The icy rock has been in a deep-freeze preservation state since the formation of our solar system 4.5 billion years ago. That flyby revealed that even little Pluto, more than three billion miles from the sun, is an active world with shifting plains, glaciers and mountains that reach up to about 15,000 feet.
Nasa will reveal its early findings at a press conference held at 2.45pm United Kingdom time.
The Kuiper Belt is the edge of our solar system, part of the original disk from which the sun and planets formed.
Ultima Thule was chosen as the next target after an intensive search that made use of observing time on the Hubble Space Telescope.
"Anything's possible out there in this very unknown region", John Spencer, deputy project scientist for New Horizons, told reporters on Monday at the Johns Hopkins Applied Physics Laboratory in Maryland. While their fellow Earthlings counted down to midnight, they were ticking off time until 12:33 a.m., when the spacecraft would make its closest approach to its target.
Scattered amongst the crowd are those who will carry on New Horizons' legacy - I've spotted team members from Nasa's Osiris-ReX spacecraft which entered orbit around the asteroid Bennu yesterday, and the leaders of upcoming missions to a odd metal asteroid called Psyche and to the trojan asteroids that share an orbit with Jupiter. Confirmation was not expected for hours, though, given the vast distance.
The Johns Hopkins University Applied Physics Laboratory, which built and operates the spacecraft, said Tuesday it had "zipped past" the object known as 2014 MU69, or Ultima Thule.
The exact shape and composition won't be known until Ultima Thule starts sending back data in a process expected to last nearly two years.
The first signal back to Earth should come about 10 hours after the flyby, around 9:45 am (1445 GMT). However, seeing as the exploratory spacecraft is about four billion miles (6.6 billion km) from our home planet, this data takes a while to travel back to Earth.
An unmanned NASA spacecraft has sent a signal back to Earth that it has successfully made it through a risky flyby past the most distant planetary object ever studied, the U.S. space agency said on Tuesday.
It is located in the Kuiper Belt, a huge asteroid belt that surrounds our solar system and contains the leftovers of the system's formation. Ultima lies in a far-off group of objects, called the Kuiper Belt, that ring the solar system.
Ultima Thule is so dim and so distant that scientists aren't even certain what it looks like.
"We finally have reached the outskirts of the solar system, these things that have been there since the beginning and have hardly changed -we think". By the time the first images and data stream back to Earth, the borders of the known world will have expand once more. "We'll find out soon enough".
But New Horizons will finally fly by its target just after midnight on January 1, taking close-up photographs and sophisticated scientific measurements of what it sees.
It looks like a giant bowling pin.