Medium-sized satellites of large Kuiper belt objects

While satellites of mid- to small-Kuiper belt objects tend to be similar in size and brightness to their primaries, the largest Kuiper belt objects preferentially have satellites with small fractional brightness. In the two cases where the sizes and albedos of the small faint satellites have been measured, these satellites are seen to be small icy fragments consistent with collisional formation. Here we examine Dysnomia and Vanth, the satellites of Eris and Orcus, respectively. Using the Atacama Large Millimeter Array, we obtain the first spatially resolved observations of these systems at thermal wavelengths. We find a diameter for Dysnomia of 700+/-115 km and for Vanth of 475+/-75 km, with albedos of 0.04_+0.02_-0.01 and 0.08+/-0.02 respectively. Both Dysnomia and Vanth are indistinguishable from typical Kuiper belt objects of their size. Potential implications for the formation of these types of satellites are discussed.