.RU

39.09: steroid Age Distributions determined by Space Weathering and Collisional Evolution Models

39.09: steroid Age Distributions determined by Space Weathering and Collisional Evolution Models



Author Block: Robert Jedicke

1, M. Willman 1
1Institute for Astronomy.

Presentation Time:

10/6/2010 4:26 PM - 4:28 PM

Location:

Exhibit Hall

Abstract.Abstract: 

We provide evidence of consistency between asteroid ages determined from their dynamical evolution and their color evolution due to space weathering. The dynamical age of an asteroid's surface is estimated as the time since its last catastrophic disruption event which is a function of the object's diameter. The age of an S-complex asteroid's surface may also be determined from its color using a space weathering model. We used a sample of 95 S-complex asteroids from SMASS and obtained their absolute magnitudes and u,g,r,i,z filter magnitudes from SDSS. The absolute magnitudes yield a size-derived age distribution. The u,g,r,i,z filter magnitudes lead to the principal component color which yields a color-derived age distribution by inverting our color-age relationship, an enhanced version of the `dual tau' space weathering model of Willman et al. 2010. We fit the size-age distribution to an 'enhanced dual tau' model and found characteristic weathering and gardening times of 2050+/-80 Myr and 4400+700-500 Myr respectively. The fit also suggests an initial principal component color of -0.05+/-0.01 for fresh asteroid surface with a maximum possible change of the probable color due to weathering of dPC1 = 1.34+/-0.04. Our predicted color of fresh asteroid surface matches the color of fresh ordinary chondritic surface of PC1 = 0.17+/-0.39.

39.10: sing The Youngest Asteroid Clusters To Constrain The Space Weathering And Gardening Rate On S-complex Asteroids



Author Block: Mark Willman

1, R. Jedicke 1, N. Moskovitz 2, D. Nesvorny 3, D. Vokrouhlicky 4, T. Mothé-Diniz 5
1Univ. Of Hawaii, 2Department of Terrestrial Magnetism, 3Dept. of Space Studies, SwRI, 4IoA, Charles University, Czech Republic, 5UFRJ / Observatorio do Valongo, Brazil.

Presentation Time:

10/6/2010 4:28 PM - 4:30 PM

Location:

Exhibit Hall

Abstract:   

We extended our earlier work on space weathering of the youngest S-complex asteroid families to include results from asteroid clusters with ages ≤10 6 years and to newly identified asteroid pairs with ages ≤5×10 5 years. We identified three S-complex asteroid clusters amongst the set of clusters with ages in the range 10 5-6 years---(1270)Datura, (21509)Lucascavin and (16598)1992YC2. Their average color agrees with the color predicted by the space weathering model. SDSS 5-filter photometry of members of the very young asteroid pairs with ages ≤10 5 years was used to determine their taxonomic classification. Their types are consistent with the background population near each object. The average color of the S-complex pairs is PC 1=0.49±0.03, over 5σ redder than predicted. This may indicate that the most likely pair formation mechanism is a gentle separation due to YORP spin-up leaving much of the aged and reddened surface undisturbed. If so then our color measurement allows us to set an upper limit of ~0.64 on the fraction of surface disturbed in the separation process. Using pre-existing color data and our new results for the youngest S-complex asteroid clusters we have extended our space weather model to explicitly include the effects of regolith gardening and fit separate weathering and gardening characteristic timescales of τ w=960±160My and τ g=2000±290My respectively. The first principal component color for fresh S-complex material is PC 1=0.37±0.01 while the maximum amount of local reddening is Δ PC 1=0.33±0.06. Our first-ever determination of the gardening time is in stark contrast to our calculated gardening time of τ g~270My based on main belt impact rates and reasonable assumptions about crater and ejecta blanket sizes. A possible resolution for the discrepancy is through a `honeycomb' mechanism in which the surface regolith structure absorbs small impactors without producing significant ejecta. This mechanism could also account for the paucity of small craters on (433)Eros.

39.11: ining The Sdss-moc Database For Main-belt Asteroid Solar Phase Behavior.



Author Block: Thien-Tin Truong

1, M. D. Hicks 2
1California State University - LA, 2JPL/Caltech.

Presentation Time:

10/6/2010 4:30 PM - 4:32 PM

Location:

Exhibit Hall

Abstract:   

The 4th Release of the Sloan Digital Sky Survey Moving Object Catalog (SDSS-MOC) contains 471569 moving object detections from 519 observing runs obtained up to March 2007. Of these, 220101 observations were linked with 104449 known small bodies, with 2150 asteroids sampled at least 10 times. It is our goal to mine this database in order to extract solar phase curve information for a large number of main-belt asteroids of different dynamical and taxonomic classes. We found that a simple linear phase curve fit allowed us to reject data contaminated by intrinsic rotational lightcurves and other effects. As expected, a running mean of solar phase coefficient is strongly correlated with orbital elements, with the inner main-belt dominated by bright S-type asteroids and transitioning to darker C and D-type asteroids with steeper solar phase slopes.
We shall fit the empirical H-G model to our 2150 multi-sampled asteroids and correlate these parameters with spectral type derived from the SDSS colors and position within the asteroid belt. Our data should also allow us to constrain solar phase reddening for a variety of taxonomic classes. We shall discuss errors induced by the standard "g=0.15" assumption made in absolute magnitude determination, which may slightly affect number-size distribution models.

39.12: pplication of the AutoClass Artificial Intelligence Program to Asteroidal Data



Author Block: Julie Ziffer

1, M. Walker 1, H. Campins 2, Y. Fernandez 2, T. Harvell 1, M. Reulliard 1
1University of Southern Maine, 2University of Central Florida.

Presentation Time:

10/6/2010 4:32 PM - 4:34 PM

Location:

Exhibit Hall

Abstract:   

As our digital databases grow, datasets become less tractable and investigating alternative analysis techniques such as artificial intelligence algorithms becomes more important. One such program, AutoClass, which was developed by NASA’s Artificial Intelligence Branch, uses Bayesian classification theory to automatically choose the most probable classification distribution to describe a dataset. To investigate its usefulness to the Planetary community, we tested its ability to reproduce the taxonomic classes as defined by Tholen and Barucci (1989). We started our evaluation by entering all Tholen identified C, S, or X type Eight Color Asteroid Survey asteroids with a color difference error of less than +/- 0.05 magnitudes. Of these 406 asteroids, AutoClass was able to firmly classify 346 (85%), identifying the remaining 60 asteroids as belonging to more than one class. Of the 346 asteroids that AutoClass classified, all but 3 (<1%) were classified as they had been in the Tholen classification scheme. The three that were misclassified had color errors estimated to be greater than +/- 0.04 magnitudes (though several other asteroids with such errors were classified correctly). To further test AutoClass, we expanded our reach to include all taxonomic types in the ECAS data, and further to include the nine wavelengths used to create the Bus and Binzel taxonomic superclasses (2002), with similar results. The initial successes of AutoClass and its ability to scan large domains for natural classes, showcase its exciting potential as a new discovery tool for Planetary scientists.

39.13: lgorithms for Determining Multiple Periodicities in Sparsely and Unevenly Sampled Data



Author Block: Nathaniel B. Lust

1, D. T. Britt 1
1University of Central Florida.

Presentation Time:

10/6/2010 4:34 PM - 4:36 PM

Location:

Exhibit Hall

Abstract.Abstract: 

Accurately determining asteroid periods from light curves is critical in a range of investigations, including shape determination, rotation rates, and the determination of mass in multiple systems. Fourier transforms of one form or another are commonly used for period determination. While these do work quite well in some instances, they can produce more ambiguous results in cases of low signal-to-noise data or low sampling rates. To this end we have been exploring alternative methods of period determination using well-proven statistical methods as well as alternative transformations that have been used for period determination in other fields. These methods of period determination are explored for both synthetic and less than ideal real data sets, comparing the results with the published values. These techniques can give us a wider range of tools to extract additional information from noisy or poorly sampled light curves.

39.14:  Proposed Standard For Reporting Asteroid Lightcurve Data



Author Block: Robert D. Stephens

1, B. D. Warner 2, A. W. Harris 3
1Goat Mountain Astronomical Research Station/MoreData!, 2Palmer Divide Observatory/MoreData!, 3MoreData!.

Presentation Time:

10/6/2010 4:36 PM - 4:38 PM

Location:

Exhibit Hall

Abstract:   

Unlike astrometric data on asteroids, there is no generally accepted central repository for raw asteroid lightcurve data. There are several sites that accept such data, but each one has its own formatting and minimum data requirements, making it a time-consuming task to prepare raw data for submission. This is especially true for the “backyard astronomers”, the group that has contributed the vast majority of asteroid lightcurves in the last 5-7 years. As a first step towards correcting this problem and making asteroid lightcurve data “universal”, we propose a standard reporting format for asteroid lightcurve observations, much as the Minor Planet Center uses for astrometric observations. Like the FITS standard, metadata and data would be defined by a set of mandatory keyword/value pairs to assure the data’s usability along with optional pairs to further describe the observations and data. This would allow someone to write a single parsing program to extract data from any number of observers to be used as he needs. We will establish a central upload point for lightcurve observations. Those data submitted using the new format will be immediately parsed into a set of database tables and then regularly converted as to needed to be used on a long-term archiving service, e.g., Strasbourg. Eventually, we hope that the Minor Planet Center or other entity might take over the entire system: standard, upload point, and data. Archival data and those not in the standard format could still be uploaded to our intermediate site. We will enlist volunteers to convert the data whenever possible and make it part of the archive. Those data that cannot be converted will remain available “as-is” for a researcher to do with them as he can.

39.15: ew Asteroid Models Based on Combined Dense and Sparse Photometry



Author Block: Josef Hanuš

1, J. Durech 2
1Astronomical Institute of Charles University, Czech Republic, 2Astronomical Institute of Charles University, Prague, Czech Republic.

Presentation Time:

10/6/2010 4:38 PM - 4:40 PM

Location:

Exhibit Hall

Abstract:   

For thousands of asteroids we investigated several ten thousands of sparse photometric data from astrometric projects. These data are available on AstDyS server (Asteroids -- Dynamic Site, http://hamilton.dm.unipi.it). We picked 7 astrometric surveys and used their calibrated photometry in lightcurve inversion method for determination of asteroid's convex shapes and rotational states. We present nearly 100 new asteroid models derived from combined dense and sparse data sets, where sparse photometry is taken from AstDyS server and dense lightcurves are from the Uppsala Asteroid Photometric Catalogue (UAPC) and from several individual observers.

39.16: steroidal Occultation Silhouettes Combined with Asteroid Models Derived by Lightcurve Inversion



Author Block: Josef Durech

1, M. Kaasalainen 2, D. Herald 3, D. Dunham 4, B. Timerson 4, J. Hanus 1, E. Frappa 5, J. Talbot 6, T. Hayamizu 7, B. D. Warner 8, F. Pilcher 9, A. Galad 10
1Charles University in Prague, Czech Republic, 2Tampere University of Technology, Finland, 3IOTA, Australia, 4IOTA, 5Asteroid Occultation Observers in Europe, France, 6Royal Astronomical Society of New Zeland, New Zealand, 7Japan Occultation Information Network, Japan, 8Palmer Divide Observatory, 9Organ Mesa Observatory, 10Modra Observatory, Slovakia.

Presentation Time:

10/6/2010 4:40 PM - 4:42 PM

Location:

Exhibit Hall

Abstract.Abstract: 

Asteroid sizes can be directly measured by observing occultations of stars by asteroids. When there are enough observations across the path of the shadow, the asteroid's projected silhouette can be reconstructed. We present our analysis of occultation data we combined with convex asteroid models. Asteroid shape models derived from photometry by the lightcurve inversion method enabled us to compute the orientation of an asteroid for the time of occultation. By scaling the shape models to fit the occultation chords, we determined the asteroid sizes with a relative accuracy of typically 10%. We combined shape and spin state models of 44 asteroids (14 of them were new or updated models) with the available occultation data to derive asteroid effective diameters. In many cases, occultations allowed us to reject one of two possible pole solutions that were derived from photometry. Our results demonstrate the possibility of deriving unique physical models of asteroids by combining shape models obtained from lightcurve inversion with occultation timings.

39.17: lbedos of Small Hilda Asteroids



Author Block: Erin L. Ryan

1, C. E. Woodward 1
1Univ. of Minnesota.

Presentation Time:

10/6/2010 4:42 PM - 4:44 PM

Location:

Exhibit Hall

Abstract:   

We present albedo results for ~70 small Hilda dynamical family members detected by the Spitzer Space Telescope in multiple archival programs. This Spitzer data samples Hildas with diameters between 2 and 11 kilometers. Our preliminary analysis reveals that the mean geometric albedo for this sample is p v = 0.05, matching the mean albedo derived for large (20 to 160 km) Hilda asteroids observed by IRAS (Ryan and Woodward 2010). This mean albedo is significantly darker than the mean albedo of asteroids in the outer main belt (2.8 AU < a < 3.5 AU), possibly suggesting that these asteroids did not originate from the outer main belt . This is in direct conflict with some dynamical models which suggest that the HIldas are field asteroids trapped from an inward migration of Jupiter (Franklin et al. 2004), and may provide additional observation support for delivery of dark Kuiper Belt contaminants to the inner solar system as per the Nice Model (Levison et al. 2009).

39.18: steroid 65 Cybele: Detection Of Small Silicate Grains, Water-Ice And Organics



Author Block: Zoe Landsman

1, J. Licandro 2, H. Campins 1, M. Kelley 3, K. Hargrove 1, N. Pinilla-Alonso 4, D. Cruikshank 4, A. S. Rivkin 5, J. Emery 6
1Physics Department, University of Central Florida, 2Instituto de Astrofisica de Canarias, Spain, 3Department of Astronomy, University of Maryland, 4NASA Ames Research Center, 5Johns Hopkins University Applied Physics Laboratory, 6Earth and Planetary Science Department, University of Tennessee.

Presentation Time:

10/6/2010 4:44 PM - 4:46 PM

Location:

Exhibit Hall

Abstract:   

We have carried out a spectroscopic study of asteroid 65 Cybele in the 2 - 4 μm and 5 - 14 μm regions, using NASA’s IRTF and Spitzer Space Telescope, respectively. We compare these spectra with those of Trojan asteroids and asteroid 24 Themis. Our target is the largest member of the Cybele population; these asteroids represent a transition group between primitive inner-main belt asteroids and Trojans, and may provide clues to the origin of the asteroid belt and the solar system.
Our 2 - 4 μm spectrum shows a clear absorption band centered at 3.1 μm, similar to that in the spectrum of 24 Themis (Campins et al. 2010, Rivkin & Emery 2010), which can be modeled using water-ice. Absorption bands in 65 Cybele’s 3.2 - 3.6 μm region also suggest the presence of complex organic solids. No hydrated silicates are detected. We fit the 5 - 14 μm continuum using the NEATM (Harris 1998) and derived a diameter D = 290 ± 5 km (consistent with Müller and Bloemmaert 2004), beaming parameter η = 0.967 ± 0.014, and albedo pV = 0.05 ± 0.01. Once the continuum has been divided out, the 5 - 14 μm emissivity spectrum clearly exhibits an emission plateau at about 9 - 12 μm with a spectral contrast of about 5%. A similar feature is seen in the spectra of Trojan asteroids and may be due to fine-grained silicates imbedded in a transparent matrix, or to a very under-dense surface structure (Emery et al. 2006). We conclude that 65 Cybele is covered by fine anhydrous silicate grains, with a small amount of water-ice and complex organic solids. This is similar to comets where non-equilibrium phases coexist, e.g. water-ice and anhydrous silicates; thus we conclude that this is a very primitive object.

39.19: 379) Huenna’s Satellite: A Chip Off The Block



Author Block: Francesca E. DeMeo

1, B. Carry 1, F. Marchis 2, M. Birlan 3, R. P. Binzel 4, S. J. Bus 5, P. Descamps 3, A. Nedelcu 3, H. Bouy 6
1Observatoire De Paris, France, 2University of California at Berkeley, 3IMCCE, France, 4MIT, 5Institute for Astronomy, 6ESAC, Spain.

Presentation Time:

10/6/2010 4:46 PM - 4:48 PM

Location:

Exhibit Hall

Abstract:   

We present near-infrared spectral measurements of Themis family asteroid (379) Huenna (D ∼98km) and its 6km satellite using SpeX on the NASA IRTF. The companion was farther than 1.5" from the primary at the time of observations and was 5 magnitudes dimmer. We describe a method for separating and extracting the signal of a companion asteroid when the signal is not entirely resolved from the primary. The spectra of (379) Huenna and its secondary are very similar, suggesting the secondary is either a fragment of the primary from a past collision or is a fragment from the collision that created the Themis family.

39.20:  Search for OH Outgassing from Icy Asteroid (24) Themis



Author Block: Amy J. Lovell

1, Y. R. Fernández 2, H. Campins 2, D. G. Schleicher 3
1Agnes Scott College, 2University of Central Florida, 3Lowell Observatory.

Presentation Time:

10/6/2010 4:48 PM - 4:50 PM

Location:

Exhibit Hall

Abstract:   

(24) Themis is a Main-Belt asteroid and the largest member of a family that includes at least two Main-Belt Comets. Recent reports have announced the existence of water ice on Themis's surface (Rivkin and Emery 2010, Campins et al. 2010). This suggests that Themis may itself possess a faint gas coma, where sublimated water would photo-dissociate into OH. We report here radio and UV observational searches for OH emission around Themis, made using the National Radio Astronomy Observatory (NRAO) 100-m Green Bank Telescope (GBT) and with the Lowell Observatory 1.1-m Hall Telescope. Despite the wide coverage of ice on Themis's surface, the asteroid did not appear active, and we did not detect the emission in either wavelength regime. The most sensitive 3σ upper-limit suggests Themis has a (water) gas production rate less than ~1x10 28 molecules/s. This result can constrain the physical state, temperature, and durability of Themis's ice.

39.21: he Spin Vector of (832) Karin



Author Block: Stephen M. Slivan

1, L. A. Molnar 2
1Wellesley College, 2Calvin College.

Presentation Time:

10/6/2010 4:50 PM - 4:52 PM

Location:

Exhibit Hall

Abstract:   

We observed rotation lightcurves of Koronis family and Karin cluster member (832) Karin during its four consecutive apparitions in 2006-2009, and combined the new observations with previously published lightcurves to determine its spin vector orientation and preliminary model shape. Karin is a prograde rotator with a period of 18.352 h, spin obliquity near 41°, and pole ecliptic longitude near either 51° or 228°. Although the two ambiguous pole solutions are near the clustered pole solutions of four Koronis family members whose spins are thought to be trapped in a spin-orbit resonance (Vokrouhlický et al., 2003), Karin does not seem to be trapped in the resonance; this is consistent with the expectation that the 6 My age of Karin (Nesvorný et al., 2002) is too young for YORP torques to have modified its spin since its formation. The spin vector and shape results for Karin will constrain family formation models that include spin properties, and we discuss the Karin results in the context of the other members of the Karin cluster, the Karin parent body, and the parent body's siblings in the Koronis family.

39.22: axonomic Classification of Asteroids via Broadband Near-Infrared Photometry



Author Block: Eric Petersen

1, C. Thomas 2, D. Trilling 2, J. Emery 3, M. Delbo 4, M. Mueller 4, R. Dave 3
1Simon Fraser University, Canada, 2Northern Arizona University, 3University of Tennessee, 4Observatoire de la Cote d'Azur, France.

Presentation Time:

10/6/2010 4:52 PM - 4:54 PM

Location:

Exhibit Hall

Abstract.Abstract: 

For faint asteroids, it is not practical to obtain near-infrared spectra. However, it may be possible to use broadband photometry to infer spectral classifications and study composition. As a test of this, we processed SpeX near-infrared asteroid spectral data to simulate colors that would be obtained through photometry. We have found that certain color combinations (for example, z-J and H-K) can prove diagnostic in asteroid spectral classification. To this end, we have defined certain color-color regions that make it possible to define an asteroid as being a likely candidate for a certain spectral type. The regions identified define V and D type asteroids, the S-Q group, and the C-X group. Knowledge and use of these regions will significantly increase the usefulness of NIR broadband photometry in the study of near earth objects and allow characterization of asteroids that are too faint to be observed spectroscopically. Work on this project is made possible through the NSF REU program at Northern Arizona University and by funding from the Spitzer Space Telescope/JPL/Caltech.

39.23: id-IR Spectroscopy Of Main-belt Asteroids From MIRSI At The IRTF



Author Block: Lucy F. Lim

1, J. P. Emery 2, A. S. Rivkin 3, J. L. Hora 4, M. Kassis 5
1NASA / GSFC, 2University of Tennessee, 3JHU/APL, 4Harvard-Smithsonian Center for Astrophysics, 5Keck Observatory.

Presentation Time:

10/6/2010 4:54 PM - 4:56 PM

Location:

Exhibit Hall

Abstract:   

Thermal emission spectra (8-13 microns) of 19 Fortuna, 4 Vesta, 1 Ceres, and several other large main-belt asteroids were measured at IRTF in 2009 and 2010. The new "MIRSI 2" (2008) interface to the Mid-Infrared Spectrometer and Imager ("MIRSI") instrument (Kassis et al. 2008, PASP 120) was used to acquire these data. The thermal emission spectra of these asteroids will be presented together with evaluations of the S/N achievable in low-resolution spectroscopy with MIRSI at IRTF. Techniques for addressing instrumental issues particular to MIRSI (Kassis et al. 2008; Sako et al. 2003, PASP 115) in the context of spectroscopic observations will also be discussed.

39.24: otation Properties of Three Jovian Trojan Asteroids



Author Block: Susan M. Lederer

1, L. M. French 2, D. A. Rohl 2, K. Friedrich 3, T. Hufford 4, F. Luzia Jasmim 5, A. Khairunnisa 6, J. Šilha 7
1NASA JSC/Cal State Univ SB, 2Illinois Wesleyan Univ., 3Universität Bonn, Germany, 4Loyola Univ., Univ. of Georgia, 5Observatório Nacional, Brazil, 6Institut Teknologi Bandung, Indonesia, 7Comenius University, Slovakia.

Presentation Time:

10/6/2010 4:56 PM - 4:58 PM

Location:

Exhibit Hall

Abstract:   

The spin properties of the Jovian Trojan asteroids have not been extensively studied due to their distance from the Sun and their low albedos. We present a progress report on a program to determine well-sampled light curves for the Jovian Trojans. Rotation data will be presented for 884 Priamus, 4489 88AK, and 4834 Thoas.
Data were obtained in October 2009 and July 2010 using the SMARTS 1.0-meter and 0.9-meter telescopes, respectively, at Cerro Tololo Interamerican Observatory near La Serena, Chile. Images were processed and measured with IRAF, and periods determined using the Canopus software package.
This research was supported by Cottrell College Science Award grants from the Research Corporation (LMF and DAR; SML) and support from the Lunar and Planetary Institute (SML).

39.25: pdated Masses and Densities of Asteroids from Recent Mars Orbiter Range Data



Author Block: Eugene G. Fahnestock

1, W. M. Folkner 1, A. S. Konopliv 1
1NASA Jet Propulsion Laboratory.

Presentation Time:

10/6/2010 4:58 PM - 5:00 PM

Location:

Exhibit Hall

Abstract:   

We present new solutions for the masses of asteroids through new solutions for the Mars ephemeris determined at JPL using recent tracking data from spacecraft orbiting Mars. This work builds upon the previous solutions for the Mars ephemeris and asteroid masses presented in (Konopliv et al., 2010, Icarus, in press), by incorporating about one additional year of range data from each of MRO, Mars Odyssey, and Mars Express spanning from July 2009 through June 2010. Doppler data also provide each spacecraft's orbit with respect to Mars along with the Mars global gravity solution of (Konopliv et al., 2010). This, added to the tracking station to spacecraft range, provides the distance between Earth and Mars centers to one to two meter accuracy. Each asteroid's mass is estimated along with Earth and Mars orbits, while the solar mass parameter, solar J2, Earth-Moon mass ratio, Sun's mass loss rate, and PPN parameters are also estimated concurrently. The studied asteroids are selected as those which most perturb the orbit of Mars on a timescale matching the span of all data used. A larger set of other perturbing asteroids have their orbits and masses used as consider parameters. A still larger set of perturbing asteroids have their orbits fixed and assumed perfectly known, but masses computed from IRAS nominal diameters and (also estimated) average bulk densities for the three main taxonomic classes. We also use IRAS and MSX nominal diameters and all more recent asteroid photometry observations we could obtain for the asteroids to update their volumes, which together with the new masses constrain their bulk densities and likely macroporosities.

39.26: pitzer/IRS Observations Of Multiple Main-Belt And Binary Near-Earth Asteroids



Author Block: J. Emilio Enriquez

1, F. Marchis 2, J. P. Emery 3, S. Im 4
1SETI Institute, 2UC-Berkeley/SETI Institute, 3U. of Tennessee, 4NASA - Ames Research Center / SETI Institute.

Presentation Time:

10/6/2010 5:00 PM - 5:02 PM

Location:

Exhibit Hall

Abstract:   

Since the discovery of Ida’s companion in 1993, 195 companions of asteroids have been discovered. To understand the formation process of these interesting bodies, their physical properties such as their bulk density, size, shape, and surface roughness need to be determined.
During the Spitzer Cycle-4, we obtained IRS thermal emission spectra (5-42 um) of 23 known binary systems. The majority of asteroids are from the main-belt (16), while the rest are NEOs (7). After extracting the thermal spectra, we used a modified Standard Thermal Model (STM) to calculate their equivalent diameter (from 0.8 km to 237 km), their albedo (from 0.04 for C-type to 0.394 for a V-type) and their beaming factor related to the surface roughness and thermal inertia. We derive their emissivity spectra, which is useful to detect silicate features.
Combining these measurements with 3D-models of these multiple asteroid systems obtained by lightcurve inversion, we should be able to derive an accurate estimate of their bulk-density and contrast them with their taxonomic classes. Preliminary studies by Marchis et al. (2008) 1, suggested a relationship between bulk density and the taxonomic class of asteroids, which varies from 0.9 g/cc for C-complex to 2.4 g/cc for S-complex asteroids.
The National Science Foundation supported this research under award number AAG-0807468. It was conducted with the Spitzer space telescope, which is operated by JPL under a contract with NASA.
1 Marchis et al. , 2008, “Mid-infrared Spectra of Binary Asteroids With Spitzer/IRS”, 40th DPS Meeting, Bulletin of the American Astronomical Society, 40, 508

39.27: xploring the Enigma of 4709 Ennomos



Author Block: Mark Reuillard

1, T. Harvell 1, J. Ziffer 1, Y. Fernandez 2, M. Walker 1
1University of Southern Maine, 2University of Central Florida.

Presentation Time:

10/6/2010 5:02 PM - 5:04 PM

Location:

Exhibit Hall

Abstract:   

Large Trojan asteroids are characteristically dark, having albedos that are typically in the range 0.03 to 0.08 (Fernandez et al., 2003). One notable exception is 4709 Ennomos with an unusually high measured albedo of about 0.13 (Fernandez et al., 2003). This corresponds to an albedo of more than 10 standard deviations above the mean of the group of 32 large Trojans sampled by Fernandez et al (2003). There are two main explanations for the anomalous albedo: Ennomos’s surface composition may truly be different from similarly-sized Trojans and be richer in more highly-reflective species, or the assumptions that go into the modeling used to derive diameter and albedo are inapplicable to Ennomos because of unusual physical or thermal properties. For the first hypothesis, so far only upper limits to compositional signatures have been found (e.g. Yang and Jewitt 2007). In this work we address the second hypothesis. One plausible explanation is that Ennomos’ rotation period is sufficiently fast or its thermal inertia is sufficiently high so as to preclude the use of a zero-thermal memory thermal model (Lebofsky and Spencer 1989, Harris 1998) i.e. the model actually used to calculate its albedo. An alternative explanation is that shape or topographic anomalies conspired to reduce the thermal emission, causing the model - which assumes a spherical body - to underestimate the diameter. To address these issues, we obtained BVRI time-series CCD photometry of Ennomos with the University of Hawaii’s 88 inch telescope on February 8, 9, and 10, 2003. The goals were to determine Ennomos’ rotation period, basic shape, and visible colors, and we will present these results. We will also discuss what the results imply about the nature of Ennomos’s surface.

39.28: he Contribution Of The Oblateness Of An Asteroid On The Orbital Evolution Of Its Satellites



Author Block: Othon Cabo Winter

1, L. G. Boldrin 1, E. Vieira Neto 1, R. Gomes 2
1UNESP, Brazil, 2Observatorio Nacional, Brazil.

Presentation Time:

10/6/2010 5:04 PM - 5:06 PM

Location:

Exhibit Hall

Abstract:   

About five years ago was discovered the first asteroid to have a pair of small satellites. That was 87 Sylvia with its satellites Romulus and Remus (Marchis et al. 2005). Nowadays a few more cases are known. A preliminary study of the long term dynamics of Romulus and Remus (Winter et al. 2009) showed that their trajectories would be highly unstable if Sylvia had a symmetrically spherical gravitational potential. However, Sylvia is highly oblate, J2=0.17. Consequently, the inclusion of J2 in the dynamics results in very stable trajectories for Romulus and Remus. That is mainly due to the fact that the orbital angular momenta of the satellites increase significantly due to J2. In the present work we show the results of a study of the evolution of the spin axis of the main asteroid and its effects on the orbital motion of the small satellites. In the dynamics of the spin is taken into account the perturbation of the Sun and Jupiter. The orbital planes of the satellites follow the equatorial plane of the asteroid.
Acknowledgements: This work has the support of FAPESP, CNPq and CAPES

39.29:  Census of Asteroid Families Between the J5/2 and J7/3 Resonances



Author Block: Lawrence A. Molnar

1, C. L. McKay 1
1Calvin College.

Presentation Time:

10/6/2010 5:06 PM - 5:08 PM

Location:

Exhibit Hall

Abstract:   

Significant theoretical progress has been made in recent years in modeling the history of the main asteroid belt. In particular, the size-frequency distributions of individual collisions (as a function of impact properties) and the collisional rate (as a function of target size) have been computed.
In this work we test these results against observations of asteroids between the J5/2 and J7/3 mean motion resonances. We select this particular zone because 1) the asteroid density is low enough that the extent and membership of all families may be determined unambiguously, 2) the boundary resonances isolate it from families in other zones, and 3) other resonances within the zone have had relatively little effect on the orbits of its families.
In particular, we computed synthetic proper elements for 16,111 multiopposition asteroids. We used proper a, e, i and absolute magnitude to identify membership in 19 collisional families. For two of these_the Karin and Koronis clusters_we also used proper longitudes of node and perihelion. The absolute magnitude is important for excluding large interlopers when a Yarkovsky signature clearly identifies them as such. For example, (293) Brasilia is probably not a member of the family previously identified as the Brasilia family. We corrected size-frequency distribution (SFDs) using the local background rates. Finally, we inferred collisional ages (or limits) from evidence of Yarkovsky broadening of the distribution in a.
We will present and discuss the empirical collision rate function implied by our results and the SFDs of the families and of the background asteroids. We will also present and discuss the average Sloan Digital Sky Survey colors for each family.
This work was funded by a Kuiper endowment and a Calvin Research Fellowship.
2010-07-19 18:44 Читать похожую статью
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