NASA's New Horizons probe successfully flew past a space rock called 2014 MU69 on New Year's Day.

MU69, also called Ultima Thule, is 4 billion miles away from Earth, 1 billion miles beyond Pluto, and now the most distant object humanity has ever visited.

The nuclear-powered New Horizons spacecraft recorded hundreds of photos of MU69, and it will beam that data home over the next two years.

The flyby may reveal secrets of how planets in our solar system formed some 4.5 billion years ago.

Scientists just flew NASA's nuclear-powered New Horizons probe past a mysterious, mountain-sized rock beyond the orbit of Pluto.

Known formally as (486958) 2014 MU69, though more frequently called "Ultima Thule" (a controversial nickname; see editor's note), the object is located more than 4 billion miles from Earth. It's 1 billion miles beyond Pluto, which New Horizons visited in July 2015.

The spacecraft phoned home at 10:39 a.m. ET, confirming it wasn't destroyed by the maneuver. This makes MU69 the most distant object that humanity has ever visited.

New Horizons pulled off the maneuver on New Year's Day, taking hundreds of photos in a highly choreographed, pre-programmed sequence. The space probe reached its closest point to the space rock — about 2,200 miles — at 12:33 a.m. ET. New Horizons then turned around to photograph its exit at a speed of about 32,200 mph.

"We're here to tell you that last night, overnight, the United States spacecraft New Horizons conducted the farthest exploration in the history of humankind, and did so spectacularly," Alan Stern, who leads the New Horizons mission, said during a press conference on Tuesday after the flyby. "Thousands of operations onboard the spacecraft had to work correctly in order for us to be able to tell you this, and now we know that it all did."

The mission was as surprising as it was ambitious: When NASA launched New Horizons toward Pluto in 2006, nobody knew MU69 existed. There wasn't even a reliable way to detect the object until astronauts plugged an upgraded camera into the Hubble Space Telescope in 2009.

But a signal acquired by NASA and Johns Hopkins University's Applied Physics Laboratory confirmed the spacecraft survived.

During the press conference on Tuesday, scientists shared the best images yet, which New Horizons recorded on Monday before its closest approach. They later released an imaginative illustration of how MU69 might appear up-close:

A composite of two images taken on December 31, 2018 (left) and an illustration (right) of 2014 MU69. NASA/JHUAPL/SwRI; sketch courtesy of James Tuttle Keane

Stern said the image tells them MU69 is about 10 miles by 20 miles, and its irregular shape could either be because it's "bi-lobate" with two different-sized ends, or it's actually two objects orbiting each other at close range.

"If we knew what to expect, we wouldn't be going to Ultima Thule," Stern previously told Business Insider. "This is what what exploration is about."

Better images should come through in the coming days, with the highest-resolution color ones expected in February. Stern said the team would start writing its first scientific paper on the data next week.

Alice Bowman, the New Horizons mission operations manager, said during the press conference that there's "a bit of all of us on that spacecraft that will just continue after we're long gone here on Earth."

"What is striking home with me is that we can build a spacecraft on Earth, and we send it out billions of miles away from Earth, and it sends us back all this wonderful data that we get to look at and learn more about our world, our solar system," she added.

What MU69 is and where it's located

An illustration of NASA's New Horizons probe visiting 2014 MU69. NASA/JHUAPL/SwRI/Steve Gribben

New Horizons coasted through a zone called the Kuiper Belt, a region where sunlight is about as weak as the light from a full moon on Earth. That far away, frozen leftovers of the solar system's formation called Kuiper Belt Objects, or KBOs, lurk in vast numbers. (Pluto is one of them.)

MU69 is one of these pristine remnants. It has presumably remained in its distant and icy orbit for billions of years. Stern said the object's nickname, Ultima Thule, comes from a Norse phrase that means "beyond the farthest frontiers" (i.e. not the Nazi appropriation of the ancient term "Thule").

The unprecedented data acquired by New Horizons might therefore reveal new clues about how the solar system evolved to form planets like Earth, Stern said.

"Ultima is the first thing we've been to that is not big enough to have a geological engine like a planet, and also something that's never been warmed greatly by the sun," he said. "It's like a time capsule from 4.5 billion years ago. That's what makes it so special."

Stern compared the flyby to an archaeological dig in Egypt.

"It's like the first time someone opened up the pharaoh's tomb and went inside, and you see what the culture was like 1,000 years ago," he said. "Except this is exploring the dawn of the solar system."

An artist's rendering of an asteroid swarm. NASA/JPL-Caltech

Another analogy: Stern said he thinks of MU69 as a "planetary embryo," since it's a building block of larger planets that never became one.

"In that sense, it's like a paleontologist finding the fossilized embryo of a dinosaur," Stern said. "It has a very special value."

Journey into the unknown

Following New Horizons' historic visit to Pluto in 2015, NASA added the bonus mission to MU69.

During a press briefing on Monday, NASA revealed New Horizons' first image showing MU69's shape.

"It's a blob only a couple of pixels cross, but you can see just from that blob that it's an elongated blob — it's not round," John Spencer, a New Horizons project scientist, said during the briefing before the flyby. "This is just the first glimpse of what's going to get rapidly better from here."

An image of Ultima Thule. The image revealed the objects are elongated. NSA/JHUAPL/SwRI via YouTube

Stern previously told Business Insider the first images that New Horizons captured during the flyby will each take two hours to transmit. Then each bit of data, moving at the speed of light as radio waves, will take about six hours to reach antennas on Earth. Those early photos will be released to the public on Wednesday.

However, it will take much longer to receive the most detailed, full-resolution images due to physical limitations of the New Horizons spacecraft and its location.

In fact, it could take up to 20 months for the researchers to get all of the flyby data, Helene Winters, the project manager of New Horizons, said during the press briefing before the flyby.

Stern shied away from making predictions about what images might show, citing how shocking the first close-up pictures of Pluto were.

"If it's anything as surprising as Pluto, though, it will be wonderful," he said.

'10,000 times harder than reaching Pluto'

This flyby was dramatically more difficult than New Horizons' Pluto visit, Stern said.

An image composite of Pluto (right) and its moon Charon (left). NASA/JHUAPL/SwRI

"Rendezvousing with something the size of a large, filthy mountain covered in dirt, a billion miles away from Pluto, and honing in on it is about 10,000 times harder than reaching Pluto," Stern said. "That's because it's about 10,000 times smaller. The achievement of getting to it is unbelievable."

Pinpointing exactly where MU69 would be in space when New Horizons could fly near it required a "two and a half week odyssey" of telescope observations around the world, mission scientist Simon Porter said on Twitter.

To see MU69 block the light of a distant star — a way to confirm the space rock's precise orbit — the researchers had to fly an airplane-based telescope called SOFIA and deploy dozens of telescopes in Argentina.

In a New York Times op-ed published on New Year's Eve, Stern described the encounter as "mind-boggling."

"As you celebrate New Year's Day, cast an eye upward and think for a moment about the amazing things our country and our species can do when we set our minds to it," Stern wrote.

Watch live coverage of New Horizons' flyby of MU69

Members of the New Horizons science team react to seeing the spacecraft's last and sharpest image of Pluto before closest approach. AP

Anyone interested in watching New Horizons flyby events can tune into live video broadcasts being hosted over the next several days. Segments that feature the first images and science results will be released on Wednesday and Thursday.

The Applied Physics Laboratory, which manages the New Horizons mission, is hosting a suite of broadcasts through the lab's YouTube channel. Additionally, NASA TV and NASA Live are mirroring some of the coverage, even though the government shutdown has sent many NASA workers home.

You can watch the main New Horizons events via the NASA Live video player below.

Editor's note: After a public campaign, the New Horizons team selected Ultima Thule as a nickname for (486958) 2014 MU69. However, we've de-emphasized it here because the Nazi party used the word "Thule" as a tenet of its ideology.