Quaoar: A Rock in the Kuiper Belt

Here we present WFPC2 observations of the Kuiper belt object Quaoar, and its satellite Weywoot. The best-fit orbit of Weywoot to these data and the ACS discovery images, is an elliptical one. The data however, are consistent with a circular orbit, consistent with expectations from tidal forces. Assuming that Quaoar has surface properties similar to that of the Uranian and Neptunian satellites, we have adjust the HST size measurement and found Quaoar's diameter to be D<1100 km, consistent with the radiometric size measurement. From the circular orbit, Quaoar has a mass of 0.19 ± 0.03 Pluto masses. The data suggest that Quaoar's density must be greater than 3.5 g cm^-3, and can be as low as 2.8 g cm^-3 if the most extreme elliptical orbit is adopted. The existence of such a high density, and the satellite, Weywoot, suggest that a large collision is responsible for the formation of both. We discuss how this conclusion is problematic when the existence of methane ice on Quaoar's surface is considered.