In the hours before NASA’s New Horizons spacecraft sped past a distant object known as Ultima Thule, scientists were puzzled.
This blurry, 30-pixel image of the small planetesimal, captured on approach, had revealed Ultima Thule to be an elongated blob that might look like bowling pin. Or maybe a cigar. Or maybe not.
Nobody was really sure what it looked like because it’s only 33 kilometres across and at a distance of 6.6 billion kilometres, is nearly 50% farther away than Pluto from Earth. Then, late on New Year’s Eve 2018, New Horizons zoomed by at a speed of more than 50,000 kilometres per hour and at a distance of just 3500 kilometres.
And while it will be mid-January before the best of its images begin trickling back, the initial close-up view was stunning: Ultima Thule is a “contact binary” composed of two smaller objects stacked one on the other like a cosmic snowman.
Or, as Twitter quickly opined, like BB-8, the scene-stealing droid in the latest Star Wars movies.
That alone was an important discovery, says Jeff Moore, a planetary geologist from NASA Ames Research Centre in California, US, because it can be explained nicely by existing planetary-formation models.
In these models, objects like Ultima Thule formed early in solar system history from agglomerations of “pebbles”. These coalesced into successively larger bodies until, in the case of Ultima Thule, only two big ones remained. These then gradually spiralled toward each other until they merged in a walking-speed kiss, more like a docking manoeuvre than a collision.
“If you had an [automobile] collision at those speeds,” Moore says, “you might not even bother to fill out the insurance forms.”
Today, a narrow “neck” of material joins the two lobes, but it probably isn’t a structural feature cementing the two halves together. Rather, Moore says, it may simply be composed of debris that rolled into the gap from nearby slopes.
“You can see that these are two objects that have come together,” adds Cathy Olkin, a planetary scientist with the Southwest Research Institute (SwRI) in Colorado, US.
Silvia Protopapa, also of SwRI, adds that the lobes have the same average surface colour. Although it will be necessary to await the return of spectroscopic data to determine what, exactly, the pair are made of, it does appear that they comprise the same materials, she says, lending further support to the belief they formed from the same source.
The hope of discovering such things is exactly why NASA chose to direct New Horizons to Ultima Thule.
The target is part of a region known as the Kuiper Belt, made up of planetesimals that have spent their entire lives so far out from the sun that chemical reactions are essentially frozen, Stern explains.
“It’s the only population of anything we know of in the solar system where virtually nothing has happened in the past four billion years,” adds John Spencer, another planetary scientist from SwRI. “It’s a very special region.”{%recommended 7747%}
New Horizons’ trajectory has currently taken it behind the sun, interrupting data transmissions until 10 January. Even then, it will take 20 months to beam back all the data packed acquired in its brief flyby.
“The spacecraft has a 15-watt transmitter,” Stern says. “Not 15 kilowatts like a radio station, but 15 watts. And we’re receiving that from four billion miles away.”
Among the things scientists most want to see are high-resolution images of Ultima Thule’s surface. Current images range from 140 metres per pixel to about 300. Ultimately, that should be improved to 35 metres per pixel, Olkin says.
Also on the wishlist are images of the same parts of its surface from different angles. These will allow the construction of 3D maps of the topography which, among other things, will help scientists look for craters. The number and size of these will be important indicators of what conditions were like in the early Kuiper Belt.
The scientists are awaiting spectroscopic data to help determine what Ultima Thule is made of and to map compositional differences across its surface.
They are also on the lookout for small moons. If found, their orbits would reveal Ultima Thule’s mass and density, says Mark Showalter, of the SETI Institute in California, US. They could also help explain the process by which the two lobes came into contact.
Meanwhile, New Horizons is looking to the next stage of its mission.
In the next couple of years, it will pass close enough to several other Kuiper Belt objects to provide better views than are possible from Earth, or even the Hubble Space Telescope.
But the spacecraft is also still “very” healthy, Stern says. It still has manoeuvring fuel, and it won’t get too far out of range to communicate with Earth until sometime in the mid-to-late 2030s.
“We can do one more flyby,” he says, though he notes that until now nobody has spent much time looking for a new target.
“We’ve been very careful to stay focused on [Ultima Thule] and not get distracted by the next shiny thing,” he laughs.
But at the moment, the biggest concern might be over the cosmic snowman’s name.
In Greek and Roman lore, Ultima Thule was a term for something incredibly distant: so cold, dark, and far away that it was beyond the limits of the known world.
But Nazis and neo-Nazis have claimed the name as the origin of the Aryan race, and recent news reports have suggested that NASA may need to jettison it in favour of something free of such overtones.
Stern disagrees. “The term Ultima Thule is many centuries old — a wonderful meme for exportation. That’s why we chose it. Just because some bad guys once liked that term, we’re not going to let them hijack it.”
