Goldstone radar observations of horseshoe-orbiting near-Earth asteroid 2013 BS45, a potential mission target

We report radar observations of near-Earth asteroid 2013 BS45 obtained during the 2013 apparition. This object is in a resonant, Earth-like orbit, and it is a backup target for NASA’s Near-Earth Asteroid Scout mission. 2013 BS45 belongs to Arjuna orbital domain which currently has ~20 discovered representatives. These objects tend to be small and difficult to characterize and 2013 BS45 is only the third Arjuna object observed with radar to date. We observed 2013 BS45 on three days at Goldstone (8560 MHz, 3.5 cm) between 2013 February 10 to 13. Closest approach occurred on February 12 at a distance of 0.0126 au. We obtained relatively weak echo power spectra and ranging data at resolutions up to 0.125 s (18.75 m px-1). The Doppler broadening of the echo power spectra strongly suggests an upper bound on the rotation period of 1.9-minutes. The radar data in combination with an assumption that the asteroid's radar albedo is no higher than that of metallic near-Earth asteroid 1986 DA constrain the equivalent diameter to 15 m≤D≤ 38 m. The circular polarization ratio is 0.210.06, which implies a near-surface that is relatively smooth on decimeter spatial scales. We bounded the OC radar albedo to OC0.09 and the optical albedo remains relatively unconstrained at 0.05pV0.35. The Yarkovsky acceleration has not been detected but, due to the object's rapid rotation, would be dominated by a seasonal component whose direction and magnitude depends on the currently unknown pole orientation and thermal inertia.