A top-secret space plane returned to Earth yesterday after almost two years in orbit.
The unmanned plane spent more than 500 days circling Earth on a classified mission.
Known as the X-37B, it resembles a mini space shuttle. And nobody knows what it's for.
Its mission began in December 2012, when the X-37B lifted off from Cape Canaveral, Florida, atop an Atlas Five rocket and it safely touched down at 9:24 a.m. Friday, officials at Vandenberg Air Force Base said.
The 29-foot-long craft is built by Boeing, and is only officially explained as a test vehicle for technology. Which could mean literally anything. The widespread assumption is that the vehicle is used for some kind of surveillance, though that has not been confirmed.
Just what the plane was doing during its 674 days in orbit has been the subject of sometimes spectacular speculation.
Several experts have theorised it carried a payload of spy gear in its cargo bay. Other theories sound straight out of a James Bond film, including that the spacecraft would be able to capture the satellites of other nations or shadow China's space lab.
In a written release announcing the return of the craft, the Air Force only said it had been conducting "on-orbit experiments."
The X-37B program has been an orphan of sorts, bouncing since its inception in 1999 between several federal agencies, NASA among them. It now resides under the Air Force's Rapid Capabilities Office.
Story continues below
The plane that landed Friday is one of two built by Boeing. This is the program's third mission, and began in December 2012.
The plane stands 9 1/2 feet tall and is just over 29 feet long, with a wingspan under 15 feet. It weighs 11,000 pounds and has solar panels that unfurl to charge its batteries once in orbit.
The Air Force said it plans to launch the fourth X-37B mission from Cape Canaveral, Florida, next year.