X-37B Space Plane Returns: 5 Theories About Its Secret Mission

After spending nearly two years in orbit on a secret mission, the U.S. Air Force's mysterious X-37B space plane landed today (Oct. 17) at Vandenberg Air Force Base in California. The unmanned, reusable vehicle logged an unprecedented 675 days in space, but very little is known about the record-setting flight.

The X-37B, also known as the Orbital Test Vehicle, touched down at the Vandenberg Air Force Base today at 9:24 a.m. local Pacific Time (12:24 p.m. EDT). It was the third in a series of flights that the Air Force has conducted using its two X-37B planes.

This most recent flight, called OTV-3, was the third one to make it into orbit and was the longest mission, at 675 days.  The program's inaugural mission launched in April 2010 and lasted 225 days. The space plane's second mission lasted 469 days.