Just after midnight on January 1st, a NASA spacecraft will whiz past the most distant space rock that’s ever been visited in our Solar System. This remote interplanetary flyby will be over in a blink. But if successful, the event could tell us a whole lot about the objects that dominate the far reaches of our cosmic neighborhood.

The robotic spacecraft making this daring visit is called New Horizons, and it’s been traveling through space for the last 13 years. You may remember this famous bot: it was the first human-made object to ever visit Pluto in the summer of 2015. Ever since that flyby, New Horizons has been plunging farther into the Solar System. Three years later, it’s ready to meet up with another faraway target, a rock nicknamed Ultima Thule located 1 billion miles beyond Pluto. That’s 4.1 billion miles from Earth.

This new rock is unlike anything we’ve ever visited before

This new rock is unlike anything we’ve ever visited before. It’s a tiny frigid object about the size of New York City, orbiting in an area of the Solar System known as the Kuiper Belt. This region of space, located beyond the orbit of Neptune, is filled with possibly millions of small frozen objects. It’s a bit like a super distant Asteroid Belt. Except the bodies in the Kuiper Belt are thought to be incredibly primitive — leftover remnants from the birth of the Solar System. When the planets first formed 4.5 billion years ago, the materials in the Kuiper Belt region didn’t join together to form new worlds but instead remained as tiny fragments. And they’ve mostly stayed the same ever since.

That’s because they’re so small and so far from the Sun. Kuiper Belt objects are incredibly cold — just 35 degrees Kelvin above absolute zero. At this temperature, the objects don’t change very much on the surface. They’ve essentially been frozen in time over billions of years. Plus they’re so small that they don’t evolve much inside either. Bigger worlds like Neptune or Uranus ultimately form internal engines, where materials deep inside the objects reshape under the forces of pressure and heat over millions of years. But the Kuiper Belt objects just aren’t big enough to do that. That means they’re like tiny time capsules, providing snapshots of what materials lurked about when our planetary system first came into being. “We’ve never been to anything like this that’s been kept in such a deep freeze so long,” Alan Stern, principal investigator for the New Horizons mission, tells The Verge. “Anything so perfectly preserved from the early days of the Solar System.”

Successfully passing by Ultima Thule is going to be an incredibly challenging task. Because it’s so small and so far away, the object is hard to see and track from Earth. And the New Horizons team only has one chance to get this flyby right. The spacecraft is currently traveling through space at 32,000 miles per hour. If something gets botched, there’s no way to turn New Horizons around and try again. “Like Pluto, it’s a one shot,” says Stern. “We don’t have a second spacecraft coming by a week later. And because it’s a very complex enterprise to do one of these flybys, there are literally hundreds of variables that all have to choreograph perfectly.”

New Horizons wasn’t always guaranteed a visit to an object like Ultima Thule

New Horizons wasn’t always guaranteed a visit to an object like Ultima Thule. The vehicle’s primary mission was to fly by Pluto, with the possibility of passing by a second Kuiper Belt object later. But when the spacecraft first launched from Earth in January 2006, astronomers didn’t even know if there was an object close enough to Pluto that New Horizons could target. All the objects we knew about at the time were out of reach. So while the New Horizons team prepped for Pluto, they also searched the sky for the spacecraft’s second destination. Finally in 2014, they found a target using NASA’s Hubble Space Telescope: a Kuiper Belt object named 2014 MU69.

MU69, now nicknamed Ultima Thule, was ideally placed on New Horizons’ path out of the Solar System. The spacecraft still carried enough fuel to fire its thrusters and reach the object. So in October and November of 2015, before NASA had even officially approved of New Horizons’ new mission, the team operating the spacecraft did a few course correction maneuvers, putting the vehicle on track to meet up with Ultima Thule.

Since then, New Horizons has gradually closed in on the rock, guided by the team back on Earth. It’s also observed other distant objects in the Kuiper Belt, snapping some of the most distant pictures ever taken by a robot. But for many months at a time, New Horizons has slipped in and out of hibernation, a mode in which most of its instruments are turned off. It’s a way to prevent unneeded wear and tear on the vehicle while it travels. It’s also a bit like autopilot — a time when the spacecraft mostly just travels and doesn’t need as much oversight from the mission team on the ground.

In the meantime, the New Horizons team had the daunting task of trying to learn as much as they can about Ultima Thule. Thought to be about 20 miles across, it’s too small and faint to visualize with any of the telescopes we have at our disposal here at Earth. That’s why the team has had to get creative. In 2017, New Horizons scientists were able to observe Ultima Thule as it passed in front of a background star. It was a momentary eclipse, known as an occultation, in which Ultima Thule briefly blocked out the star’s light. From that split-second crossing, the scientists were able to get a better understanding of the rock’s shape.

All indications point to this rock being weird. In fact, it may not even be a single rock. The occultation revealed that Ultima Thule is either shaped like a rubber duck, looking a bit like two mounds squashed together. Or it could be two separate rocks orbiting super close to one another. We really won’t know for sure until just a day or two before New Horizons reaches Ultima Thule. The tiny object is 100 times smaller than Pluto, making it harder to see in advance with New Horizons’ onboard cameras. The best pictures won’t come until New Horizons is more or less on top of the object. “This is a dot in the distance until the last minute, and then you snap your fingers, and you go from a point to a world in literally a 48-hour period,” says Stern.

“This is a dot in the distance until the last minute.”

That’s if everything goes according to plan. And Stern argues that this flyby is even more challenging than the one at Pluto, when New Horizons came within 7,750 miles of the dwarf planet’s surface. At that time, radio communication from Earth took 4.5 hours to reach New Horizons. But now that the spacecraft is farther away, it takes even longer — six hours — to get a signal to the vehicle. That, coupled with the small size and faintness of Ultima Thule, creates an even more complex trial for the scientists and engineers to overcome.

And the smallest unforeseen detail can muck something up at the last minute. Just last month, for instance, the New Horizons team averted a potentially big problem when they realized that the spacecraft had gone slightly off course. They eventually realized that each time the vehicle had reoriented itself to observe a far-off object in the Kuiper Belt, the movement slightly pushed New Horizons off track. Fortunately, the team caught this early enough so that they could correct the spacecraft’s trajectory and decided to sacrifice some upcoming observations to be extra careful. “That was a late catch, but the guy that discovered it is my hero,” says Stern.

But now, there’s nothing left to be done. The final commands for the flyby sequence were uploaded to the spacecraft on Christmas Day, and those cannot be changed. New Horizons must do what it’s been designed to do: execute a series of pre-planned tasks when it flies by Ultima Thule on January 1st at 12:33AM ET, coming within 2,200 miles of the rock’s surface. It will snap pictures, map the object’s surface, look for moons and an atmosphere, and take the body’s temperature. Basically, it will try to learn all it can about Ultima Thule in a matter of a few seconds.

It’ll be a short wait before we get those first tantalizing up-close images, though. Four hours after the flyby, New Horizons will send a signal back to Earth to let the mission team know if the encounter was a success. And since radio communication takes six hours to reach our planet, we won’t know for 10 hours if the flyby went as planned. Then a few hours after that first signal, New Horizons will start the long process of sending back all of the data its collected, including the first high-resolution pictures.

“I think the fun of this is we don’t know what we’re going to see.”

Getting all of this data back to Earth will take up to two years. New Horizons will collect about 50 gigabits of data, which is the storage capacity you might find on a small flash drive. But because of the spacecraft’s low processing power, it’ll take up to 20 months to downlink everything to Earth. So new surprises and pictures will keep coming in throughout 2019 and 2020.

As for what we’ll find at Ultima Thule, the mission team is reluctant to speculate. The rock is truly in a new category of Solar System object, and there’s just no precedent for what we might find. But Stern is optimistic that we’ll see something amazing. “I don’t make predictions,” says Stern. “The only prediction I made at Pluto is we’d find something wonderful, and we did. I think the fun of this is we don’t know what we’re going to see.”