New Horizons’ departure images shed new light on Ultima Thule’s shape

Laurel Kornfeld

A new set of images showing the New Horizons spacecraft departing from Ultima Thule following its New Year’s Day closest approach reveals the Kuiper Belt Object (KBO) is shaped less like a snowman and more like a flat object, with one lobe looking like a pancake and the other like a dented walnut.

Initial images returned immediately after the flyby suggested the double-lobed object was composed of two nearly-spherical lobes, one larger than the other. Their apparent nearly round shapes were not due to the lobes being rounded by their own gravity, as both are far too small to attain that milestone.

The new images were taken at approximately 12:42 a.m. EST (05:42 GMT) Jan. 1, 2019, approximately 10 minutes after closest approach from a position 5,494 miles (8,862 kilometers) past Ultima Thule and an incredible 4.1 billion miles (6.6 billion kilometers) from Earth. They were captured from a different angle than the approach photos, which is why they provide new insight into Ultima Thule’s shape.

Mission scientists used 14 of these images to create a movie showing New Horizons’ departure from its second target. The movie clearly reveals that neither of Ultima Thule’s lobes is spherical, in part because the departure photos outline of parts of the KBO that block out background stars in spite of being in shadow.

“We had an impression of Ultima Thule based on the limited number of images returned in the days around the flyby, but seeing more data has significantly changed our view,” said mission Principal Investigator Alan Stern of the Southwest Research Institute (SwRI) in Boulder, Colorado. “It would be closer to reality to say Ultima Thule’s shape is flatter, like a pancake. But more importantly, the new images are creating scientific puzzles about how such an object could even be formed. We’ve never seen something like this orbiting the Sun.”

Also newly returned is a photo depicting a crescent view of Ultima Thule. Ten individual raw images taken by New Horizons’ Long Range Reconnaissance Imager (LORRI), which individually came out blurry due to the long exposure time required to take them, were combined to create a single photo showing a brighter, sharper view of the crescent.

Background stars visible in the individual images disappear in the combined, processed image and movie, enabling scientists to obtain a more accurate view of Ultima Thule’s two lobes.

The fact that both lobes are much flatter than anyone expected will motivate scientists to come up with new theories about the way planetesimals like Ultima Thule formed during the Solar System’s earliest days, said project scientist Hal Weaver of the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, Maryland.

All of the raw images used to create the processed image and video can be found on LORRI‘s website, where new photos are published every Friday.

“This really is an incredible image sequence, taken by a spacecraft exploring a small world four billion miles away from Earth,” Stern said. “Nothing quite like this has ever been captured in imagery.”

Most images and data from the flyby remain on the spacecraft, from which they will be returned over the next 19 months.

Video courtesy of NASA