BALTIMORE — Three and a half years after giving humanity its first close-up view of Pluto, and almost 13 years after launching from Earth, the New Horizons spacecraft will explore another new frontier: a reddish hunk of rock and ice known as Ultima Thule.

The object — or, perhaps, two objects (it’s so far away astronomers aren’t sure) — is thought to be a pristine remnant of the early solar system, untouched for billions of years. Its nickname conveys its significance, meaning “beyond the known world.”

Ultima Thule is 4 billion miles from Earth. New Horizons will reach it as the new year arrives on Jan. 1, with a mission to collect as many images and as much data as possible while speeding past at 32,000 miles per hour.

“This is pure exploration,” said Alan Stern, the mission’s principal investigator. “We are really flying toward something completely unknown, unlike any other object we’ve studied in the past.”

The Ultima Thule flyby is an encore for the New Horizons mission, led by the Johns Hopkins Applied Physics Laboratory. The baby grand piano-sized spacecraft designed and managed for NASA on the lab’s Laurel, Maryland, campus zipped past Pluto — its primary mission — in July 2015, revealing craggy, icy surfaces and a wisp of an atmosphere around the dwarf planet.

It will capture similar images and data as it passes even closer to Ultima Thule, itself just about 20 miles across.

But even though it’s technically an add-on phase of a mission whose main focus was Pluto, the examination of Ultima Thule could prove even more revelatory.

“It could potentially be the most primitive object ever encountered by a spacecraft,” said Hal Weaver, the mission’s project scientist. “By examining what it looks like now, we are looking back at the time of planetary formation.”

Ultima Thule is in a region at the edge of the solar system known as the Kuiper Belt. Scientists suspect the region is home to hundreds of thousands of asteroid- or cometlike objects at least as big as Ultima Thule — which is officially known as 2014 MU69.

 

'A mystery'

While Pluto and its moons are also part of the Kuiper Belt, Ultima Thule is about a billion miles farther from Earth. It took New Horizons nearly a decade to reach Pluto, even though it rocketed from Earth at up to 36,000 mph, faster than any mission before it.

Nearly 3½ years more will have passed by the time the spacecraft makes it to Ultima Thule.

As dramatic and illuminating as the Pluto flyby was, scientists know even less about what to expect from Ultima Thule. Because it’s so far away and so dim, they aren’t even sure if it’s a single mass. They suspect it is made up of two lobes, but it also could be two separate objects orbiting around each other.

“We were already getting hints of what Pluto was going to be looking like well in advance of the day of closest approach,” Weaver said. “This time, everything is going to be pretty much a mystery, we think, until the last hour or so.”

That hour is expected to come around midnight Jan. 1, when scientists predict New Horizons will begin observing Ultima Thule in earnest. The spacecraft’s moment of closest approach to the object is expected around 12:30 a.m.

 

Very risky

The flyby itself is a complicated and extremely risky process, but one that is run completely on auto-pilot. Scientists have spent more than a year fine-tuning a flurry of commands New Horizons will follow to capture images and spectroscopic data, which together will show both what Ultima Thule looks like and what it’s made of.

Scientists faced the same sort of challenge when observing Pluto, but it’s even more difficult this time because Ultima Thule is so much smaller and farther away.

New Horizons’ speed, coupled with Ultima Thule’s size, also poses problems. If the spacecraft weren’t flying so fast, it wouldn’t reach its target in any reasonable time. But because of that speed, the scientists must perfectly time the pre-programmed sequence of observations to ensure Ultima Thule is literally in the frame of New Horizons’ cameras.

“The object’s so small that if you’re off, you could miss the whole thing,” said Alice Bowman, the New Horizons mission operations manager.

But, “a rice-sized pellet hitting the spacecraft in the wrong place could destroy it,” Weaver said.