Richard A Lovett
In the first published results from the New Horizons spacecraft’s New Year’s Day 2019 flyby of an object officially called 2014 MU69, but more commonly known as Ultima Thule, scientists have reported that despite its unusual shape, it has a remarkably bland surface, without major variations in colour or composition.
Nor does it have any detectable moons, dust clouds, or atmosphere, the scientists report in the journal Science.
But that doesn’t mean it’s boring, says John Spencer, a planetary scientist with the Southwest Research Institute, Boulder, Colorado, US, who is one of the paper’s co-authors.
The most striking finding comes from the object’s odd shape, which features two flattened lobes joined edgewise via a narrow neck.
This structure, Spencer says, means that the two lobes began as separate objects that gradually spiraled inward toward each other in a merger that was more a kiss than a crash.
That’s important, because it gives us a snapshot of the process in which small objects came together to produce the larger objects that eventually made the planets, he says.
But the relative blandness of the surface is also important not for what’s there, but for what isn’t: craters.
A few craters exist, but compared to the destinations that other spacecraft such as NASA’s OSIRIS-REx and Japan’s Hayabusa2 have been visiting, Ultima Thule is nearly as smooth as the proverbial baby’s bottom.
“If you look at the average asteroid, they are covered in craters,” Spencer says. “They are completely beaten up.”
That’s because asteroids lie much closer to the sun in a region where there is not only a dense concentration of objects, but where orbits are easily stirred up by Jupiter’s gravity.
“That means that they’re coming in at each other at crazy angles,” Spencer explains.
The result is that they have not only experienced a great many collisions during their lifetimes, but that when they collide, they often hit hard.
Most asteroids, in fact, are shards of larger bodies ripped apart by ancient impacts, then peppered by innumerable smaller ones.
“They are not intact, pristine objects,” Spencer says.
Ultima Thule, however, lies in a far-distant part of the solar system known as the Cold Classical Kuiper Belt, a region beyond Neptune where objects sedately circle in widely spaced, well-ordered orbits. Like cars on the freeway, they are all cruising along at the same speed in basically the same direction, and therefore don’t run into each other very often.
This means that Ultima Thule’s surface remains largely undisturbed since the dawn of the solar system.
“We’ve never seen that before,” Spencer says.
But that doesn’t mean its surface is as dull a cue ball. Rather, it is mottled, as though made up of sub-units, each about five kilometres across. Eight of these have been counted on the larger lobe, and that’s only on the parts of it that New Horizons was able to see as it zipped past.
“You get the idea that it’s built of different pieces,” Spencer suggests.
One of his colleagues, he adds, has described it as looking like monkey bread, a pastry-like snack in which balls of dough are baked together in a single pan, fusing into a lumpy loaf.
Ultima Thule obviously wasn’t baked in a cosmic bread pan, so it’s an open question how its component parts came to fit so smoothly together. Perhaps they were initially soft and pliable and sort of smushed into each other as they came together, Spencer speculates.
But at the moment, he concedes, “We’re scratching our heads.”
As for other findings, the lack of an atmosphere or any detectable amounts of gases venting from Ultima Thule’s surface was no surprise.
The object is far out from the sun and has been in the same orbit for four-and-half-billion years. If it was ever going to have vented gases, it presumably would have done so a long time ago.
But it would have been silly to waste a once-in-a-lifetime opportunity to find surprises.
“We looked for things even if we were not expecting to see them,” he says.
The lack of moons, on the other hand, was a disappointment, because their orbits would have allowed scientists to calculate Ultima Thule’s mass, and from that its density, a useful clue to its interior composition.
Not that the scientists are certain that no moons exist.
“We’re still searching,” says Spencer. The spacecraft, he adds, is continuing to download data, though most of the high-priority science stuff has already been received.
The next round of scientific analyses will probably be published in August or September.
After that, the scientists will turn their attention to seeking a fresh destination for New Horizons, for yet another flyby, reachable with the spacecraft’s remaining fuel.
Finding such a destination, will be tough, however, because there aren’t all that many objects that far out, and those that do exist are so dim that they are hard to find.
“I think it’s a long shot,” Spencer says, “but we’re certainly leaving no stone unturned. We’ll give it our best shot.”