Written by Kenneth Chang
Ultima Thule, an icy world 4 billion miles from the sun, looks like a big snowman. At a news conference Wednesday, scientists working with NASA’s New Horizons mission released several images that the spacecraft took as it flew by Tuesday, New Year’s Day.
Planetary scientists have never before seen a close-up of a body like Ultima Thule. It is likely a fragment that coalesced more than 4.5 billion years ago and that has remained in the deep freeze of the solar system’s Kuiper belt ever since.
If it is indeed a pristine planetesimal, a building block of the planets, studying it will offer clues to how Earth and its neighbors formed.
On Tuesday, scientists released a blurry picture of this small world, also known by its official designation 2014 MU69, taken before the flyby from a distance of half a million miles. It sort of looked like a fuzzy bowling pin then.
“That was so 2018,” Stern joked.
Now, much more has come into focus. The scientists now say with confidence that Ultima Thule long ago was what they call a “contact binary.” “Two completely separate objects that are now joined together,” said S. Alan Stern, principal investigator for the mission.
It consists of two almost spherical lobes, one with about three times the volume of the other. To tell them apart, they named the larger one Ultima and the smaller one Thule. “Being scientists, we not all that creative with words,” Stern said.
Such contact binaries appear to be common in the outer solar system, but, Stern said, “This is the first object that we can clearly tell was born this way.” By contrast, he suggested, scientists did not know for sure whether other two-lobe bodies — most notably the rubber ducky-like Comet 67P/Churyumov-Gerasimenko — were two objects that came together or one larger body that had eroded into its current shape.
A contact binary fits with some theories of how the planets formed — that clouds of pebbles clumped together into larger lobe-size bodies, and then these two lobes gently bumped into each other and stuck.
“It’s actually gratifying to see these almost perfectly formed contact binaries in their native habitats,” said Jeffrey M. Moore, leader of the mission’s geology and geophysics team.
In at least the general outlines, he added, “our ideas of how these things form seem to be somewhat vindicated by these observations.” He said that the two lobes must have hit at very low speeds, a few miles per hour at most. Moore said that the mottled appearance of the surface with dark streaks were suggestive of hills and ridges. As yet, there are no obvious craters visible.
“We see no unambiguous evidence,” Moore said. “I’d be surprised if there are not at least a few.”
Because these images were taken with the sun directly behind the spacecraft, there are no shadows to highlight the topography. Color images show that Ultima Thule is reddish. A ring of brighter, less reddish material circles the connection between the two lobes, possibly loose material that rolled down and piled up at the bottom of the slopes.
The details of the shape conclusively answer the mystery of why New Horizons did not detect variations in Ultima Thule’s brightness as it approached. Typically, an irregularly shaped object will have a rhythmic pattern of brightening and dimming as it rotates. But here, the spacecraft was looking down at one of the poles, so essentially the same side of Ultima Thule was facing the spacecraft the entire time.
The first batch of science data from the flyby arrived on Earth on Tuesday afternoon. More than 100 scientists, including Heidi B. Hammel, a planetary scientist and a media liaison for the science team, gathered in the evening for a look. “Everybody was there,” Hammel said in an interview. “They all wanted to see it. The picture goes up and everybody applauds and cheers. Immediately, the chatter starts.”
A second shift of scientists worked on the data overnight, presenting more detailed analysis during a science team meeting on Wednesday morning.
Stern highlighted the precision that was required for the rendezvous with Ultima Thule. “It’s only really (the) size of something like Washington, D.C., and it’s about as reflective as garden-variety dirt,” he said. “And it’s illuminated by a sun that’s 1,900 times fainter than it is outside on a sunny day on Earth, So we’re basically chasing it down in the dark at 32,000 mph.”
He also addressed a controversy over the nickname that the New Horizons team had chosen. Ultima Thule is a Latin phrase that means “a place beyond the known world,” but it was also adopted by Nazis to refer to the birthplace of the Aryan race.
“The term, Ultima Thule, which is very old, many centuries old, possibly a thousand years old, is a wonderful meme for exploration, and that’s why we chose it,” Stern said. “Just because some bad guys once liked that term, we’re not going to let them hijack it.”