After flying past the dwarf planet Pluto in 2015, NASA’s New Horizons spacecraft shifted course to Ultima Thule (also known as 2014 MU69), a much smaller body about 22 miles (36 km) in length. Ultima Thule is part of the Kuiper Belt, a torus-shaped ensemble of objects in the outer Solar System beyond the orbit of Neptune. The New Horizons team has just published the first peer-reviewed scientific results from the flyby, and they reveal some fascinating things about Ultima Thule: it consists of two lobes that appear to have merged at low speed, producing a contact binary; this type of Kuiper Belt object is mostly undisturbed since its formation roughly 4.5 billion years ago and so will preserve clues about that process; no evidence for Ultima Thule’s satellites, rings or other dust structures, a gas coma, or solar wind interactions was detected.
This composite image of Ultima Thule was compiled from data obtained by NASA’s New Horizons spacecraft as it flew by the object on January 1, 2019. The image combines enhanced color data (close to what the human eye would see) with detailed high-resolution panchromatic pictures. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute / Roman Tkachenko.
New Horizons conducted a flyby of Pluto and its system of moons on 14 July 2015. The spacecraft then continued farther into the Kuiper Belt, adjusting its trajectory to fly close to Ultima Thule.
Ultima Thule is located about 1 billion miles (1.6 billion km) farther from the Sun than Pluto was at the time of the New Horizons flyby.
Its orbit indicates that it is a ‘cold classical’ Kuiper Belt object, thought to be the least dynamically evolved population in the Solar System.
New Horizons completed its Ultima Thule flyby on January 1, 2019, with a closest approach distance of 2,200 miles (3,538 km) — less than one-third of its closest distance to Pluto.
During the high-speed flyby, made at 9 miles/s (14.4 km/s), the probe collected about 50 Gigabits of high-resolution imaging, compositional spectroscopy, temperature measurements, and other data on this Kuiper Belt object.
Their analysis indicates that Ultima Thule has a flattened bi-lobate shape, probably formed by the gentle collision of two smaller objects.
“Ultima Thule consists of a large, strangely flat lobe (nicknamed Ultima) connected to a smaller, somewhat rounder lobe (nicknamed Thule), at a juncture nicknamed the neck,” said New Horizons principal investigator Dr. Alan Stern and colleagues.
“How the two lobes got their unusual shape is an unanticipated mystery that likely relates to how they formed billions of years ago.”
“The lobes likely once orbited each other, like many so-called binary worlds in the Kuiper Belt, until some process brought them together in what scientists have shown to be a gentle merger.”
For that to happen, much of the binary’s orbital momentum must have dissipated for the objects to come together, but scientists don’t yet know whether that was due to aerodynamic forces from gas in the ancient Solar Nebula, or if Ultima and Thule ejected other lobes that formed with them to dissipate energy and shrink their orbit.
The alignment of the axes of Ultima and Thule indicates that before the merger the two lobes must have become tidally locked, meaning that the same sides always faced each other as they orbited around the same point.
“We’re looking into the well-preserved remnants of the ancient past,” Dr. Stern said.
“There is no doubt that the discoveries made about Ultima Thule are going to advance theories of the formation of the Solar System.”
The largest depression on Ultima Thule is a 5-mile (8 km) wide feature, nicknamed Maryland crater, which likely formed from an impact.
Some smaller pits on the object, however, may have been created by material falling into underground spaces, or due to exotic ices going from a solid to a gas (called sublimation) and leaving pits in its place.
In color and composition, Ultima Thule resembles many other objects found in its area of the Kuiper Belt.
It’s very red — redder even than much larger, 1,500-mile (2,400-km) wide Pluto, and is in fact the reddest outer Solar System object ever visited by spacecraft.
Its reddish hue is believed to be caused by modification of the organic materials on its surface New Horizons scientists found evidence for methanol, water ice, and organic molecules on Ultima Thule’s surface — a mixture very different from most icy objects explored previously by spacecraft.
“The insights gained in this study are based on only about 10% of the total data collected during the flyby,” the researchers said.
“The full data transmission from the spacecraft to Earth is expected to be complete in 2020.”
The results appear in the journal Science.
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S.A. Stern et al. 2019. Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object. Science 364 (6441): eaaw9771; doi: 10.1126/science.aaw9771
