Now they will work to download and look through all of the data sent back over that long distance, a process that could take years.
"It's a snowman!" lead scientist Alan Stern informed the world at a news conference.
"I'm 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". New Horizons got up close and personal with this frozen chunk of rock on January 1st, but New Horizons has only just beamed back high-resolution images.
The images released so far are "just the tip of the iceberg", he said, adding only 1% of data stored on the spacecraft has now been received by scientists.
Pictures of Ultima Thule have already reached scientists on Earth, and a much clearer picture will develop as data reaches the New Horizons team over the next few days. "New Horizons has set a new bar for state-of-the-art spacecraft navigation".
Images taken during the spacecraft's approach - which brought New Horizons to within just 2,200 miles (3,500 kilometers) of Ultima at 12:33 a.m. EST - revealed that the Kuiper Belt object may have a shape similar to a bowling pin, spinning end over end, with dimensions of approximately 20 by 10 miles (32 by 16 kilometers).
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This image from video made available by Nasa shows a diagram describing the size and shape of the object Ultima Thule.
Another possibility is Ultima could be two objects orbiting each other. But that didn't happen with Ultima Thule, which has remained pristine for eons in a cosmic deep-freeze, more than 4 billion miles (6.4 billion km) from the sun.
Scientist Jeff Moore says that the two spheres formed when small, icy pieces coalesced in space billions of years ago.
Carly Howett, New Horizons co-investigator, said: "We can definitively say that Ultima Thule is red".
That's why New Horizons team members were so keen to fly by the object, and why they're so excited by the early science returns from the epic New Year's Day encounter. The centre image taken by the Long-Range Reconnaissance Imager (LORRI) has a higher spatial resolution than MVIC by approximately a factor of five.
"Reaching Ultima Thule from 4 billion miles away is an incredible achievement". Initial images suggested it resembled a bowling pin.