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The main belt asteroids 458271 (2010 UM26) and 2010 RN221 share almostidentical orbital elements and currently appear as comoving objects 30 arcsecapart in the plane of the sky. They are products of the breakup of a parentobject, or the splitting of a binary, with a separation age measured in decadesrather 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 ofthe breakup and separation of the components are unknown. We obtained deep,high resolution imaging using the Hubble Space Telescope to characterize thepair and to search for material in addition to the main components that mighthave been released upon breakup. The primary and secondary have absolutemagnitudes $H$ = 17.98 and 19.69, respectively, and effective diameters 760 mand 350 m (assuming geometric albedo 0.20). The secondary/primary mass ratio is0.1, assuming equal densities. Time-series photometry shows that the primaryrotates with period 5.9 hour and has a small photometric range (0.15magnitudes), while the period of the secondary is undetermined (but >20 hours)and its lightcurve range is at least 1 magnitude. The primary rotation periodand component mass ratio are consistent with a simple model for the breakup ofa rotationally unstable precursor. However, unlike other observationallysupported instances of asteroid breakup, neither macroscopic fragments norunresolved material are found remaining in the vicinity of this asteroid pair.We suggest that the pair is a recently dissociated binary, itself formedearlier by rotational instability of 2010 UM26.

Physical Properties of the Young Asteroid Pair 2010 UM26 and 2010 RN221