NASA's Mission to Pluto and the Kuiper Belt
This composite image of the primordial contact binary Kuiper Belt object 2014 MU69 (officially named Arrokoth) was compiled from data obtained by NASA's New Horizons spacecraft as it flew by the object on Jan. 1, 2019. The image combines enhanced color data (close to what the human eye would see) with detailed high-resolution panchromatic pictures.Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute//Roman Tkachenko
The New Horizons team used the powerful Hubble Space Telescope to search for its post-Pluto, Kuiper Belt flyby target. Using observations made with Hubble on June 26, 2014, the science team (led by co-investigator Marc Buie) discovered an object that New Horizons could reach with its available fuel. The object was subsequently designated 2014 MU69, given the minor planet number 485968 and, with public input, nicknamed "Ultima Thule" (which means "beyond the known world") in March 2018.
The New Horizons team had always planned to propose a permanent name for MU69 after the flyby, based in part on whether MU69 was found to be a single body, a binary pair, or perhaps a system of multiple objects. -- and that happened on November 12, 2019, when the mission team announced MU69's official name as "Arrokoth," a Native American term that means "sky" in the Powhatan/Algonquin language.
Arrokoth is located in the Kuiper Belt, beyond the orbit of Neptune. At 12:33 a.m. (EST) on January 1, 2019, New Horizons flew just 2,200 miles (3,500) kilometers from the object's surface, when it was about 4 billion miles (6.6 billion kilometers) from the Sun -- the most distant planetary flyby in history and the first close-up look at a solar system object of this type.
Arrokoth is the first unquestionably primordial contact binary ever explored. Approach pictures hinted at a strange, snowman-like shape, but further analysis of images, taken near closest approach, uncovered just how unusual the KBO's shape really is. At 22 miles (35 kilometers) long, the binary consists of a large, flat lobe (nicknamed "Ultima") connected to a smaller, rounder lobe (nicknamed "Thule").
This strange shape was the biggest surprise of the flyby. Nothing like it has been anywhere in the solar system -- sending the planetary science community back to the drawing board to understand how planetesimals (the building blocks of the planets) form.
Because it is so well preserved, Arrokoth offered our clearest look back to the era of planetesimal accretion and the earliest stages of planetary formation. Apparently the two lobes once orbited each other, like many so-called binary worlds in the Kuiper Belt, until something brought them together in a "gentle" merger.
In color and composition, New Horizons data revealed that MU69 resembles many other objects found in its region of the Kuiper Belt. Consistent with pre-flyby observations from the Hubble Telescope, Arrokoth is very red – redder even than Pluto, which New Horizons flew past on the inner edge of the Kuiper Belt in 2015 – and about the same color as many other so-called "cold classical" KBOs. ("Cold" referring not to temperature but to the circular, uninclined orbits of these objects; "classical" in that their orbits have changed little since forming, and represent a sample of the primordial Kuiper Belt.)
New Horizons scientists have also seen evidence for methanol, water ice and organic molecules on the surface -- a spectrum similar to some of the most extreme objects we've seen in the outer solar system.
Flyby data transmission continues, with all data expected on the ground by late 2020.