  The peculiar object P/2010 A2 was discovered by the LINEAR near-Earth
asteroid survey in January 2010 and given a cometary designation due to the
presence of a trail of material, although there was no central condensation or
coma. The appearance of this object, in an asteroidal orbit (small eccentricity
and inclination) in the inner main asteroid belt attracted attention as a
potential new member of the recently recognized class of 'Main Belt Comets'
(MBCs). If confirmed, this new object would greatly expand the range in
heliocentric distance over which MBCs are found. Here we present observations
taken from the unique viewing geometry provided by ESA's Rosetta spacecraft,
far from the Earth, that demonstrate that the trail is due to a single event
rather than a period of cometary activity, in agreement with independent
results from the Hubble Space Telescope (HST). The trail is made up of
relatively large particles of millimetre to centimetre size that remain close
to the parent asteroid. The shape of the trail can be explained by an initial
impact ejecting large clumps of debris that disintegrated and dispersed almost
immediately. We determine that this was an asteroid collision that occurred
around February 10, 2009.
