What scientists hope to gather from what the ship will be sending back is an insight into how the solar system was formed.
The object is so far away that it takes more than six hours for a command sent from Earth to reach New Horizons.
In July of 2015, after travelling for over nine years in space, the New Horizons probe made history, by giving us our very first close-up look at Pluto and its moons. The Ultima Thule rendezvous was more complicated, given the distance from Earth, the much closer gap between the spacecraft and its target, and all the unknowns surrounding Ultima Thule.
These bodies are time capsules, preserved in a deep freeze for the past 4.6 billion years.
Ultima Thule (officially 2014 MU69) is believed to be a pristine remnant of the birth of the solar system.
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.
This explains why, in earlier images taken before Ultima was resolved, its brightness didn't appear to vary as it rotated.
The spaceship was to collect 900 images over the course of a few seconds as it shaved by.
Stern said his bet is that the object is a single body, not two pieces orbiting each other, but he would wait until more, clearer images arrive Wednesday to say for sure.
Every 20 minutes, the spacecraft´s cameras and infrared imagers are capturing shots of the space rock, "so as it rotates and we get closer we will get good data on all parts of it as we approach", said John Spencer, a scientist with the Southwest Research Institute. In 2014, astronomers found Thule using the Hubble Space Telescope and the following year selected it for New Horizon's extended mission.
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).
Ultima Thule is named for a mythical, far-northern island in medieval literature and cartography, according to NASA.
Ultima lies 1 billion miles beyond Pluto, in a ring of icy worlds known as the Kuiper Belt.
"This is another great step in the exploration of our solar system".
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. For more updates, follow NASA New Horizons' Twitter account here.
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 unusual metal asteroid called Psyche and to the trojan asteroids that share an orbit with Jupiter. The mission was launched in 2006 and took a 9½-year journey through space before reaching Pluto.