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. At the Johns Hopkins University Applied Physics Laboratory, which built and operates the spacecraft, scientists will count down to the moment of New Horizons' closest approach, at 12:33 a.m.
The New Horizons probe successfully captured images of Ultima Thule, a small, frigid world about 19 miles long that lies about 4 billion miles from Earth.
Scientists believe that objects like Ultima nearly certainly hold clues to the formation conditions of the Solar System 4.6 billion years ago. 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. Measurements taken Saturday showed that the spacecraft was within 20 miles of its intended flyby distance from Ultima Thule, and that the timing of the encounter will be within 2 seconds of what was expected.
NASA launched the spacecraft in 2006; it's about the size of a baby grand piano.
New Horizons spacecraft as it approaches Pluto and its largest moon, Charon, in July 2015. Out in the Kuiper belt, where sunlight is 0.05percent as strong as it is on Earth, and temperatures hover close to absolute zero, the primitive building blocks of planets have persisted unchanged for 4.6billion years. A processed version of the raw photo shows the object as an elongated blob.
But even as the world awaits news from the far reaches of the solar system, New Horizons has once again made it to the history books.
The Kuiper Belt object was discovered with the Hubble Space Telescope in 2014. In March 2018, NASA called it Ultima Thule, which means "The Edge of the World", on the basis of Internet voting.
Ultima Thule is named for a mythical, far-northern island in medieval literature and cartography, according to NASA.
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. Good close-up pictures should be available the day after the flyby.
FILE - This composite image made available by NASA shows the Kuiper Belt object nicknamed "Ultima Thule," indicated by the crosshairs at center, with stars surrounding it on August 16, 2018, made by the New Horizons spacecraft. Data from watching this KBO pass in front of known background stars suggests that this is the likely shape of Ultima Thule.
"I've never seen so many people so excited about two pixels", Stern said, his face advertising his own excitement.
The first signal back to Earth should come about 10 hours after the fly-by, around 9:45am (10:45pm Hong Kong time).
Scientists say there are two possibilities for this: Ultima Thule is either one object with two connected lobes, sort of like a spinning bowling pin or peanut still in the shell, or two objects orbiting surprisingly close to one another.