On one of the rare decent nights in the month, I followed Apollo 2010 XM56 for 5.9 hours on December 16th. It had been discovered a week earlier by LINEAR and with an estimated diameter of only about 30 meters it was predicted to reach mag. +15 as it passed by Earth at slightly less than 2 Lunar distances. During the night its apparent speed accelerated from about 160"/min up to 250"/min and only stayed in the same field of view for 4 minutes at the start of the night and 3 minutes by the end. It was obviously varying substantially in brightness with a period of about 2 hours. I stacked all the usable images from each of the separate fields of view taken during the night and ended up with 96 photometric measures. The raw lightcurve shows a plot of relative magnitude against fractional Julian Day (0.5 = December 17.0 UT) and shows the amplitude of the brightness variation increasing from an already substantial 2.5 magnitudes at the start to 4.4 magnitudes by the end of the night! By the end, the object was changing from being very well recorded on individual 2 second exposures at maximum, then fading within 1/2 an hour to being completely invisible on individual images at the deep minimum, though the multiple image stacks made at minimum still recorded it well. Some of the increase in amplitude will be due to the rapidly increasing phase angle (47° increasing to 65°) as shadows lengthened on the surface of the object although there may also be some variation due to tumbling. The period was determined to be 2.35 +/- 0.02 h.
|Raw lightcurve of 2010 XM56 from 2010 Dec 16/17th|