Lance A. M. Benner

Radar Imaging of Binary Near-Earth Asteroid (66391) 1999 KW4

High-resolution radar images reveal near-Earth asteroid (66391) 1999 KW4 to be a binary system. The ∼1.5-kilometer-diameter primary (Alpha) is an unconsolidated gravitational aggregate with a spin period ∼2.8 hours, bulk density ∼2 grams per cubic centimeter, porosity ∼50%, and an oblate shape dominated by an equatorial ridge at the objects potential-energy minimum. The ∼0.5-kilometer secondary (Beta) is elongated and probably is denser than Alpha. Its average orbit about Alpha is circular with a radius ∼2.5 kilometers and period ∼17.4 hours, and its average rotation is synchronous with the long axis pointed toward Alpha, but librational departures from that orientation are evident. Exotic physical and dynamical properties may be common among near-Earth binaries.

Orbit and Bulk Density of the OSIRIS-REx Target Asteroid (101955) Bennu

The target asteroid of the OSIRIS-REx asteroid sample return mission, (101955) Bennu (formerly 1999 RQ 36), is a half-kilometer near-Earth asteroid with an extraordinarily well constrained orbit. An extensive data set of optical astrometry from 1999 to 2013 and high-quality radar delay measurements to Bennu in 1999, 2005, and 2011 reveal the action of the Yarkovsky effect, with a mean semimajor axis drift rate da=dt ¼ð � 19:0 � 0:1 Þ� 10

Dynamical Configuration of Binary Near-Earth Asteroid (66391) 1999 KW4

Dynamical simulations of the coupled rotational and orbital dynamics of binary near-Earth asteroid 66391 (1999 KW4) suggest that it is excited as a result of perturbations from the Sun during perihelion passages. Excitation of the mutual orbit will stimulate complex fluctuations in the orbit and rotation of both components, inducing the attitude of the smaller component to have large variation within some orbits and to hardly vary within others. The primarys proximity to its rotational stability limit suggests an origin from spin-up and disruption of a loosely bound precursor within the past million years.

ExploreNEOs. V. Average Albedo by Taxonomic Complex in the Near-Earth Asteroid Population

Examining the albedo distribution of the near-Earth object (NEO) population allows for a better understanding of the relationship between absolute (H) magnitude and size, which impacts calculations of the size frequency distribution and impact hazards. Examining NEO albedos also sheds light on the differences between the NEO and Main Belt populations. We combine albedo results from the ExploreNEOs Warm Spitzer Exploration Science program with taxonomic classifications from the literature, publicly available data sets, and new observations from our concurrent spectral survey to derive the average albedos for C-, D-, Q-, S-, V-, and X-complex NEOs. Using a sample size of 118 NEOs, we calculate average albedos of 0.29+0.05 –0.04, 0.26+0.04 –0.03, and 0.42+0.13 –0.11 for the Q-, S-, and V-complexes, respectively. The averages for the C- and D-complexes are 0.13+0.06 –0.05 and 0.02+0.02 –0.01, but these averages are based on a small number of objects (five and two, respectively) and will improve with additional observations. We use albedos to assign X-complex asteroids to one of the E-, M-, or P-types. Our results demonstrate that the average albedos for the C-, S-, V-, and X-complexes are higher for NEOs than the corresponding averages observed in the Main Belt.

The Dynamical Environment About Asteroid 25143 Itokawa, Target of the Hayabusa Mission

This paper explores the dynamical environment about Asteroid 25143 Itokawa, using the shape and rotation state model recently estimated from radar imaging data by Ostro et al. (2004). In 2005 the Japanese spacecraft Hayabusa will reach Itokawa and begin a period of vicinity operations about that body. The existence of this model can be used to prepare for the Hayabusa spacecraft rendezvous with that asteroid, enabling a more rapid transition from arrival to the start of intensive science and sampling operations. The Hayabusa mission also yields a unique opportunity to validate the shape inversion procedures from radar data described in Hudson (1993). We find that there are stable orbiting options for the Hayabusa spacecraft about Itokawa.

Radar imaging and characterization of the binary near-Earth asteroid (185851) 2000 DP107

Potentially hazardous asteroid (185851) 2000 DP107 was the first binary near-Earth asteroid to be imaged. Radar observations in 2000 provided images at 75 m resolution that revealed the shape, orbit, and spin-up formation mechanism of the binary. The asteroid made a more favorable flyby of the Earth in 2008, yielding images at 30 m resolution. We used these data to obtain shape models for the two components and to improve the estimates of the mutual orbit, component masses, and spin periods. The primary has a sidereal spin period of 2.7745 +/- 0.0007 h and is roughly spheroidal with an equivalent diameter of 863 m +/- 5 %. It has a mass of 4.656 +/- 0.43 x 10^11 kg and a density of 1381 +/- 244 kg m^{-3}. It exhibits an equatorial ridge similar to the (66391) 1999 KW4 primary, however the equatorial ridge in this case is not as regular and has a ~300 m diameter concavity on one side. The secondary has a sidereal spin period of 1.77 +/- 0.02 days commensurate with the orbital period. The secondary is slightly elongated and has overall dimensions of 377 x 314 x 268 m (6 % uncertainties). Its mass is 0.178 +/- 0.021 x 10^{11} kg and its density is 1047 +/- 230 kg m^{-3}. The mutual orbit has a semi-major axis of 2.659 +/- 0.08 km, an eccentricity of 0.019 +/- 0.01, and a period of 1.7556 +/- 0.0015 days. The normalized total angular momentum of this system exceeds the amount required for the expected spin-up formation mechanism. An increase of angular momentum from non-gravitational forces after binary formation is a possible explanation. The two components have similar radar reflectivity, suggesting a similar composition consistent with formation by spin-up. The secondary appears to exhibit a larger circular polarization ratio than the primary, suggesting a rougher surface or subsurface at radar wavelength scales.

Radar observations of Asteroids 64 Angelina and 69 Hesperia

Context. In the past decade, more than one hundred asteroid models wer e derived using the lightcurve inversion method. Measured by the number of derived models, lightcurve inversion has be come the leading method for asteroid shape determination. Aims. Tens of thousands of sparse-in-time lightcurves from astro metric projects are publicly available. We investigate the se data and use them in the lightcurve inversion method to derive new asteroid models. By having a greater number of models with kn own physical properties, we can gain a better insight into the na ture of individual objects and into the whole asteroid popul ation. Methods. We use sparse photometry from selected observatories from t he AstDyS database (Asteroids – Dynamic Site), either alone or in combination with dense lightcurves, to determine new a steroid models by the lightcurve inversion method. We inves tigate various correlations between several asteroid parameters and char acteristics such as the rotational state and diameter or fam ily membership. We focus on the distribution of ecliptic latitudes of pole di rections. We create a synthetic uniform distribution of lat itudes, compute the method bias, and compare the results with the distributi on of known models. We also construct a model for the long-ter m volution of spins. Results. We present 80 new asteroid models derived from combined data sets where sparse photometry is taken from the AstDyS database and dense lightcurves are from the Uppsala Asteroi d Ph tometric Catalogue (UAPC) and from several individual observers. For 18 asteroids, we present updated shape solutions based o n new photometric data. For another 30 asteroids we present t h ir partial models, i.e., an accurate period value and an estimate of the ecliptic latitude of the pole. The addition of new models inc reases the total number of models derived by the lightcurve inversion m ethod to∼200. We also present a simple statistical analysis of physic al properties of asteroids where we look for possible correlat ions between various physical parameters with an emphasis o n the spin vector. We present the observed and de-biased distribution s of ecliptic latitudes with respect to di fferent size ranges of asteroids as well as a simple theoretical model of the latitude distribut ion and then compare its predictions with the observed distr ibutions. From this analysis we find that the latitude distribution of small asteroids ( D < 30 km) is clustered towards ecliptic poles and can be explained by the YORP thermal e ff ct while the latitude distribution of larger asteroids ( D > 60 km) exhibits an evident excess of prograde rotators, probably of primordial origin.

Recent radar observations of asteroid 1566 Icarus

Abstract We report Doppler-only radar observations of Icarus at Goldstone at a transmitter frequency of 8510 MHz (3.5 cm wavelength) during 8–10 June 1996, the first radar detection of the object since 1968. Optimally filtered and folded spectra achieve a maximum opposite-circular (OC) polarization signal-to-noise ratio of about 10 and help to constrain Icarus physical properties. We obtain an OC radar cross section of 0.05 km 2 (with a 35% uncertainty), which is less than values estimated by Goldstein, 1969 and by Pettengill et al., 1969 , and a circular polarization (SC⧸OC) ratio of 0.5±0.2. We analyze the echo power spectrum with a model incorporating the echo bandwidth B and a spectral shape parameter n , yielding a coupled constraint between B and n . We adopt 25 Hz as the lower bound on B , which gives a lower bound on the maximum pole-on breadth of about 0.6 km and upper bounds on the radar and optical albedos that are consistent with Icarus tentative QS classification. The observed circular polarization ratio indicates a very rough near-surface at spatial scales of the order of the radar wavelength.

Radar imaging and physical characterization of near-Earth Asteroid (162421) 2000 ET70

We observed near-Earth Asteroid (162421) 2000 ET70 using the Arecibo and Goldstone radar systems over a period of 12 days during its close approach to the Earth in February 2012. We obtained continuous wave spectra and range-Doppler images with range resolutions as fine as 15 m. Inversion of the radar images yields a detailed shape model with an effective spatial resolution of 100 m. The asteroid has overall dimensions of 2.6 km � 2.2 km � 2.1 km (5% uncertainties) and a surface rich with kilometer-scale ridges and concavities. This size, combined with absolute magnitude measurements, implies an extremely low albedo (� 2%). It is a principal axis rotator and spins in a retrograde manner with a sidereal spin period of 8.96 ± 0.01 h. In terms of gravitational slopes evaluated at scales of 100 m, the surface seems mostly relaxed with over 99% of the surface having slopes less than 30, but there are some outcrops at the north pole that may have steeper slopes. Our precise measurements of the range and velocity of the asteroid, combined with optical astrometry, enables reliable trajectory predictions for this potentially hazardous asteroid in the interval 460–2813.

Goldstone Solar System Radar Observatory: Earth-Based Planetary Mission Support and Unique Science Results

The Goldstone Solar System Radar (GSSR) facility is the only fully steerable radar in the world for high-resolution ranging and imaging of planetary and small-body targets. These observations provide information on surface characteristics, orbits, rotations, and polar ices for a wide variety of solar system objects. The resulting data are used not just for scientific studies of these objects, but also for frequent support of the National Aeronautics and Space Administration (NASA) flight projects, including many solar system exploration missions over the last three decades. For example, the GSSR has contributed to the Mars Exploration Rovers (MERs), Cassini, Hayabusa (MUSES-C), MESSENGER, NEAR, SOHO recovery, Mars Pathfinder, Lunar Prospector, Clementine, Magellan, and Viking. Other recent examples include measurement of lunar topography at high resolution near the lunar south pole, which is of particular interest concerning the impact site of the Lunar Crater Observation and Sensing Satellite (LCROSS) mission, and the characterization and orbit refinement of near-Earth asteroids, both for asteroid impact hazard mitigation and for identification of potential targets for future spacecraft missions. We also present important radar scientific results including near-Earth object (NEO) radar imaging of especially interesting objects, and the results from high accuracy determination of Mercury rotation via radar speckle displacement (RSD).