LAUREL, Md. — A couple of days before the New Horizons spacecraft made its flyby of a small, icy world far beyond Pluto, scientists working on the mission finally got a picture of the body, nicknamed Ultima Thule, that was more than a single dot. It looked a bit elongated, but that was really all that could be detected from the image.
“I’ve never seen so many people so excited about two pixels,” said S. Alan Stern, the principal investigator of the mission, during a news conference on Monday.
Two days later, the scientists unveiled images from the flyby with some 28,000 pixels. They could finally make out some meaningful details, which could eventually advance scientific understanding of the solar system’s earliest days.
The New Horizons team will have to wait 20 months for all of the spacecraft’s data and images to return to Earth, but here is what they’ve learned so far.
It’s a snowman (or BB-8)
Ultima Thule, known also by its official designation of 2014 MU69, turns out to be what planetary scientists call a “contact binary” — two bodies that formed separately and then gently touched and stuck together. It’s a bit more than 21 miles long. To label the two parts, the scientists named the larger one Ultima and the smaller one Thule.
The New Horizons scientists described it as a snowman, but people on Twitter noticed a resemblance to a robot character from recent Star Wars movies.
At Thursday’s news conference, scientists released stereo images of the asteroid giving a somewhat better sense of this shape. Brian May, the astrophysicist who was also the lead guitarist of the rock band Queen, made one of these images. On Friday, Dr. Stern tweeted a video showing how the New Horizons team expressed their appreciation for Dr. May’s work.
It looks as old and pristine as scientists hoped
Planetary scientists are intrigued by the region known as the Kuiper belt — the home of Ultima Thule and other objects — because it is perhaps the only place where some of the solar system’s earliest building blocks are preserved.
The lack of sharp corners and apparently smooth surface of Ultima Thule suggests that it has not changed much in the last 4.5 billion years. What the scientists find there could tell them a lot about how the sun and planets formed.
Bilobate and red all over
Color measurements confirm that Ultima Thule is reddish like many objects in the outer solar system. Indeed, in hue, it is an unremarkable member of what astronomers call cold classicals, which all seem to have remained pristine and undisturbed since they formed.
“It displays an average color that falls well within the average colors displayed by the cold classical Kuiper belt objects,” said Silvia Protopapa, a member of the science team, said during a news conference on Thursday.
The two lobes, Ultima and Thule, are the same shade of red, suggesting that they formed near each other before merging. The “neck” where the lobes connect is brighter and less red, which could indicate bits of debris that slid down the slopes.
It rotates once every 15 hours
In the past five months, as New Horizons approached Ultima Thule, the spacecraft looked for rhythmic variations in the brightness that would reveals how fast the body was rotating. However, the brightness seemed to remain steady.
They now know that the spacecraft is roughly looking down at one of Ultima Thule’s poles, so that it is mostly the same side of the body that was always facing the spacecraft.
With the world’s two lobes now visible, the mission’s scientists can finally calculate a rotation rate: once every 15 hours, give or take an hour.
Things they haven’t seen
Scientists have not spotted any craters yet, but that does not mean there are none. In the images seen so far, the sun is behind the spacecraft and thus shadows would not be visible, hiding the topography. Higher resolution images should be more revealing.
In addition, craters might not look like craters because of the low density of Ultima Thule and the slow speed of collisions in the Kuiper belt.
“It’s almost like styrofoam balls running into each other at low velocity,” said Richard P. Binzel, a New Horizons science team member. “You don’t so much make craters as dents.”
Also not seen yet are any moons or rings. The researchers are also looking for an atmosphere, although they do not expect to find one.
The New Horizons team hit its mark
The prediction of New Horizons’ closest approach to Ultima Thule was off by only 2 seconds. By contrast, for the spacecraft’s flyby of Pluto in 2015, the prediction was off by about 80 seconds. Even though Ultima Thule is smaller and farther away, the navigators were able to plot a more precise course this time, because in 2017 and 2018, astronomers on the mission team were able to pin down Ultima Thule’s location by observing the object passing in front of a few distant stars.
At closest approach, at two seconds after 12:33 a.m., New Horizons was just under 2,200 miles from Ultima Thule, traveling at 32,290 miles per hour.
Beyond Ultima Thule
New Horizons has completed its study of Ultima Thule.
As the spacecraft returns its data, there is great interest in the highest resolution images taken right around the time of closest approach.
Given how precisely the spacecraft seems to have performed, Dr. Stern said he was “guardedly optimistic” that Ultima Thule will be in some of those images.
The spacecraft has also resumed measurements of dust, gases and radiation in the Kuiper belt, which could help scientists understand the processes that turn materials red in the Kuiper belt. New Horizons will also make long-distance images of other small worlds there to compare with Ultima Thule.