It will become the most distant world ever explored by humankind.
Scientists chose to study Ultima Thule with New Horizons after the spaceship, which was launched in 2006, completed its main mission of flying by Pluto in 2015, returning the most detailed images ever taken of the dwarf planet. For just a moment, the craft will fly within 2,200 miles (3,540 km) of Ultima Thule, a primitive space rock from the Kuiper Belt far beyond Neptune. Confirmation won't come for hours, though, given the vast distance. "We are straining the capabilities of this spacecraft", Stern said at a news conference Monday.
'Because of where it was formed and the fact that Ultima is not large enough to have a geologic engine like Pluto and larger planets, we expect that Ultima is the most well-preserved sample of a planetary building block ever explored.
A huge spill-over crowd in a nearby auditorium joined in the loud celebration, cheering each green, or good, status update.
Scientists are already split on whether it's elongated or even two objects - but it might be even weirder than expected.
The Ultima Thule rendezvous was more complicated, given its 4 billion-mile (6.4 billion-kilometre) distance from Earth, the much closer gap between the spacecraft and its target, and all the unknowns surrounding Ultima Thule.
The crowd ushered in 2019 at midnight, then cheered, blew party horns and jubilantly waved small USA flags again 33 minutes later, the appointed time for New Horizons' closest approach to Ultima Thule.
Stern said the goal was to take images of Ultima that are three times the resolution the team had for Pluto.
As for its shape, scientists say there are two possibilities. A single body is more likely, they noted. The first post-flyby images will be unveiled on Wednesday, kicking New Horizons' long-running campaign of discovery back into high gear.
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The flyby took place about a billion miles beyond Pluto, which was until now the most faraway world ever visited up close by a spacecraft. The best colour close-ups, though, won't be available until later in January and February.
"I'm excited to see the surface features of this small world, particularly the craters on the surface". The instruments on New Horizons will create geologic and compositional maps of Ultima Thule, as well as searching for any rings, debris, or even small satellites orbiting the object.
"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".
It was discovered in 2014 with the help of the Hubble Space Telescope, and is believed to be 12-20 miles in size.
The almost circular orbit of Ultima Thule indicates it originated at its current distance from the Sun.
The report was greeted with cheers and hugs at APL's mission control center. The close encounter marks the farthest spacecraft flyby in history. But NASA will be sharing updates throughout the day, so be sure to check back for more exciting New Horizons news.
From New Horizons' location, a radio transmission traveling at light speed requires just over six hours to reach Earth.
While celebrating at NASA's headquarters, principal investigator Alan Stern said it would take months to receive the data from today's fly-by.
"There's a bit of all of us on that spacecraft", she said, "and it will continue after we're long gone here on Earth".