Astronomy Cast

Astronomy Cast takes a fact based journey through the cosmos as it offers listeners weekly discussions on astronomical topics ranging from planets to cosmology.

2012 LZ1 - a big chunk of rock

Finally on Sunday, June 17, 2012, after a long period of unstable weather, I loaded my target list of minor planet followup observations for an unattended imaging run. The list contained two recently discovered NEOS: 2012 LD1 and 2012 LT7, PHA 2004 CL which was only seen for two nights in that opposition at that time, NEO 2012 BC20 deep in a crowded milkyway starfield, and numbered NEO (317685). The run was supposed to start at 21:30 UT. I was a little ahead of time. Flatfield and bias images ready, initial focus done. At 21:05 UT it was not dark enough for those faint objects. So I decided to take a shot on PHA 2012 LZ1 which was discovered on June 10 and passed the earth on June 14 in 14.5 LD (lunar distances). Only a couple of days after closest encounter this object was fairly bright with a magnitude of 15.0 but still pretty fast with an angular speed of about 30 arcseconds per minute. With an exposure time of 3-5 seconds it sould be possible to make it visible on a single exposure but preventing the object from trailing. The pointing of the telescope was a matter of a minute and a sequence of 80 images was taken from 21:15 - 21:24 UT.

First the size of 2012 LZ1 was thought to be around 500 meters according to its brightness and distance. Because of its size and proximity to the earth this object was selected as radar target for the famous Arecibo radio telescope in Puerto Rico and the results where a bit surprising: 2012 LZ1 is almost twice as big. Scientists where able to determine that this space rock was only reflecting 2-4 percent of the sunlight that hit its surface. A surface as dark as coal.