Bramson, Stern J.

208996 Achlys (provisional designation 2003 AZ84) is a large trans-Neptunian object orbiting the Sun in the Kuiper belt, a region of icy bodies beyond Neptune. It was discovered on 13 January 2003 by Chad Trujillo and Michael E. Brown at Palomar Observatory. Achlys has an elongated shape that is believed to be distorted by its rapid 6.8-hour rotation. Its diameter is estimated to be roughly 940 km (580 mi) across its equator to 490 km (300 mi) across its poles. After Pluto and Orcus, Achlys is the third largest member of the plutinos—a population of Kuiper belt objects following a 2:3 orbital resonance with Neptune, in which they complete two orbits for every three orbits completed by Neptune. The surface of Achlys is dark gray and mostly composed of water ice. Observations of stellar occultations show that Achlys's surface bears a chasm or depression between 8 and 13 km (5.0 and 8.1 mi) deep, similar to those seen on Pluto and its moon Charon. Achlys has one known moon that is about 80 km (50 mi) in diameter. The mass and density of Achlys has not been measured, though predictions based on its shape and rotation suggest that it has a density lower than that of water ice.