Four months ago, we flew by the most distant body we’ve ever studied up close and discovered an object that was far more mysterious than anticipated.

NASA’s New Horizons spacecraft performed the flyby of 2014 MU69, informally known as Ultima Thule, at New Year 2019 and the first peer-reviewed analysis of the data was published today in Science.

Ultima Thule is an astonishing four billion miles from Earth and getting close to it offers us our first real glimpse of the birth of our Solar System. The two distinct lobes of Ultima Thule are called a planetesimal, basically a remnant of the rocks that came together to form planets.

This composite image of the primordial contact binary Kuiper Belt Object 2014 MU69 (nicknamed Ultima Thule) – featured on the cover of the May 17 issue of the journal Science – 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.
(Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko)

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko

“We’re looking into the well-preserved remnants of the ancient past,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute, Boulder, Colorado, in a statement. “There is no doubt that the discoveries made about Ultima Thule are going to advance theories of solar system formation.”

In this body’s case, the two lobes were once distinct bodies in a binary system, that first became tidally-locked and then eventually came together in a gentle merger, rather than a cataclysmic impact.

Somehow, the orbital momentum between the bodies slowed down and they came together. Potentially because of the aerodynamic forces from gas in the ancient solar nebula, or because Ultima and Thule, as they are fondly nicknamed, got closer together when they ejected other smaller space rocks and lost energy.

Solving this mystery is just one branch of study in the data. Scientists are also looking at the topography of the planetoid, including its bright spots, hills, troughs and craters.

The larger depressions will no doubt prove to be impact craters, but smaller pits could have been caused by exotic ices going from solid to gas or material falling into underground spaces.

Like most Kuiper Belt objects, Ultima Thule is red, but it is a lot redder than its colleagues. The data shows evidence of methanol, water ice and organic molecules on the body’s surface, a very different mixture to other icy objects we’ve managed to study up-close.

Data from the flyby is still being transmitted back to Earth and will continue until late summer next year. And New Horizons is not out of the game yet. It is now 4.1 billion miles from Earth and speeding deep into the Kuiper Belt at nearly 33,000 miles an hour. While it is too far away from other objects to do another flyby, it is mapping the radiation and dust environment as well as measuring distant objects’ brightness.


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