Look at that thing. Ultima Thule is clearly two separate objects, both nearly spherical, that came together into one body. Each is so distinctive, mission scientists have split the name and assigned “Thule” to the smaller one and “Ultima” to the other. The bodies seem to be barely touching but are in fact firmly held together by gravity. It reminds me of a snowman that’s missing its head.
The most distant object ever explored by humankind also appears to be one of the most ancient and primitive objects in the solar system. Theories of the solar system’s formation start from the ground up with smaller pebbles sticking together to form larger planetesimals, the building blocks of the planets. Ultima Thule formed at the beginning of it all from small, icy chunks that coalesced into two planetesimals. The two spiraled into toward one another and joined to form what astronomers call a “contact binary.” If you want to know what things looked like shortly after the first solid materials coalesced from the solar nebula, Ultima Thule appears to be as close as we’ve come so far. Like taking a ride back in a time machine.
The reason the two objects still appear so neatly intact is that they approached each other at about the same speed you back into a parking spot — 1 to 2 miles an hour — and slowly fused together. Take a look again at color photo and focus on the “neck” of the asteroid. The rest of Ultima Thule is reddish-brown, probably from methane or other organic compounds exposed to sunlight, but the neck is pale, almost white and indicates lots of fine-grained material. During today’s press conference, mission scientists suggested it could be material that rolled downhill from the steep slopes where the two bodies make contact. Much more detailed photos are on the way along with compositional measurements that will help answer that question in more detail.
The asteroid looks rather flat and shadowless in the photo because the sun was at the spacecraft’s back and in direct, face-on sunlight. As New Horizons passed the asteroid, it continued to take pictures as the lighting angle changed. In those pictures the sun will be off to one side, and we’ll see lots more detail including the topography of hills and valleys. While some of the round spots could be craters, no one can say for sure until the side-lit images make it back to Earth.
Reporters asked what the asteroid was made of, and although the scientists demurred, one suggested it could be methane and nitrogen ices. Cold stuff! Another interesting detail that came up was where New Horizons will go next. It still has years worth of nuclear fuel, enough to travel nearly 2.5 times further than it is now. Alan Stern, the prinicipal investigator for the project, and his team will be making proposals for new targets in the coming months. Tune in for more science information and possibly more photos during a press briefing Wednesday (Jan. 3) from 1-2 p.m. Central Time at this site.
Keep going you little emissary from Earth!