Way, way out at the cold, dark edges of the solar system—past the rocky inner planets, beyond the gas giants, a billion miles more remote than Pluto—drifts a tiny frozen world so mysterious, scientists still aren't entirely sure if it's one world or two.

Astronomers call it Ultima Thule, an old cartography term meaning "beyond the known world." Its name is a reference to its location in the Kuiper Belt, the unexplored "third zone" of our solar system populated by millions of small, icy bodies.

Numerous though they are, no Kuiper Belt object has ever been seen up close. NASA's two Voyager probes—which traversed the third zone decades ago—might have spied a glimpse of one had they been equipped with the right instruments, except that the Kuiper Belt hadn't even been detected yet. On New Years Eve, for the first time, NASA will get a chance at some facetime with one of these enigmatic space rocks.

At 9:33 pm PST, 33 minutes past midnight on the East Coast, the agency's New Horizons probe will make a close pass of Ultima Thule, making it the most distant object ever to be visited by a spacecraft.

Astronomers have almost no idea what awaits them. “What’s it going to look like? No one knows. What’s it going to be made of? No one knows. Does it have rings? Moons? Does it have an atmosphere? Nobody knows. But in a few days we’re going to open that present, look in the box, and find out,” says Alan Stern, the mission's primary investigator.

New Horizons has traveled for 13 years and across 4 billion miles to reach this point, and the probe looks to be in fine shape: Mission planners confirmed earlier this month that it will pass within 2,200 miles of Ultima Thule after determining that large objects, like moons, and smaller ones, like dust, were unlikely to pose a threat to the spacecraft as it blazed past in excess of 31,000 miles per hour. ("When you're traveling that fast, hitting something even the size of a grain of rice could destroy the spacecraft," says Hal Weaver, the mission's project scientist.)

New Horizons' trajectory will carry it three times closer to Ultima Thule than it did Pluto, which it shot past in the summer of 2015. The photos New Horizons beamed back then were the most detailed ever captured not just of the former planet, but the outer solar system. Because of its proximity, the images the probe collects of Ultima Thule will be more detailed still, and from a billion miles deeper in space. "Pluto blew our doors off," Stern says, "but now we're heading for something much more wild and woolly."