AAS 203rd Meeting, January 2004
Session 134 Solar System and The Sun
Oral, Thursday, January 8, 2004, 2:00-3:30pm, Regency VI

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[134.02] Radar Observations of Binary Asteroid 5381 Sekhmet

C. D. Neish (The University of British Columbia), M. C. Nolan, E. S. Howell (Arecibo Observatory), A. S. Rivkin (MIT)

Radar astronomy is a unique observation technique used to determine the physical properties and orbits of asteroids. By transmitting a phase-coded radio wave at an asteroid and analysing the reflected echoes, we can generate two-dimensional images of the asteroid dispersed in range and Doppler shift.

We used this technique to image asteroid 5381 Sekhmet at Arecibo Observatory during its close approach to Earth in May 2003. These images show the presence of a previously unknown asteroidal satellite. This is the seventh binary system detected by radar in the near-Earth asteroid population (Merline et al. 2002).

Based on range extents at 45-m resolution, we find the average diameters of the two asteroids to be 1000 m and 300 m, with rotation periods of 2.7 ± 0.4 h and 10 ± 2 h, respectively. We estimate the orbital period of the binary to be 12.5 ± 0.3 h, with an orbital radius of 1.54 ± 0.12 km. The orbital velocity of the secondary was measured to be 0.215 ± 0.012 m/s. The rapid rotation rate of the primary indicates that the viewing geometry must be nearly equatorial. These measurements imply a density of 1.98 ± 0.65 g/cm3, assuming a circular orbit and the same composition for primary and satellite. A visible spectrum shows a mineral absorption band due to pyroxene and possibly olivine at 0.94 microns. Thus, asteroid Sekhmet is an S-type asteroid, with a composition similar to 433 Eros and 243 Ida. The density of Sekhmet is lower than that determined for 433 Eros (2.67 ± 0.03) which implies a porosity of about 40%, twice that of Eros (Wilkison et al. 2002). A high porosity would be expected if this binary system formed from spin-up of a strengthless body following a tidal encounter with a planet.

This research was supported by the NSF through an REU summer internship at the Arecibo Observatory.

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