Researchers discovered Ultima in 2014, hoping to find an object on New Horizon's existing flight path, and have been counting down to this rendezvous for years.
"It sounds like science fiction, but it's not", Alan Stern, the lead planetary scientist on the New Horizons mission, wrote in The New York Times on Monday.
For that reason, Stern said he and his colleagues were "on pins and needles to see how this turns out".
The flyby is expected to happen at 12:33 a.m. ET on January 1.
Scientists suspect Ultima Thule is a single object no more than 20 miles (32 kilometers) long, though there's a chance it could prove to be two smaller bodies orbiting each other or connected by a slender neck.
But until it came into New Horizons' view, Ultima remained little more than a distant dot hardly different from the stars behind it.
But beginning January 1, 2019, Ultima Thule will be far less mysterious, once the first images arrive.
In this realm of frozen worlds, known as the Kuiper Belt, scientists believe objects have been preserved for some 4 billion years in their primordial state, when the universe formed.More news: Sarri On Hazard's Future: We Need To Solve The Problem
While the mission marked the farthest close-encounter of an object within our solar system, Nasa's Voyager 1 and 2, a pair of deep space probes launched in 1977, have reached greater distances on a mission to survey extrasolar bodies.
In an editorial in The New York Times, Stern recalled that December 2018 marks the 50th anniversary of the first time humans ever explored another world, when USA astronauts orbited the Moon aboard Apollo 8.
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). Data from watching this KBO pass in front of known background stars suggests that this is the likely shape of Ultima Thule.
'In effect, Ultima should be a valuable window into the early stages of planet formation and what the solar system was like over 4.5 billion years ago'.
Even clearer images should be in hand over the next few days.
"From mission design to navigation to operations, our team worked very hard to get the spacecraft where it needs to be for this historic exploration in the Kuiper Belt", said New Horizons mission operations manager Dr. Alice Bowman, also from the Johns Hopkins University Applied Physics Laboratory.
"There's a lot of chatter in the science team room", Spencer said.
The Queen guitarist, 71, is releasing New Horizons, his first solo work in over two decades, as a "tribute" to the spacecraft of the same name.
He described the New Horizons spaceship as an "amazing piece of American workmanship, operating essentially flawlessly in space for well over a decade".
"Who knows what we might find?".