Photometric observations of Earth-impacting asteroid 2008 TC3

A calibrated lightcurve is presented of the near-Earth asteroid 2008 TC3, obtained before it impacted Earth on October 7, 2008. The asteroid was observed in unfiltered images from the end of astronomical twilight until the object entered Earth's shadow about 2 h later. The observations covered a wide range of phase angles from 14.79 degrees to 2.93 degrees, during which the asteroid ranged from 82,000 km to 29,000 km distance from the observer. A method is presented for obtaining photometrically filtered brightness values for the asteroid using unfiltered imaging techniques. Over 1,700 images of the asteroid produce a lightcurve with a peak-to-peak variation in V of 0.76 magnitude. Analysis of the lightcurve yields values for H = 30.86 +/- 0.01 and G = 0.33 +/- 0.03. Combined with other constraints on the kinetic energy and diameter of the asteroid, which suggest a low 1.8 g cm-3 density and albedo 0.05 +/- 0.01, the value of H implies an asteroid of about 4.1 m in diameter, 28 m3 in volume, and 51,000 kg in mass. The determined value of G is out of range for normal, larger asteroids of albedo 0.05-0.15.