomic Size Frequency Distributions By Dynamical Zone

We investigate the size-frequency distribution of major asteroid taxonomic classes by combining data from the recent Wide-field Infrared Survey Explorer (WISE) and Carvano et al.\u27s (2010) Sloan Digital Sky Survey (SDSS) based taxonomy. Prior to WISE the limited scope of albedo and diameter studies restricted our understanding of taxonomic size frequency distributions. Our research using the new WISE database extends the number of asteroids analyzed from 4,000 to over 8,000 and expands on previous work by focusing on asteroid classes S, C, X (and its subclasses), and D. We remove asteroids belonging to the major asteroid families as defined by Nesvorny (2010) and control for collisional environment by using dynamical zones as defined by Cellino et al. (1991). Understanding the taxonomic size frequency distributions provides insight into the collisional evolution of asteroids in each class. Our results can be extrapolated to infer the overall volume of each class

A Comparison of Autoclass and Principal Component Analysis as Applied to Asteroid Families

We present and compare the Sloan Digital Sky Survey (SDSS) colors of the Themis family, an older (2.5 Gya) outer-belt asteroid family, with that of the Beagle family (15 Mya), Themiss younger second-generation family. Comparing Beagle family colors with that of its older progenitor\u27s family\u27s colors allows a direct test of space weathering effects on C-complex asteroids. The question of space weathering among CComplex asteroids is an unsettled one. Nesvorny et al[1] found clear color differences between C-complex asteroids of different ages, which could be explained by space weathering processes. The trend with age that they determined based upon Principal Component Analysis (PCA) was anchored with the older Themis family at one extreme. Critical to the weathering hypothesis is the assumption that these asteroids would have similar un-weathered compositions. The catastrophic disruption of the Beagle family eliminates this question since its composition is that of its progenitor Themis member. The size of the SDSS dataset allows us to analyze these families using two statistical methods, PCA and Autoclass. Combined, these powerful methods provide a detailed picture of the role of space weathering among these asteroid