Ultima Thule is more than four million miles from Earth, making it the farthest ever object ever explored by humanity.
Hurtling through space at a speed of 32,000 miles per hour, the spacecraft aimed to make its closest approach within 2,200 miles of the surface of Ultima Thule.
Brian May of Queen rung in the new year with fellow astronomers.
Although the image provided a new look at Ultima Thule, it lacks detail.
The probe flew by the rock during a mission to gather information about the creation of the solar system.
These bodies are time capsules, preserved in a deep freeze for the past 4.6 billion years. Now, it is heading towards the edge of the solar system and will shortly reach Ultima Thule, where it will complete a historic flyby.
But the encounter itself was 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. You can also follow along on NASA TV.
Scientists at John Hopkins Applied Science Laboratory whooped with joy and applauded as the first signals came back from the spacecraft, which left Earth more than a decade ago.
Prof Weaver added that the path is looking all clear for New Horizons, and is confident the mission will be a resounding success.
What does it look like?
This artist's impression of Ultima Thule depicts it as a contact binary, two smaller objects that orbit each other and are so close that they touch.
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Scientist have puzzled over why Ultima Thule does not appear to change in brightness, which would be expected of an elongated object that alternately shows its short and long sides as it rotates.
Scientists made a decision to study it with New Horizons after the spaceship, which launched in 2006, completed its main mission of flying by Pluto in 2015, returning the most detailed images ever taken of the dwarf planet.
The flyby will be fast, at a speed of nine miles per second.
As for the "cold, classical" part, Ultima Thule's orbit has a very low "inclination", meaning that it travels around the Sun in roughly the same plane as all the planets (except Pluto), and its orbit is almost circular (unlike Pluto).
In classic and medieval literature, Thule was the most distant, northernmost place beyond the known world.
About 10 hours earlier, NASA celebrated the New Year's flyby, at 12:33am (05:33 GMT) when the New Horizons spacecraft aimed its cameras at the space rock 6.4 billion km away in a dark and frigid region of space known as the Kuiper Belt.
"There's a lot of chatter in the science team room", Spencer said.
"This science will help us understand the origins of our solar system". "This flyby marks a first for all of us - APL, NASA, the nation and the world - and it is a great credit to the bold team of scientists and engineers who brought us to this point".