We don't know much about the US Air Force's X-37B uncrewed space plane. Although it resembles the space shuttle, it's much smaller (at about 30 feet in length), and it has a cargo bay that could hold something about the size of a standard refrigerator. It seems to fly in a relatively low orbit below the International Space Station. Oh—and it can fly in space for a long, long time before it needs to return to Earth.

Since 2010, two identical space planes have completed three missions of increasing lengths: 224 days, 468 days, and 674 days. One of the planes most recently launched on May 20, 2015, and there are signs that this space plane might finally be coming home soon, perhaps even on Tuesday. According to NASASpaceFlight.com, the X-37B may land in Florida on Tuesday after 636 days in space. Notably, it would land at Kennedy Space Center's historic Shuttle Landing Facility for the first time.

However, Spaceflight Now reports that the orbital maneuvers interpreted as preparation for landing may have just been an exercise. “The X-37 is still on-orbit. The program is conducting a regularly scheduled exercise this week,” Capt. Annmarie Annicelli, media operations officer at the Pentagon’s Air Force Press Desk, told the publication on Tuesday.

So what has the X-37B been doing up in space? The military isn't saying, but about a year ago Air & Space spoke with a number of space plane experts to get a sense of what the Air Force might be up to. According to the magazine, the vehicle is likely testing autonomous systems for navigation and other functions, including landing. Additionally, most of the experts believe the Air Force is interested in using the vehicle as an on-orbit test bed for developing advanced surveillance sensors, as the military looks to transition from massive, expensive, and vulnerable observation satellites to smaller, cheaper, but just-as-capable reconnaissance satellites.

The experts also speculated that the Air Force might be testing technologies that could be incorporated into a human-rated version of the vehicle that could carry a flight crew. Among the applications contemplated for the X-37B would be the recovery of satellites for repair on Earth.