  The main belt asteroids 458271 (2010 UM26) and 2010 RN221 share almost
identical orbital elements and currently appear as comoving objects 30 arcsec
apart in the plane of the sky. They are products of the breakup of a parent
object, or the splitting of a binary, with a separation age measured in decades
rather than thousands or millions of years as for most other asteroid pairs
(Vokrouhlicky et al.~2022). The nature of the precursor body and the details of
the breakup and separation of the components are unknown. We obtained deep,
high resolution imaging using the Hubble Space Telescope to characterize the
pair and to search for material in addition to the main components that might
have been released upon breakup. The primary and secondary have absolute
magnitudes $H$ = 17.98 and 19.69, respectively, and effective diameters 760 m
and 350 m (assuming geometric albedo 0.20). The secondary/primary mass ratio is
0.1, assuming equal densities. Time-series photometry shows that the primary
rotates with period 5.9 hour and has a small photometric range (0.15
magnitudes), while the period of the secondary is undetermined (but >20 hours)
and its lightcurve range is at least 1 magnitude. The primary rotation period
and component mass ratio are consistent with a simple model for the breakup of
a rotationally unstable precursor. However, unlike other observationally
supported instances of asteroid breakup, neither macroscopic fragments nor
unresolved material are found remaining in the vicinity of this asteroid pair.
We suggest that the pair is a recently dissociated binary, itself formed
earlier by rotational instability of 2010 UM26.
