Christopher Magri

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.

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.

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.

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.