  We revisited a mass ejection phenomenon that occurred in asteroid P/2010 A2
in terms of the dynamical properties of the dust particles and large fragments.
We constructed a model assuming anisotropic ejection within a solid cone-shaped
jet and succeeded in reproducing the time-variant features in archival
observational images over ~3 years from 2010 January to 2012 October. When we
assumed that the dust particles and fragments were ejected in the same
direction from a point where no object had been detected in any observations,
the anisotropic model can explain all of the observations including (i) the
unique dust cloud morphology, (ii) the trail surface brightness and (iii) the
motions of the fragments. Our results suggest that the original body was
shattered by an impact with the specific energy of Q* <~ 350 J/kg, and remnants
of slow antipodal ejecta (i.e., anisotropic ejection in our model) were
observed as P/2010 A2. The observed quantities are consistent with those
obtained through laboratory impact experiments, supporting the idea that the
P/2010 A2 event is the first evidence of the impact shattering occurred in the
present main asteroid belt.
