The mutual orbit, mass, and density of transneptunian binary Gǃkúnǁ'hòmdímà (229762 2007 UK126)

Abstract We present high spatial resolution images of the binary transneptunian object Gǃkunǁ'homdima (229762 2007 UK 126 ) obtained with the Hubble Space Telescope and with the Keck observatory on Mauna Kea to determine the orbit of Gǃo'e ǃhuGǃo'e ǃhu, the much smaller and redder satellite. Gǃo'e ǃhu orbits in a prograde sense, on a circular or near-circular orbit with a period of 11.3 days and a semimajor axis of 6000 km. Tidal evolution is expected to be slow, so it is likely that the system formed already in a low-eccentricity configuration, and possibly also with the orbit plane of the satellite in or close to the plane of Gǃkunǁ'homdima's equator. From the orbital parameters we can compute the system mass to be 1.4 × 10 20  kg. Combined with estimates of the size of Gǃkunǁ'homdima from thermal observations and stellar occultations, we can estimate the bulk density as about 1 g cm −3 . This low density is indicative of an ice-rich composition, unless there is substantial internal porosity. We consider the hypothesis that the composition is not unusually ice-rich compared with larger TNOs and comet nuclei, and instead the porosity is high, suggesting that mid-sized objects in the 400 to 1000 km diameter range mark the transition between small, porous objects and larger objects that have collapsed their internal void space as a result of their much higher internal pressures and temperatures.