New Horizons reveals Kuiper Belt object is a contact binary


LAUREL, Md. — The distant object that NASA’s New Horizons spacecraft flew past Jan. 1 is now taking shape as a body — or bodies — unlike any visited by a spacecraft to date.

At a Jan. 2 press conference at the Johns Hopkins University Applied Physics Laboratory (APL) here, scientists working on the mission released new images showing that the Kuiper Belt object known as 2014 MU69, and nicknamed Ultima Thule, is a “contact binary,” two objects touching one another, with an appearance some likened to a snowman.

“Just like with Pluto, we could not be happier,” Alan Stern, the principal investigator for New Horizons, said, recalling the spacecraft’s 2015 flyby of that world. “What you’re seeing is the first contact binary ever explored by spacecraft: two completely separate objects that are now joined together.”

The shape of Ultima Thule had been the subject of speculation for years leading up to the flyby. The small object appeared as a point source in Hubble Space Telescope images, where it was discovered in 2014, and in images from New Horizons itself until a couple days before the flyby. Scientists speculated that it could be single, likely elongated object, or two objects closely orbiting each other.

The contact binary shape of Ultima Thule is consistent with models of the formation of the Kuiper Belt. “What we think we’re looking at is the end product of a process that probably took place only a few hundred thousand or maybe a few million years at the very beginning of the formation of the solar system,” said Jeff Moore of NASA’s Ames Research Center.

At that time the outer fringes of the solar system consisted of “innumerable small particles or pebbles,” Moore explained, that slowly coalesced into larger ones. That created the two bodies seen at Ultima Thule: a larger one, dubbed simply “Ultima,” that is 19 kilometers across, and a smaller one, “Thule,” 14 kilometers across.

The two bodies came together at a very low speed, he said, on the order of a few kilometers per hour, slow enough to preserve each object. “If you had a collision with another car at those speeds, you may not even bother to fill out the insurance forms,” he said.

That means that Ultima Thule is likely an object that dates back to the formation of the solar system, as scientists suspects prior to the flyby. “What we’re looking at is basically the first planetesimals,” Moore said. “These are the only remaining basic building blocks.”

There is some dispute among scientists, though, about whether Ultima Thule is the first contact binary seen. The nucleus of comet 67P/Churyumov–Gerasimenko, as seen by ESA’s Rosetta spacecraft, has two lobes that appear connected by a narrow “neck” region. Moore said that the shape of that cometary nucleus could also be explained by activity as cometary ice sublimates.

Besides the improved images, scientists also refined other knowledge of the object. Cathy Olkin, deputy project scientist, said the object has a rotation period of approximately 15 hours. The images, she said, also showed some brightness variations on the surface, including a brighter area in the neck where the two bodies meet. That could be explained if the neck contains fine-grained particles that settle there from both lobes.

The first color images, taken at lower resolution, show that Ultima Thule has a red color. That color can be explained by irradiation of ices on its surface, said Carly Howett of the Southwest Research Institute.

The images don’t yet reveal much information about topography given the sun angle at the time New Horizons took the image, on approach to Ultima Thule at a distance of about 50,000 kilometers. That will change as later images, taken at different angles, are sent back, Moore said.

Also on its way back to Earth are spectral data collected by the spacecraft. Olkin said the initial data will focus on specific bands that could help scientists identify water ice or other volatiles.

The only issue with the flyby so far has nothing to do with the spacecraft or the object but rather the Ultima Thule nickname that the mission, with NASA’s concurrence, applied to the object. In just the last day some members of the public expressed reservations about the name because of ties to Nazi ideology.

Stern defended the choice of the name, noting that it dates back to the classical era, referring to the most distant northern lands. The name, he said, “is a wonderful meme for exploration, and that’s why we chose it.”

“And I would say that just because some bad guys once liked that term, we’re not going to let them hijack it,” he said, prompting a round of applause from team members and guests in the APL auditorium.

Thomas Zurbuchen, NASA associate administrator for science, defended the name as well. “If there is a connection, it is very tenuous,” he said of any Nazi ties to the name, emphasizing the “positive message” of the name. He added that searches for the term online turn up other, innocuous uses the name.

A Google search for “Ultima Thule” found not just references to the Kuiper Belt object and the mission but also a lodge in Alaska, Finnish glassware and an Australian radio show, among other citations.