  We examine the motions of large fragments at the head of the dust tail of
active asteroid P/2010 A2. In previous work we showed that these fragments were
ejected from the primary nucleus in early 2009, either following a
hypervelocity impact or by rotationally induced break-up. Here, we follow their
positions through a series of Hubble Space Telescope images taken during the
first half of 2010. The orbital evolution of each fragment allows us to
constrain its velocity relative to the main nucleus after leaving its sphere of
gravitational influence. We find that the fragments constituting a prominent
X-shaped tail feature were emitted in a direction opposite to the motion of the
asteroid and towards the south of its orbital plane. Derived emission
velocities of these primary fragments range between 0.02 and 0.3 m/s,
comparable to the ~0.08 m/s gravitational escape speed from the nucleus. Their
sizes are on the order of decimeters or larger. We obtain the best fits to our
data with ejection velocity vectors lying in a plane that includes the nucleus.
This may suggest that the cause of the disruption of P/2010 A2 is rotational
break-up.
