The autonomous space craft, blandly called the X-37B Orbital Test Vehicle, is intended to “demonstrate technologies for a reliable, reusable, unmanned space-test platform.” Translation: It’s a space craft that can orbit for hundreds of days and then glide back to Earth so that it can be relaunched to save money.

The Air Force has not disclosed many details about the space craft other than to say it’s designed for “low Earth orbit altitudes where it can perform long duration space technology experimentation and testing.” It has also said that the X-37B Orbital has a wingspan of 14 feet, 11 inches, is nearly 30 feet long, and weighs 11,000 pounds.

Lift off! @usairforce OTV-5 on its way to orbit. pic.twitter.com/rEDpOUFnsD

— Space Launch Delta 45 (@SLDelta45) September 7, 2017

The Air Force said that Thursday’s launch was the X-37B program’s fifth space flight and the first to be sent aloft on a rocket by Elon Musk’s private space cargo company SpaceX. Its first space flight was in 2010.

Boeing (BA) built the spacecraft, which was based on NASA’s designs of an earlier vehicle, according to the space agency.

In a sign of how long the X-37B can stay aloft, the third mission lasted 674 days while the fourth, which concluded in May, spent 718 days in orbit. Over all four previous missions, the space craft have spent a total of 2,085 days in space.

Typically, the space plane lands at the Vandenberg Air Force Base in California. But it has also landed at NASA’s Kennedy Space Center as it did in its fourth mission.

Get Data Sheet, Fortune’s technology newsletter.

Although the Air Force has stayed mostly silent about the X-37B’s details, a little information has trickled out from other sources. For example, a Boeing engineer said during a 2010 aerospace meeting that project was intended to evaluate the spacecraft’s autonomous navigation systems, according to Air & Space Magazine.

The Boeing engineer then speculated that the X-37B could be altered so that it could carry passengers. Additionally the space magazine reported that some aerospace experts believe the Air Force is testing small satellites that could be cheaper than current technologies.

“I think that’s probably what they’re not telling you, that there are payloads in there that might be part of the design for future reconnaissance satellites,” aerospace expert James Andrew Lewis told the magazine.