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The New Horizons spacecraft is about to explore the farthest object ever: Ultima Thule

New Horizons will fly by a rock a billion miles past Pluto on New Year’s Day.

Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

A billion miles past Pluto, in the farthest reaches of the solar system, lies an object called Ultima Thule. Its name means “beyond the known world.”

Though scientists have detected it with telescopes, they don’t know a lot about it. What they have gleaned from their observations is that Ultima Thule is a rock in the solar system’s Kuiper belt, the area beyond Neptune. It’s irregularly shaped and about 18 miles across. It might look like two potatoes that have been sandwiched together, like so:

An artist’s depiction of Ultima Thule.
Johns Hopkins Applied Physics Lab

On January 1, the New Horizons spacecraft, the one famous for flying by Pluto, will pass by Ultima Thule to explore this strange rock and try to learn more about the very formation of our solar system.

After New Horizons flew past Pluto in 2015, its mission scientists chose Ultima Thule as its next stop in the Kuiper belt, mainly because New Horizons had the ability to reach it with its remaining fuel.

But the New Years flyby is significant because Ultima will be the very farthest object in the solar system humanity has ever explored, and one of the most primitive.

Because Ultima is in such a cold, remote, and quiet region of the solar system, it’s probably been orbiting the sun undisturbed for nearly the entire age of the solar system.

“We expect that Ultima is the most well-preserved sample of a planetary building block ever explored,” Alan Stern, the principal investigator of New Horizons, writes in a blog post. “What will Ultima reveal? No one knows. To me, that is what’s most exciting — this is pure exploration and fundamental science!”

Ultima Thule is very, very far away.
Johns Hopkins University Applied Physics Laboratory

To Stern, the flyby — and the images that come from it — will be akin to the famous “Earthrise” photo taken by the Apollo 8 mission 50 years ago. This photo was taken during the first time humans orbited the moon. It was the first time we saw our own planet from the perspective of another world.

This view of the rising Earth greeted the Apollo 8 astronauts as they came from behind the Moon after the lunar orbit insertion burn.
SSPL via Getty Images

“Over all those years between Apollo 8’s December exploration and ours, NASA has made history by exploring farther and farther,” Stern writes. “As a result we have made Carl Sagan’s prediction that in just a generation or two the planets would be transformed from points of light to real and explored worlds.”

For now, Ultima Thule is still just a point of light. Here’s how it looks from New Horizons’s camera, as the spacecraft approaches closer and closer.

Johns Hopkins Applied Physics Laboratory/Southwest Research Laboratory/Henry Throop

But soon it will be much more than a point of light. The images of the object might be dimmer than those of Pluto (it is a billion miles farther from the sun). But New Horizons has a camera that can make out stunning delicate details. Just see the images it took of Pluto.

Recall that Pluto used to be just a point of light too. New Horizons revealed it to be an incredible, dynamic world with a beautiful heart-shaped icy plane. This GIF shows the very best image of Pluto we had from 2015 transform into what New Horizons saw on its historic flyby. It went from a smudgy blob to a thing of beauty.


(And yes, scientists are still debating whether Pluto deserves to be designated a planet instead of a dwarf planet.)

New Horizons will pass by Ultima Thule around 12:33 am Eastern on January 1, right after the start of the New Year. You can follow along with NASA on the NASA TV live stream (see below). New Horizons will also be carrying messages from the public (submissions ended on December 21) to celebrate the far-flung arrival.