The US probe's images acquired as it approached Ultima hinted at the possibility of a double body, but the first detailed picture confirms it. It is 6.4 billion km (4 billion miles) from Earth. The object, the most-distant ever explored, is known as a "contact binary".
And the images confirmed what observations using the Hubble Space Telescope had suggested as New Horizons scientists scouted Ultima Thule - that, like parts of Pluto and its moon Charon, it has a rusty hue.
Ultima Thule sits in the Kuiper Belt, about 1.6 billion km past Pluto. To tell them apart, they named the larger one Ultima and the smaller one Thule. The volume ratio is three to one.
Team members and guest at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland, gathered to celebrate both the new year and the fly-by of New Horizons, which launched in 2006.
This lends support to a theory of planet formation that holds that planets are formed by a gentle accumulation of cosmic objects, rather than through catastrophic collisions between them.
This New Year the spacecraft happened to be the icebreaker and gave the world a clear and vivid idea about the Ultima Thule.
"It's only really size of something like Washington, D.C., and it's about as reflective as garden-variety dirt", he said.
"We were basically chasing it down in the dark at 32,000 miles per hour".
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Images were taken by onboard instruments as the spacecraft zoomed past the world some 2,200 miles from its surface on New Year's Day, just past midnight.
We have a lot more data to download from New Horizons over the coming months at the painfully slow rate of one kilobit per second.
The plutonium-powered probe should be capable of flying through the Kuiper Belt for another 10 years. Ultima Thule rotates about once every 15 hours, the scientists determined. On average, the object is about as bright as the darkest spot on Pluto's surface, Verbiscer says. The resulting deep freeze hinders or completely prevents all chemical reactions within the rock, keeping it exactly as it was shortly after the solar system first formed.
At that time the outer fringes of the solar system consisted of "innumerable small particles or pebbles", Moore explained, that slowly coalesced into larger ones.
A third factor is just the nature of the environment.
The object is so old and pristine that it's essentially like going back in time to the beginning of our solar system. The Kuiper belt hasn't been stirred up.
There are no obvious signs Ultima Thule has been significantly disturbed since it came together as the solar system formed, the researchers said. Stern said the New Horizons team would start writing scientific papers next week, based on the data already in hand, and nearly certainly propose another mission extension to NASA by 2020. At the time, and again today, New Horizons personnel defended the choice, citing its original connotations. Critically, it should also have sufficient electrical reserves to keep operating its instruments into the 2030s. The New Horizons team is already pushing for another flyby in the 2020s while the spacecraft systems are still working. Some of the data acquired by New Horizons might shed light on that.