A Nasa spacecraft has conducted a historic flyby of the farthest, and quite possibly the oldest, cosmic body ever explored by humankind.
The US space agency was hoping to take a closer look at a tiny, distant world called Ultima Thule early on Tuesday, hoping the frozen cosmic object will reveal some clues as to how planets took shape 4.6 billion years ago.
It rang in the New Year with a live online broadcast to mark New Horizons' zoom past the mysterious object located about four billion miles away in a dark and frigid region of space known as the Kuiper Belt.
The flyby at 12.33 am on Tuesday (5.33am GMT) took place about a billion miles beyond Pluto, which was until now the most faraway world ever visited up close by a spacecraft.
"Go New Horizons!" said lead scientist Alan Stern as a crowd including kids dressed in space costumes blew party horns and cheered at the Johns Hopkins Applied Physics Laboratory in Maryland to mark the moment when the spacecraft aimed its cameras at the distant space rock.
Real-time video of the actual flyby was impossible, since it takes more than six hours for a signal sent from Earth to reach the spaceship and another six hours for the response to arrive.
"Anything's possible out there in this very unknown region," John Spencer, deputy project scientist for New Horizons, told reporters on Monday at the Johns Hopkins Applied Physics Laboratory in Maryland.
A solo track recorded by legendary Queen guitarist Brian May – who also holds an advanced degree in astrophysics – was released just after midnight to accompany a video simulation as Nasa commentators describe the close pass.
"This is a night none of us are going to forget," May said.
What could mission reveal?
Hurtling through space at a speed of 32,000 miles per hour, the spacecraft aims to make its closest approach within 2,200 miles of the surface of Ultima Thule.
Dr Stern, the lead planetary scientist on the New Horizons mission, told reporters that Ultima Thule is unique because it is a relic from the early days of the solar system and could provide answers about the origins of other planets.
In less than 48 hours, New Horizons will make history! The team at @JHUAPL is preparing for the #NewYears flyby of #UltimaThule, the farthest object explored by a spacecraft ever - 4 billion miles from the Sun and ~1 billion miles from Pluto. pic.twitter.com/3EiB2bmOKy— NASA New Horizons (@NASANewHorizons) December 30, 2018
Temperatures are freezing - almost absolute zero or -273C - which means scientists hope it proves to be a time capsule.
"The object is in such a deep freeze that it is perfectly preserved from its original formation," he said.
"Everything we are going to learn about Ultima – from its composition to its geology to how it was originally assembled, whether it has satellites and an atmosphere and those kinds of things – are going to teach us about the original formation conditions of objects in the solar system."
The encounter itself is risky, and if the spacecraft were to collide with a speck of space debris as small as a grain of rice, it could be destroyed instantly, mission managers warned.
For that reason, Dr Stern said he and his colleagues are "on pins and needles to see how this turns out."
The first signal back to Earth should come about 10 hours after the flyby, around 9.45 am (1445 GMT). Only then will Nasa know if New Horizons survived the close pass.
Seven instruments on board will hopefully record high-resolution images and gather data about its size and composition.
But the flyby will be fast - at a speed of nine miles per second. As a result, many of the pictures taken will likely be of empty space. That's because the team on the ground will be trying to capture a 12- to 22-mile-wide world some four billion miles away.
At closest approach, New Horizons is expected to take nearly 900 photos at the highest resolution - and Ultima Thule is expected to appear in a few.
"We're rendezvousing with something that's a mountain draped in black velvet in almost pitch-dark conditions, and we're screaming up to it [..] within 2 seconds of perfection," Dr Stern told the Washington Post. "You can't get any better than that."
Despite the tough task, the team is confident. “I think we're good,” Marc Buie, an astronomer working to pin down Ultima Thule’s position, told the New York Times. “I think we're better than good.”
What does it look like?
Scientists are not sure what Ultima Thule (pronounced TOO-lee) looks like – whether it is cratered or smooth, or even if it is a single object or a cluster.
It was discovered in 2014 with the help of the Hubble Space Telescope, and is believed to be 12-20 miles in size.
A blurred and pixelated image released on Monday, taken from 1.2 million miles away, has intrigued scientists because it appears to show an elongated blob, not a round space rock.
Astronomers believe it is either made up of two objects touching each other, or perhaps even a binary system, in which two objects orbit each other.
“It’s really puzzling, because we know the shape is irregular,” Dr Stern said.
Even clearer images should be in hand over the next three days.
Scientists decided to study it with New Horizons after the spaceship completed its main mission of flying by Pluto in 2015, returning the most detailed images ever taken of the dwarf planet.
Dr Stern said the goal is to take images of Ultima that are three times the resolution the team had for Pluto.
Launched in January 2006, New Horizons embarked on a 4 billion mile journey toward the solar system's frigid, faraway edge to study the dwarf planet Pluto and its five moons.
During a 2015 fly-by, the probe found Pluto to be slightly larger than previously thought. In March, it revealed that methane-rich dunes were on the icy dwarf planet's surface.
Frontier of planetary science
Ultima Thule is named for a mythical, far-northern island in medieval literature and cartography, according to Nasa.
"Ultima Thule means ’beyond Thule’ – beyond the borders of the known world – symbolising the exploration of the distant Kuiper Belt and Kuiper Belt objects that New Horizons is performing, something never before done," the US space agency said in a statement.
According to project scientist Hal Weaver of the Johns Hopkins Applied Physics Laboratory, humans didn’t even know the Kuiper Belt – a vast ring of relics from the formation days of the solar system – existed until the 1990s.
"This is the frontier of planetary science," said Weaver.
"We finally have reached the outskirts of the solar system, these things that have been there since the beginning and have hardly changed – we think. We will find out."
In an editorial in The New York Times, Dr Stern recalled that December 2018 marks the 50th anniversary of the first time humans ever explored another world, when US astronauts orbited the Moon aboard Apollo 8.
"New Horizons will continue in that legacy," Dr Stern wrote.
"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."
Listen to Brian May's new music
The Queen guitarist is releasing his first official solo work in two decades to mark the occasion.
May, who is as passionate about science as he is about music, said he didn't know if "anyone's going to like it yet".
Dr Stern, who first suggested creating the track, was among the few who had listened to it.
“I have been bouncing it off Alan all the way," he told Newsweek. "He's made some comments - some very interesting comments, because of course he comes from a completely different world from me. And he's been liking it, which is great.
“I find myself in an unusual place because I’m deeply immersed in music - and have been all my life - but I’m also deeply immersed in astronomy and astrophysics.”
Creating a single inspired by the New Horizons probe was “an amazing opportunity for me to combine the two,” May added.
Woven into the recording are words from Stephen Hawking, including a message from the scientist to the New Horizons team in 2015 after the successful Pluto mission.
“He said exactly what was in my mind,” May said. “In another part of his message - which actually nobody has heard yet - he said, ‘we do this because we are human and because we need to know.'"
What comes next?
The huge trove of data that New Horizons sends back is hoped to keep scientists busy for the next one and a half years.
But its journey is far from over - barring any disasters.
Nasa is hoping New Horizons can go on to investigate other objects in the Kuiper Belt.
"The spacecraft is very healthy, it's not using any of its back-up systems and it has power and fuel to operate for close to 20 more years," Dr Stern said.
"There's a lot of future exploration ahead for New Horizons."