updated 10:20 AM EDT, Sun October 19, 2014

STORY HIGHLIGHTS

Los Angeles (CNN) -- Whatever it was doing up in space, we may never know, but the U.S. Air Force's unmanned X-37B space plane returned to Earth this week, with still no details from the military on the nearly two-year mission.

"The X-37B Orbital Test Vehicle mission 3 (OTV-3)," as the military calls it, touched down at Vandenberg Air Force Base in California on Friday morning after conducting experiments in orbit for 674 days, the military said.

Conspiracy theorists endlessly conjecture on what the Pentagon is doing with "the newest and most advanced re-entry spacecraft." The Air Force's two vehicles resemble small space shuttles, and have now logged a combined 1,367 days in space, the military said.

In the latest mission, the X-37B lifted off from Cape Canaveral in Florida on December 11, 2012. At the time, the Air Force said its mission would last about nine months.

The military has spoken only in generalities about the spacecraft and its mission.

Air Force X-37B space plane

Air Force X-37B space plane

Air Force X-37B space plane

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Mysterious space mission ends