The closest approach between the probe and the asteroid occurred at 05:33 GMT. Good close-up pictures should be available the day after the flyby.
"As you celebrate New Year´s Day, cast an eye upward and think for a moment about the fantastic things our country and our species can do when we set our minds to it".
While celebrating at NASA's headquarters, principal investigator Alan Stern said it would take months to receive the data from today's fly-by. As distant as it is, Pluto is barely in the Kuiper Belt, the so-called Twilight Zone stretching beyond Neptune.
He called it an auspicious beginning to 2019, which will mark the 50th anniversary of Neil Armstrong and Buzz Aldrin's footsteps on the moon in July 1969.
There was a bit of a party here last night, and accompanying the team at mission control was everyone from scientific celebrities - Walter Alvarez, discoverer of the K-Pg boundary (formerly known as the K-T boundary) that provided evidence of the asteroid impact that did for the dinosaurs - to, well, actual celebrities. Never before has a spacecraft explored anything so far away.
Brian May, the guitarist for the legendary rock band Queen and an astrophysicist, is also a participating scientist in the New Horizons mission.
Its seven science instruments were to continue collecting data for four hours after the flyby. This configuration limits the spacecraft's communication with Earth, commanding it to quickly address any technical issues on its own, then get back to science.
New Horizons acquired gigabytes of photos and other observations during the pass, however, because of the vast distance between the spacecraft and Earth, it will take until September 2020 to retrieve all of the data stored on the probe. Some of those discoveries may take a long time, but they'll be worthwhile if they shed light on the Solar System and the cosmos at large.
Prior to closest approach, project officials were optimistic that the spacecraft would perform the flyby as planned.
"I can't promise you success". "There's some danger and some suspense", Stern said at a fall meeting of astronomers.
"Because this is a flyby, we only get one chance to get it right", Bowman said.
The risk added to the excitement. The amount of light it gives off is completely flat, and mission team doesn't know why!
And even the U.S. government shutdown couldn't stop NASA from celebrating such an extraordinary feat.
New Horizons examined Pluto when it flew past the dwarf-planet three years ago.
Ultima Thule was discovered in 2014 with the help of the Hubble Space Telescope. The object is 20 miles long by 10 miles wide.
"It is probably the best time capsule we've ever had for understanding the birth of our solar system and the planets in it, " Stern said.
May says there's "nothing more exciting in the world of exploration than going to a place about which we know nothing".
Stern said 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.
Its mission now totalling $800 million, the baby grand piano-sized New Horizons will keep hurtling toward the edge of the solar system, observing Kuiper Belt Objects, or KBOs, from afar, and taking cosmic particle measurements. Scientists wanted the spacecraft staring down Ultima Thule and collecting data, not turning toward Earth to phone home. "We will find out".