Photometry of 2006 RH120: an asteroid temporary captured into a geocentric orbitT. Kwiatkowski1, A. Kryszczyńska1, M. Polińska1, D. A. H. Buckley2, D. O'Donoghue2, P. A. Charles2, L. Crause2, S. Crawford2, Y. Hashimoto2, A. Kniazev2, N. Loaring2, E. Romero Colmenero2, R. Sefako2, M. Still2, and P. Vaisanen2
1 Astronomical Observatory, Adam Mickiewicz University, Słoneczna 36, 60-286 Poznań, Poland
2 South African Astronomical Observatory, Observatory Road, Observatory 7925, South Africa
Received 15 September 2008 / Accepted 12 December 2008
Aims. 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.
Methods. Due to its faintness, 2006 RH120 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.
Results. From the lightcurves obtained on three nights we derived two solutions for a synodical period of rotation: P1 = 1.375 0.001 min and P2 = 2.750 0.002 min. The available data are not sufficient to choose between them. The absolute magnitude of the object was found to be H = 29.9 0.3 mag (with the assumed slope parameter G = 0.25) and its effective diameter D = 2-7 m, depending on the geometric albedo pV (with the most typical near-Earth asteroids albedo pV = 0.18 its diameter would be D = 3.3 0.4 m). The body has an elongated shape with the a/b 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.
Key words: techniques: photometric -- minor planets, asteroids
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