Ten hours after the middle-of-the-night encounter 4 billion miles (6.4 billion kilometers) away, flight controllers in Laurel, Maryland, received word from the spacecraft late Tuesday morning. "From here out the data will just get better and better!"
Previously, New Horizons swooped by Pluto in 2015, capturing the icy, mountainous world in unprecedented detail.
Ultima Thule is the first destination to be reached that was not even known until after the spacecraft's launch.
Stern and other members of the team at Johns Hopkins University's Applied Physics Laboratory, which NASA chose to carry out the mission, were very pleased with the performance of the almost 13-year-old space probe.
Familiar to many as the spacecraft that taught us so much about the dwarf planet Pluto and its "heart" on its surface, New Horizons has now taken photos of another distant object, Ultima Thule. When a solid radio link finally was acquired and team members reported that their spacecraft systems were green, or good, she declared with relief: "We have a healthy spacecraft".
"We did it again". Cheers erupted at Johns Hopkins University's Applied Physics Laboratory, home to Mission Control.
A half hour later, the New Horizons team poured into an APL auditorium to a five-minute standing ovation and a parade of high fives from APL staff.
Another NASA spacecraft, OSIRIS-REx, also set a new record on Monday by entering orbit around the asteroid Bennu, the smallest cosmic object - about 1,600 feet (500 meters) in diameter - ever circled by a spacecraft. In the meantime, NASA has released a blurry image of Ultima Thule, revealing some interesting discoveries about its shape and trajectory.
"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's also spinning, although scientists don't yet know how fast. A single body is more likely, they noted. An answer should be forthcoming on Wednesday, once new and better pictures arrive. Ultima Thule was discovered in 2014. However, it will take hours for the spacecraft to communicate its status with Earth and beam back the first images of the unexplored object.
The space object is actually called (486958) 2014 MU69 and is believed to have formed more than four billion years ago during the very beginning of our solar system's history, so is a relic of a bygone age which could help scientist unravel the mystery of how all the planets formed. After the quick flyby, New Horizons will continue on through the Kuiper Belt with other planned observations of more objects, but the mission scientists said this is the highlight.
"This mission's always been about delayed gratification", Stern reminded reporters. The mission was launched in 2006 and took a 9½-year journey through space before reaching Pluto.
The small body is known as a "cold classical" Kuiper Belt object, or KBO, meaning it is a pristine sample of that original material, circling the sun in a circular, flat orbit that indicates it has not been jostled or otherwise disturbed since the solar system's birth.