Photometry of 2006 RH₁₂₀: an asteroid temporary captured into a geocentric orbit

Abstract

<i>Aims. <i/>From July 2006 to July 2007 a very small asteroid orbited the Earth within its Hill sphere. We used this opportunity to study its rotation and estimate its diameter and shape.<i>Methods. <i/>Due to its faintness, 2006 RH<sub>120<sub/> was observed photometrically with the new 10-m SALT telescope at the SAAO (South Africa). We obtained data on four nights: 11, 15, 16, and 17 March 2007 when the solar phase angle remained almost constant at 74°. The observations lasted about an hour each night and the object was exposed for 7-10 s through the “clear” filter.<i>Results. <i/>From the lightcurves obtained on three nights we derived two solutions for a synodical period of rotation: <i>P<i/><sub>1<sub/> = 1.375 <i>±<i/> 0.001 min and <i>P<i/><sub>2<sub/> = 2.750 <i>±<i/> 0.002 min. The available data are not sufficient to choose between them. The absolute magnitude of the object was found to be <i>H<i/> = 29.9 <i>±<i/> 0.3 mag (with the assumed slope parameter ) and its effective diameter <i>D<i/> = 2-7 m, depending on the geometric albedo <i>p<i/><sub><i>V<i/><sub/> (with the most typical near-Earth asteroids albedo <i>p<i/><sub><i>V<i/><sub/> = 0.18 its diameter would be <i>D<i/> = 3.3 <i>±<i/> 0.4 m). The body has an elongated shape with the ratio greater than 1.4. It probably originates in low-eccentricity Amor or Apollo orbits. There is still a possibility, which needs further investigation, that it is a typical near-Earth asteroid that survived the aerobraking in the Earth's atmosphere and returned to a heliocentric orbit similar to that of the Earth.