The surveillance craft turned as it continued its silent orbit of the Moon.  Its sensors focussed on the surface, ready to track any movement below.  Following in its orbit, invisible to the sensors of the first spacecraft, a second angular shape slowly gained ground.  As it approached it readied a concentrated burst of electromagnetic energy.

NASA has announced that the twin GRAIL Moon satellites have started their science mission.  They will track the distance between the two craft as they pass over sections of the Moon that may be more or less dense.  This will give scientists data to model the internal structure of the Moon, and hence refine theories as to its origin and evolution.  Read the press release here:  http://www.nasa.gov/home/hqnews/2012/mar/HQ_12-070_GRAIL_Science_Begins.html

James.