EPS Colloquium: Bill McKinnon

Exploring the Kuiper Belt: NASA's New Horizons Reaches Ultima Thule

On 1 January 2019, the New Horizons spacecraft flew within 3500 km of (486958) 2014 MU₆₉ (nicknamed Ultima Thule), a “cold classical” Kuiper belt object (KBO), so called because this small world likely formed close to where it orbits today — in the far-off Kuiper belt beyond Neptune. In this talk I will describe initial results from the encounter. MU₆₉ turned out to be a bi-lobate, contact binary with a remarkably flattened larger lobe, (possibly) discrete geological units, and albedo heterogeneity. In contrast, there is apparently little surface color or compositional heterogeneity. No evidence for satellites, ring/dust structures, gas coma, or solar wind interactions was detected, though the latter two were not expected. A relative paucity of small (<1-km) impact craters matches expectations, even for an ancient body, given the size-frequency and velocity distributions of small KBOs in this remote region of the Solar System. Evidence to date suggests a very gentle merger of the two lobes of MU₆₉, as opposed to something more violent or catastrophic. Overall, MU₆₉ provides the clearest view to date of the accretion processes operative in the protosolar nebula and subsequent planetesimal disk.