NASA's Mission to Pluto and the Kuiper Belt
Ultima Thule is located in the Kuiper belt in the outermost regions of the Solar System, beyond the orbit of Neptune. In the early morning of January 1st 2019 Eastern Time, NASA's New Horizons spacecraft will fly by Ultima Thule at a distance of 3500 km (2200 miles). At this time, Ultima Thule will be at a distance of almost 6.5 billion km (4 billion miles) from the Sun, making this the most distant planetary flyby that has yet been attempted, and the first time that a Solar System object of this type has been seen close-up.
Ultima Thule measures approximately 30 km in diameter, and is irregularly shaped. In July 2017, Ultima Thule passed in front of a star as seen from Earth (a stellar occultation), allowing astronomers to determine that its shape is most likely a contact binary (two bodies that are touching) or a close binary system (two objects that are orbiting each other). An artist's impression of Ultima Thule as a contact binary is shown in the accompanying picture.
We will only know what Ultima Thule's surface looks like once New Horizons has sent back the first pictures after it has flown by, although based on observations of similar-sized Solar System objects, it will almost certainly display impact craters. The lighting environment at its surface is very dim, as it receives only about 0.05% of the light from the Sun that Earth does. We do know that Ultima Thule has a reddish color, probably caused by exposure of hydrocarbons to sunlight over billions of years. The flyby will also reveal whether it has any moons, or even a ring system. Ultima Thule belongs to a class of Kuiper belt objects called the "cold classicals", which have nearly circular orbits with low inclinations to the solar plane, and which have not been perturbed since their formation perhaps 4.6 billion years ago. Ultima Thule will therefore be the most primitive planetary object yet explored, and will reveal to us what conditions were like in this distant part of the Solar System as it condensed from the solar nebula.