Ultima Thule, filmed by New Horizons from 9000 kilometres away.
When NASA’s New Horizons Spacecraft got its first good views of Ultima Thule, the 32-kilometre-long Kuiper Belt object it zoomed by on 1 January 2019, science fiction fans thought it looked remarkably like BB-8, the rolling droid of Star Wars fame.
Closer views made it look more like a frowning snowman.
But now, as images come back from a third angle, it looks more like a spaceship from the Star Trek franchise.
Initially, the object – at 6.6 billion kilometres away, the most distant thing ever visited by humanity – looked like two spheres joined by a narrow “neck”.
But the new images, released in early February, reveal a different profile, considerably more flattened.
It’s possible to see a great many shapes in them, ranging from the Starship Enterprise at the start of a saucer separation manoeuvre to what the project’s official release more conservatively describes as a pancake merged to a dented walnut.
The images were taken nearly 10 minutes after New Horizons’ closest approach, when the fast-moving spacecraft was already looking back from a range of almost 9000 kilometres.
From that angle, all that was visible was a thin crescent, somewhat like the new moon seen from Earth – except this new moon is wing-shaped due to Ultima Thule’s two lobes.
The curvature of the wings was itself enough to prove that Ultima Thule’s lobes are flattened, not spherical. But this was confirmed by watching how the dark parts of the object blocked out background stars during a sequence of 14 such images. That allowed the scientists to trace out the rest of its silhouette, even though most of its surface was too dark to be seen.
“We had an impression of Ultima Thule based on the limited number of images returned in the days around the flyby,” says Alan Stern, the mission’s principal investigator, “but [this] has significantly changed our view.”
Diagram showing the “old” and “new” approximations of the shape of Ultima Thule.
It also poses a puzzle for scientists trying to unravel the mysteries of the early Solar System. “The new images are creating scientific puzzles about how such an object could even be formed,” Stern says. “We’ve never seen something like this orbiting the sun.”
Hal Weaver, a New Horizons project scientist from Johns Hopkins University’s Applied Physics Laboratory in Maryland, US, adds that Ultima Thule’s bizarre shape is almost certainly a relic from its formation 4.5 billion years ago.
“We think Ultima Thule has changed very little since it formed,” he says.
Its shape, he adds, may reflect the way in which each of the two lobes formed before they merged, possibly revealing information about asymmetries in the processes that formed them. But, he adds, “the details will take months to sort out”.
The elongated shapes also mean that the two lobes had less angular (or rotational) momentum when they merged than they would have had if they were spherical.
That, Weaver says, makes it easier to figure out how the merger happened in concert with a law of physics known as conservation of angular momentum, which requires that the rotational momentum the two objects had before they merged had to have gone somewhere, rather than being lost entirely.
(One theory is that any excess was carried away by one or more tiny moons, yet to be found.)
Meanwhile there are still some uncertainties about Ultima Thule’s true shape, which might be either flatter or not as flat as the current estimate.
“The number of look angles are limited for such a fast flyby,” Weaver says, “but we’re squeezing out as much as we can, using all the available evidence.”
And, he adds: “We’re still waiting for more images, which should allow us to further refine the shape. But the limited range of viewing directions and the fact that one hemisphere of Ultima Thule was never illuminated by the Sun will constrain how well we can do.”
