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Canberra, Australia

Becker T.M.,University of Central Florida | Howell E.S.,Arecibo Observatory Universities Space Research Association | Nolan M.C.,Arecibo Observatory Universities Space Research Association | Magri C.,University of Maine at Farmington | And 17 more authors.
Icarus | Year: 2015

We report radar observations (2380-MHz, 13-cm) by the Arecibo Observatory and optical light curves observed from eight different observatories and collected at the Ondřejov Observatory of the triple near-Earth asteroid system (153591) 2001 SN263. The radar observations were obtained over the course of ten nights spanning February 12-26, 2008 and the light curve observations were made throughout January 12 - March 31, 2008. Both data sets include observations during the object's close approach of 0.06558AU on February 20th, 2008. The delay-Doppler images revealed the asteroid to be comprised of three components, making it the first known triple near-Earth asteroid. Only one other object, (136617) 1994 CC is a confirmed triple near-Earth asteroid.We present physical models of the three components of the asteroid system. We constrain the primary's pole direction to an ecliptic longitude and latitude of (309°, -80°)±15°. We find that the primary rotates with a period 3.4256±0.0002h and that the larger satellite has a rotation period of 13.43±0.01h, considerably shorter than its orbital period of approximately 6days. We find that the rotation period of the smaller satellite is consistent with a tidally locked state and therefore rotates with a period of 0.686±0.002 days (Fang et al. [2011]. Astron. J. 141, 154-168). The primary, the larger satellite, and the smaller satellite have equivalent diameters of 2.5±0.3km, 0.77±0.12km, 0.43±0.14km and densities of 1.1±0.2g/cm3, 1.0±0.4g/cm3, 2.3±1.3g/cm3, respectively. © 2014 The Authors. Source

Hanus J.,Charles University | Broz M.,Charles University | Durech J.,Charles University | Warner B.D.,Palmer Divide Observatory | And 8 more authors.
Astronomy and Astrophysics | Year: 2013

Context. The current number of ∼500 asteroid models derived from the disk-integrated photometry by the lightcurve inversion method allows us to study the spin-vector properties of not only the whole population of main-belt asteroids, but also of several individual collisional families. Aims. We create a data set of 152 asteroids that were identified by the hierarchical clustering method (HCM) as members of ten collisional families, among which are 31 newly derived unique models and 24 new models with well-constrained pole-ecliptic latitudes of the spin axes. The remaining models are adopted from the DAMIT database or a few individual publications. Methods. We revised the preliminary family membership identification by the HCM according to several additional criteria: taxonomic type, color, albedo, maximum Yarkovsky semi-major axis drift, and the consistency with the size-frequency distribution of each family, and consequently we remove interlopers. We then present the spin-vector distributions for asteroidal families Flora, Koronis, Eos, Eunomia, Phocaea, Themis, Maria, and Alauda. We use a combined orbital-and spin-evolution model to explain the observed spin-vector properties of objects among collisional families. Results. In general, for studied families we observe similar trends in (ap, β) space (proper semi-major axis vs. ecliptic latitude of the spin axis): (i) larger asteroids are situated in the proximity of the center of the family; (ii) asteroids with β > 0 are usually found to the right of the family center; (iii) on the other hand, asteroids with β < 0 to the left of the center; (iv) the majority of asteroids have large pole-ecliptic latitudes (|β| â‰30); and finally (v) some families have a statistically significant excess of asteroids with β > 0 or β < 0. Our numerical simulation of the long-term evolution of a collisional family is capable of reproducing the observed spin-vector properties well. Using this simulation, we also independently constrain the age of families Flora (1.0 ± 0.5 Gyr) and Koronis (2.5-4 Gyr). © 2013 ESO. Source

Harris A.W.,MoreData Inc. | Pravec P.,Academy of Sciences of the Czech Republic | Galad A.,Academy of Sciences of the Czech Republic | Galad A.,Comenius University | And 13 more authors.
Icarus | Year: 2014

Most asteroid lightcurves are dominated by the second harmonic of the rotation period, caused by elongated shape. However, if the shape is not very elongate, other harmonics may dominate, leading to ambiguity of which is the true rotation period. We argue from geometry that at low phase angle, harmonics other than the second with amplitude exceeding ~0.4 magnitude are nearly impossible, so lightcurves with larger amplitude than that suggest a unique period dominated by the second harmonic, unless the spin is complex, non-principal axis rotation. On the other hand, lightcurves with amplitude less than 0.2-0.3 magnitudes can be dominated by other harmonics, especially the 4th and 6th, so the period may be ambiguous unless odd harmonics can be found to identify the true rotation period. We present examples of each, low and high amplitude ambiguities. © 2014 Elsevier Inc. Source

Hanus J.,Charles University | Durech J.,Charles University | Broz M.,Charles University | Marciniak A.,Adam Mickiewicz University | And 70 more authors.
Astronomy and Astrophysics | Year: 2013

Context. The larger number of models of asteroid shapes and their rotational states derived by the lightcurve inversion give us better insight into both the nature of individual objects and the whole asteroid population. With a larger statistical sample we can study the physical properties of asteroid populations, such as main-belt asteroids or individual asteroid families, in more detail. Shape models can also be used in combination with other types of observational data (IR, adaptive optics images, stellar occultations), e.g., to determine sizes and thermal properties. Aims. We use all available photometric data of asteroids to derive their physical models by the lightcurve inversion method and compare the observed pole latitude distributions of all asteroids with known convex shape models with the simulated pole latitude distributions. Methods. We used classical dense photometric lightcurves from several sources (Uppsala Asteroid Photometric Catalogue, Palomar Transient Factory survey, and from individual observers) and sparse-in-time photometry from the U.S. Naval Observatory in Flagstaff, Catalina Sky Survey, and La Palma surveys (IAU codes 689, 703, 950) in the lightcurve inversion method to determine asteroid convex models and their rotational states. We also extended a simple dynamical model for the spin evolution of asteroids used in our previous paper. Results. We present 119 new asteroid models derived from combined dense and sparse-in-time photometry. We discuss the reliability of asteroid shape models derived only from Catalina Sky Survey data (IAU code 703) and present 20 such models. By using different values for a scaling parameter cYORP (corresponds to the magnitude of the YORP momentum) in the dynamical model for the spin evolution and by comparing synthetic and observed pole-latitude distributions, we were able to constrain the typical values of the c YORP parameter as between 0.05 and 0.6. © 2013 ESO. Source

Magri C.,University of Maine at Farmington | Howell E.S.,Arecibo Observatory | Nolan M.C.,Arecibo Observatory | Taylor P.A.,Arecibo Observatory | And 23 more authors.
Icarus | Year: 2011

We observed near-Earth Asteroid (8567) 1996 HW1 at the Arecibo Observatory on six dates in September 2008, obtaining radar images and spectra. By combining these data with an extensive set of new lightcurves taken during 2008-2009 and with previously published lightcurves from 2005, we were able to reconstruct the object's shape and spin state. 1996 HW1 is an elongated, bifurcated object with maximum diameters of 3.8 × 1.6 × 1.5. km and a contact-binary shape. It is the most bifurcated near-Earth asteroid yet studied and one of the most elongated as well. The sidereal rotation period is 8.76243 ± 0.00004. h and the pole direction is within 5° of ecliptic longitude and latitude (281°, -31°). Radar astrometry has reduced the orbital element uncertainties by 27% relative to the a priori orbit solution that was based on a half-century of optical data. Simple dynamical arguments are used to demonstrate that this asteroid could have originated as a binary system that tidally decayed and merged. © 2011 Elsevier Inc. Source

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