  While having a comet-like appearance, P/2012 F5 (Gibbs) has an orbit native
to the Main Asteroid Belt, and physically is a km-sized asteroid which recently
(mid 2011) experienced an impulsive mass ejection event. Here we report new
observations of this object obtained with the Keck II telescope on UT 2014
August 26. The data show previously undetected 200-m scale fragments of the
main nucleus, and reveal a rapid nucleus spin with a rotation period of 3.24
$\pm$ 0.01 hr. The existence of large fragments and the fast nucleus spin are
both consistent with rotational instability and partial disruption of the
object. To date, many fast rotators have been identified among the minor
bodies, which, however, do not eject detectable fragments at the present-day
epoch, and also fragmentation events have been observed, but with no rotation
period measured. P/2012 F5 is unique in that for the first time we detected
fragments and quantified the rotation rate of one and the same object. The
rapid spin rate of P/2012 F5 is very close to the spin rates of two other
active asteroids in the Main Belt, 133P/Elst-Pizarro and (62412), confirming
the existence of a population of fast rotators among these objects. But while
P/2012 F5 shows impulsive ejection of dust and fragments, the mass loss from
133P is prolonged and recurrent. We believe that these two types of activity
observed in the rapidly rotating active asteroids have a common origin in the
rotational instability of the nucleus.
