Scientists think Ultima Thule is a building block of larger objects in the Kuiper Belt, a band of frigid worlds that orbit the Sun beyond Neptune. Objects in the belt serve as time capsules from the dawn of the solar system, when a cloud of dust and gas collapsed to form the Sun, planets, and other small bodies. The temperature on Ultima Thule is expected to be just 40 or 50 degrees above absolute zero, said Alan Stern, the principal investigator of the New Horizons mission.

"It's a big deal because we're going 4 billion years into the past," he said. "Nothing that we've ever explored in the entire history of space exploration has been kept in this kind of deep freeze the way Ultima has."

New Horizons made worldwide headlines when it flew past Pluto in 2015, revealing a complex world with icy mountains and frozen plains. The big reveal of Ultima Thule probably won't garner as much publicity — but that hardly means it's going to be a cakewalk.

"Everything about this flyby is tougher," said Stern.

For the Pluto encounter, the team knew exactly where to aim, thanks to decades of telescopic observations that refined our understanding of its orbit. That won't be the case with Ultima Thule, which was only discovered in 2014 by the Hubble Space Telescope. The New Horizons team must continually track the object using optical navigation cameras, and make the appropriate course corrections.

"The orbit is not very well characterized," said Alice Bowman, the mission operations manager for New Horizons. "That's why we're doing optical navigation measurements, continuously looking to make sure we know the point in space we want to target."

That's a tricky task because Ultima Thule is dark and reddish, and only reflects about as much light as potting soil, Stern said. The object is also set against a particularly crowded region of space that is flush with background stars.