Keck Observatory Laser Guide Star Adaptive Optics Discovery and Characterization of a Satellite to the Large Kuiper Belt Object 2003 EL61

The newly commissioned laser guide star adaptive optics system at Keck Observatory has been used to discover and characterize the orbit of a satellite to the bright Kuiper Belt object 2003 EL₆₁. Observations over a 6 month period show that the satellite has a semimajor axis of 49,500+/-400 km, an orbital period of 49.12+/-0.03 days, and an eccentricity of 0.050+/-0.003. The inferred mass of the system is (4.2+/-0.1)×10²¹ kg, or ~32% of the mass of Pluto and 28.6%+/-0.7% of the mass of the Pluto-Charon system. Mutual occultations occurred in 1999 and will not occur again until 2138. The orbit is fully consistent neither with one tidally evolved from an earlier closer configuration nor with one evolved inward by dynamical friction from an earlier more distant configuration.