Ultima Thule emerges as contact binary, “cosmic snowman,” in new spacecraft images

History has been made at the New Horizons operations headquarters at the Johns Hopkins Applied Physics Lab on January 2, 2019. The team unveiled new images received late in the day yesterday, and it’s now clear that Ultima Thule, a distant, icy Kuiper Belt Object lying 4 billion miles away, is a contact binary. That means it is a single object, with two lobes, but the lobes are gently in contact. The scenario to form such a body is that probably numerous small planetesimals came together in the outer solar system in a small region early on, the two of them more or less nudged together, gently fused by gravity. This created a snowman-shaped, two-lobed binary object.

We now know the basics of this strange object, the most distant solar system body ever explored by a spacecraft. It is approximately 21 miles long. It has a rotation period of about 15 hours. It is very dark, reflecting only about 10 percent of the light that strikes it. It is reddish in color, probably from the radioactive decay of organic compounds on its icy surface.

Scientists can also now infer some geological properties of Ultima Thule. The body has a mottled appearance due to some surface irregularities or differences in elevation. It appears not to have any impact craters. It does show the tight, squeezed area of a belt, the small region where the two lobes are in contact. It has some brighter regions, and some darker ones. We’ll continue to learn much more about the object’s geology in the coming hours and days.

Planetary scientists have now nicknamed the two lobes, giving the name Ultima to the larger one and Thule to the smaller one. We believe that Ultima Thule, which represents pristine, unchanged icy material from the early days of the solar system, formed when small, icy bodies came together, accreting into larger ones. Two of the larger bodies stuck together gently as much smaller ones cleared out. The remaining two lobes formed Ultima Thule, and with its material we have a window into the early solar system.

The best summary of this day came from a team slide: “Think of New Horizons as a time machine that has brought us back to the very beginning of the solar system, to a place where we can observe the most primordial building blocks of the planets.”