Even in the final days of the approach, however, Ultima Thule is playing things close to the vest. "We've never in the history of spaceflight gone to a target that we've known less about," Alan Stern, principal investigator of New Horizons and a researcher at the Southwest Research Center in Colorado, told reporters Sunday (Dec. 30). [Ultima Thule Flyby! Full Coverage]

But when the spacecraft arrives, it will turn a suite of instruments onto the mysterious object, and many of its mysteries will be unveiled. It's the second historic rendezvous for New Horizons, which zipped by Pluto in July 2015 on the first-ever flyby of that world.

"We are ready to science the heck out of Ultima Thule," Stern said.

Lying at the outskirts of the solar system, Pluto may seem alone at first glance. Only in the last few decades have scientists realized that it is one of a collection of objects that make up the Kuiper Belt, a band of rocky, icy objects that border the solar system. Smaller than planets, most of the inhabitants are left over from the formation of the solar system. They provide a glimpse of the early ingredients of the solar system, the bits and pieces that formed the larger worlds we know today. [NASA's New Horizons Mission in Pictures]

When New Horizons launched in 2006, no one knew anything about Ultima Thule. Scientists didn't discover the spacecraft's next target until 2014, about a year before the epic flyby of Pluto. Originally named 2014 MU69, Ultima Thule was one of three objects proposed for an extended mission target.

Since its 2015 selection as the next object for New Horizons to visit, researchers have scrambled to learn as much about Ultima Thule as possible. In 2017, they learned that the Kuiper Belt Object (KBO) was not spherical but seemed elongated, or possibly even made up of two orbiting bodies.

As New Horizons closed in on Ultima Thule over the last three months, it began snapping hundreds of photos of the object. But while the images revealed no sign of potential dangers that could harm the spacecraft, such as unexpected moons or clouds of debris that could slam into the spacecraft as it blew by the KBO, they also gave no hint about the shape of the body it is about to visit.

In fact, it's still possible that Ultima Thule is made up of not one but two objects, closely packed together. Stern told Space.com Saturday (Dec. 29) that the timeline for resolving whether the KBO is alone or part of a duet depends on how close the potential pair lie to one another.

"If they are touching, it will take until the last day," he said.

It's too late now to divert the spacecraft to a higher altitude, where it would be safe from any potential dangers. The last chance to make that decision passed on Dec. 13, and team member Will Grundy of Lowell Observatory in Arizona said that it takes months to come up with a new path.

At 9 a.m. EST on Sunday (Dec. 30), engineers sent the spacecraft its last command before the flyby, redirecting the closest image by two seconds, Bowman said. That readjusted the aim point for the flyby by about 19 miles (30 km). For comparison, Hersman said that during the Pluto encounter, the spacecraft was off by about 80 seconds. The team didn't find out until 9 p.m. EST last night (Sun. Dec 30) that the spacecraft successfully received the new commands. Had it not gone well, they would have immediately resent the same update.

New Horizons will fly only 2,200 miles (3,500 km) above the surface of the KBO, three times closer than it buzzed Pluto. To help conserve power, several components of the spacecraft will be temporarily turned off, according to Chris Hersman, Missions Systems Engineer at JHU APL. The student dust counter, which picks up roughly one micrometer-sized dust particle per day, and the transmitting portion of one of the radio transmitters. By turning off these tools, the spacecraft will be able to operate its scientific instruments.