Definition of "Ultima"

For the purposes of discussion, MU69’s larger and smaller lobes have been informally designated “Ultima” and “Thule,” respectively, by the New Horizons team. Ultima and Thule have best-fit measured diameters of ~19.5 and ~14.2km, respectively, with errors of only a few percent at this time.

2019 January 24, Stern, S. A., J. R. Spencer, H. A. Weaver, C. B. Olkin, J. M. Moore, W. Grundy, R. Gladstone et al., “Overview of initial results from the reconnaissance flyby of a Kuiper Belt planetesimal: 2014 MU69”, in arXiv‎

Ultima and Thule have similar colors with measured albedos ∼0.06, indicating that UT is a typical member of the Cold Classical Kuiper Belt class of objects.

2020, Umurhan, O. M., J. T. Keane, R. A. Beyer, M. Bird, I. Linscott, S. B. Porter, J. R. Spencer et al., “Thermophysical, Gravitational, and Geomorphology Properties of 2014 MU69”, in Abstracts of the 235th AAS Meeting (Honolulu, HI), page 717