New Horizons completes flyby of Ultima Thule


Updated 1 p.m. Eastern with press conference comments.

LAUREL, Md. — NASA’s New Horizons completed a close approach to a small body in the distant Kuiper Belt early Jan. 1, collecting data that may reveal new insights about the formation of the solar system.

New Horizons made its closest approach to 2014 MU69, also known as Ultima Thule, at 12:33 a.m. Eastern, passing approximately 3,500 kilometers from the Kuiper Belt object. While the approach was celebrated at the time during an event at the Johns Hopkins University Applied Physics Laboratory (APL) here, the spacecraft was not in communications with the Earth.

NASA’s Deep Space Network received a signal from the spacecraft at 10:29 a.m. Eastern. That initial transmission contained no science data but rather telemetry about the health of the spacecraft and its performance during the flyby, including how much data it collected. Future downlinks, including one scheduled for later Jan. 1, will start returning science data.

“We have a healthy spacecraft,” said Alice Bowman, the New Horizons mission operations manager, after reviewing that initial burst of telemetry from the spacecraft, 6.6 billion kilometers from the Earth. “We’ve just accomplished the most distant flyby. We are ready for Ultima Thule science transmission.”

Prior to closest approach, project officials were optimistic that the spacecraft would perform the flyby as planned. “We’re very confident in the spacecraft and very confident in the plan that we have for the exploration of Ultima,” said Alan Stern, principal investigator for New Horizons, at a Dec. 31 press conference. “But I’d be kidding you if I didn’t tell you that we’re also on pins and needles to see how this turns out.”

“The New Horizons team makes it look easy. It’s not easy,” Stern said at a press conference here Jan. 1 after hearing back from the spacecraft. “From everything that we can tell, they scored a 100 on the test.”

While the signal contained no science data, project scientists did release some findings from data collected prior to closest approach and transmitted to Earth Dec. 31. That includes a better image of Ultima Thule, which appears similar in shape to a peanut or bowling pin, about 35 kilometers long and 15 kilometers wide.

MU69 shape and sketch
A composite image (left) of 2014 MU69, aka Ultima Thule, taken by New Horizons Dec. 31, along with a sketch showing the estimated rotation axis of the object. Credit: NASA/JHUAPL/SwRI; James Tuttle Keane

Stern said scientists can’t rule out that Ultima Thule is a binary object, with two bodies orbiting very close to each other, but believe it’s more likely it is a single body with two distinct lobes.

That shape would not be surprising. “Most of the small bodies in the solar system are highly elongated,” said Hal Weaver, New Horizons project scientist, because the objects aren’t massive enough that gravity can shape them into something more spherical. “We were expecting to have something along these lines.”

Those pre-flyby observations also resolved a mystery about the lack of brightness variations as it rotated. Weaver said images taken as New Horizons approached revealed that the rotational pole of Ultima Thule was pointed almost directly at the spacecraft, as some scientists speculated in recent days. “It was almost like a propeller blade,” Weaver said, showing the same profile as it spun around. “There’s no change in the brightness because we’re always seeing the same side.”

What’s not yet clear, he added, is the rotation rate of the object. The observations so far can be explained by rotation periods of 15 and 30 hours, but upcoming data should resolve that difference.

Better images of Ultima Thule will come in the next few days, with at least one such image scheduled for release at a Jan. 2 briefing. “It’s going to be dramatically different,” Weaver said. “Ultima Thule will be turned into a real world.”

It will take about 20 months for New Horizons to transmit the estimated seven gigabytes of data it collected during the flyby. The next few days will see some high-resolution images, Stern said, although the sharpest images — with an estimated resolution as good as 33 meters per pixel — won’t come down until February. Some initial spectral data will also be returned this week.

The science team will work with those data this week, then take a break next week when the spacecraft is in solar conjunction, with the sun blocking communications with the spacecraft. The science team will reconvene around Jan. 15, Stern said, after communications resume.

“The images that will start to come down this week will already reveal the basic geology and structure of Ultima,” Stern said. “We’re going to start writing our first scientific paper next week.”

What makes Ultima Thule interesting to scientists is that is part of a population of “cold classical” Kuiper Belt objects whose orbits, with low inclinations and eccentricities, suggest that they are pristine objects unaltered since the formation of the solar system 4.6 billion years ago.

“Nothing has happened to these things since they formed,” said John Spencer, a member of the New Horizons science team, at a pre-flyby briefing. “It’s a very special region that we’re very excited to explore.”