LAUREL, Maryland — The first well-resolved image of the faraway chunk of rock fleetingly visited by NASA’s New Horizons spacecraft on New Year’s Day reveals the object — officially named 2014 MU69 but nicknamed Ultima Thule — is made of two lobes scientists say came together in an ancient slow-speed collision just as the solar system’s planets were forming.
Roughly the size of a large metropolitan area, the frozen world located 4.1 billion miles (6.6 billion kilometers) is a relic left over from the birth of the solar system some 4.5 billion years ago. The plutonium-powered New Horizons spacecraft targeted Ultima Thule after zipping past Pluto in July 2015.
Scientists revealed the sharpest image of Ultima Thule, nicknamed for an ancient term meaning “beyond the known world,” in a press conference Wednesday at the Johns Hopkins University Applied Physics Laboratory here, where New Horizons was built and home of the mission control center. The new images, taken just before the craft’s closest approach, were the first to reveal details about Ultima Thule’s shape, a day after the previously-best picture only resolved the object as a fuzzy blob.
“That image is so 2018,” said Alan Stern, New Horizons principal investigator from the Southwest Research Institute, referring to the New Year’s Eve pre-flyby picture. “Meet Ultima Thule! Just like with Pluto, we could not be happier. What you’re seeing is the first contact binary ever explored by spacecraft. It’s two completely separate objects that are now joined together.”
The bigger of the two lobes has been nicknamed Ultima, and the smaller one Thule, Stern said. The bigger section is around three times the volume of the smaller one.
Shaped like a charcoal-colored snowman in black-and-white images, Ultima Thule actually appears reddish in color imagery captured by the New Horizons spacecraft, according to Carly Howett, a co-investigator on the mission from the Southwest Research Institute.
Scientists have also nailed down Ultima Thule’s rotation rate, a basic characteristic that eluded the New Horizons team during the spacecraft’s approach.
“We now know that Ultima rotates with a period of 15 hours, give or take one hour,” said Cathy Olkin, the mission’s deputy project scientist from the Southwest Research Institute.
Olkin said the object’s rotation axis was tilted about 30 degrees from the spacecraft’s approach vector, meaning much of the same face of Ultima Thule always faced New Horizons as it sped toward the New Year’s Day Encounter.
Stern said the spacecraft’s one-shot encounter went according to plan, with the probe zooming past Ultima Thule at a distance of around 2,200 miles (3,500 kilometers) at 12:33 a.m. EST (0533 GMT) on New Year’s Day. He called the flyby “a technical success beyond anything ever attempted before in spaceflight.”
Discovered by the Hubble Space Telescope in 2014, Ultima Thule is the most distant planetary body ever explored up-close, and a building block of planetary formation scientists said has been undisturbed for billions of years, holding clues about the conditions in the early solar system.
“What this spacecraft and this team accomplished is unprecedented,” Stern said Wednesday. “The object that we rendezvoused with at midnight on New Year’s Day wasn’t even known until the summer of 2014.
“It’s only really the size of something like Washington D.C., and it’s about as reflective as garden variety dirt, and it’s illuminated by a sun that’s 1,900 times fainter than it is outside on a sunny day here on the Earth,” Stern continued. “So we were basically chasing it down in the dark at 32,000 mph (14 kilometers per second), and all that had to happen just right, as did the entire sequence.”
The results discussed in Wednesday’s press conference are preliminary. According to Stern, far less than 1 percent of the data collected during the flyby has been downlinked to Earth, a time-consuming process expected to take 20 months because of the slow transmission rate over the vast distance to New Horizons.
Jeff Moore, New Horizons geology and geophysics team lead from NASA’s Ames Research Center, said Ultima Thule likely formed by a low-speed merger between two bodies more than 4 billion years ago, at the earliest stages of the birth of the solar system’s planets.
“We think what we’re looking at is perhaps the most primitive object that has yet been seen by any spacecraft, and may represent a class of objects which are the oldest and most primitive objects that can be seen anywhere in the present solar system,” Moore said.
“What we think we’re looking at is the end product of a process which probably took place in only a few hundred thousand or maybe a few million years at the very beginning of the formation of the solar system, where initially you have innumerable small particles or pebbles that swirl together into little nodes,” Moore said.
Through collisions and gravitational interactions, the debris grew into larger clumps, forming the two bodies that make up Ultima Thule, and countless similar objects that formed the planets.
“Maybe that’s how Ultima and Thule, as separate objects, formed, and then as the last few bits and pieces of their local swirl are ejected or else collide, the amount of energy that’s still left in the system is so little that the two lobes come together,” Moore said.
The two bodies that formed Ultima Thule must have merged at slow speeds — likely at only a few miles per hour — to neatly come together without disturbing their individual near-spherical shapes, Moore said.
Pluto’s small moons are the shard-like leftovers from a violent collision, whereas Ultima Thule’s lobes retain their original shape.
“So what we’re looking at is basically the first planetesimals, and these first planetesimals, where they were more commonly found in the inner solar system, came together to form the planets and the moons and everything else we see,” Moore said. “But these are the only remaining basic building blocks in the backyard of the solar system that everything else we live on, or can see with our telescopes, or visit with our spacecraft, were formed from.
“I think we should think about New Horizons as a time machine, like the ‘wayback machine’ that we set to time zero that has brought us back to the very beginning of solar system history, to a place where we can observe the most primordial building blocks of the planets,” he said.
Even better images are expected to be sent back to Earth by New Horizons in the coming weeks, including views from different angles that will help scientists sort out the topography, cratering and roughness of Ultima Thule. At first glance, no obvious craters are visible on the object.
The first glimpse at Ultima Thule’s chemical composition should be available as soon as Thursday, when scientists plan to hold another press conference.
“It’s just going to get better and better,” Stern said.
The mission’s chief scientist also addressed a controversy over the nickname Ultima Thule during Wednesday’s press briefing. Ultima Thule’s roots are in Latin, but Nazis used the term to refer to the mythological birthplace of the Aryan race.
“I think New Horizons is an example, one of the best examples of our time, of raw exploration,” Stern said.
Stern noted the name’s ancient origins, and called it a “wonderful meme for exploration.”
“That’s why we chose it,” he said. “Just because some bad guys once liked that term, we’re not going to let them hijack it.”
Formally known as 2014 MU69, the object’s official name — and the names of its surface features — must be approved by the International Astronomical Union, and Stern told reporters there are plans to begin submitting names to the IAU later this year.
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