Ultima Thule — from blurry bowling pin to world at the edge of our understanding
The most distant object ever explored by spacecraft is a reddish, snowmanshape rock 6.4 billion km from Earth.
The object, nicknamed Ultima Thule, was photographed by Nasa’s New Horizons spacecraft during a late-night rendezvous on Tuesday. It is the first inhabitant of the Kuiper belt — the ring of rocky relics that surrounds the outer solar system — that scientists have seen up close.
Its odd shape, which scientists term a “contact binary”, indicates that it formed as two spherical rocks slowly fused together in the early days of the solar system. This finding lends support to a theory of planet formation that suggests worlds are born from slow accumulation, rather than catastrophic collisions, researchers said.
“This is exactly what we need to move the modelling work on planetary formation forward,” said Cathy Olkin, the mission’s deputy project scientist. “Ultima is telling us about our evolutionary history.”
New Horizons’s encounter with Ultima Thule happened so far away that it took six hours for signals travelling at the speed of light to reach Earth. Scientists didn’t receive confirmation that the spacecraft survived until Wednesday, and the first scientific results didn’t start streaming in until that night.
Researchers at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, from where New Horizons is operated, were up late, working to transform those bits of data into the first highresolution image of a Kuiper belt object.
The black-and-white photo was taken from about 50,000km away, as New Horizons sped toward its target at 51,500km/h.
“Spectacular,” principal investigator Alan Stern said at a news conference yesterday at which he displayed the early images from the flyby. He described watching his colleagues jump out of their seats and embrace one another upon seeing the compelling, crystal clear image. “That’s elation,” he said. “And it’s just the tip of the iceberg.”
Scientists had suspected that Ultima Thule would not be perfectly round since mid-2017, when a global network of observers found the rock passing in front of a distant star. But the Kuiper belt object is so distant and
so dim that even the most powerful telescopes saw it only as a flicker of light in the sky. Even as New Horizons sped toward its target, in the hours before closest approach, Ultima Thule resembled little more than a blurry bowling pin.
But now “it’s a world”, Stern said — with shape, character and implications for our understanding of planetary science.
Jeff Moore, New Horizons geology team lead, said Ultima Thule likely formed in the first few million years of the solar system from a swirl of smaller objects. Over time, dust and pebbles clumped together to form the object’s two lobes, which eventually combined to form a single body. The lack of evidence for damage at the sight of the collision suggests that the joining was probably gentle, like tapping someone’s bumper as you fit
into a tight parking space, Moore said. “You don’t need to fill out any paperwork.” This would make Ultima Thule a lot like the early planetesimals from which larger worlds — including our own — ultimately formed. But unlike the planets, which have undergone dramatic geologic change, and comets, which are heated and transformed by the sun, the Kuiper belt object has existed in a “deep freeze” (-273.15C) since it first formed, 4.6 billion years ago.
“What we think we’re looking at is the end product of a process that took place at the very beginning of the formation of the solar system,” Moore said. He called New Horizons “a time machine”, capable of taking scientists
back to the moment of our origins.
Colour images from New Horizons reveal that, like other Kuiper belt objects, Ultima Thule has a dark reddish hue. This is something of a mystery, because Ultima Thule is thought to be made mostly of ice. But researchers think radiation in this dark and distant part of the solar system could interact with contaminants in the ice, changing their colour. Early observations about its chemical composition were expected to arrive today, and they may help explain the phenomenon more.
And there are many more oddities to be explored, scientists said. Olkin pointed out dramatic variations in brightness that speckle the object. Moore noted that the early images did not show any solid evidence of impact craters.
What we think we’re looking at is the end product of a process that took place at the very beginning of the formation of the solar system. Jeff Moore
Additional images may reveal whether Ultima Thule has been struck in the past or is worn smooth.
It will take as long as 20 months for scientists to download and process all the data collected during their brief encounter with Ultima Thule, scientists said. That includes a brief delay next week, when the sun comes between the Earth and the spacecraft, blocking all transmissions.
And New Horizons’ mission isn’t over, Stern said.
The spacecraft’s subsystems are healthy, and it has sufficient fuel to operate for another 15 to 20 years.
Stern and his colleagues plan to apply for Nasa approval to extend their mission, either to conduct another Kuiper belt object flyby or explore other aspects of the outer solar system.