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 . `` Technologies being tested in the program include advanced guidance , navigation and control , thermal protection systems , avionics , high temperature structures and seals , conformal reusable insulation , lightweight electromechanical flight systems , and autonomous orbital flight , re-entry and landing , '' an Air Force statement said . Theorists speculate the spacecraft is a space bomber , a spy plane against such targets as the Chinese space station , or merely an experiment as the government states , according to a Popular Mechanics story in 2012 . The previous mission of the X-37B , which landed at Vandenberg on June 16 , 2012 , lasted 469 days , according to the Air Force . That mission was flown by the second of the orbiters . The latest mission is the second for the first of the orbiters , which was refurbished after it spent 224 days in orbit following an April 2010 launch . When the latest mission launched , the Air Force said it might not be the last . `` Officials anticipate multiple missions will be required to satisfy the test program objectives , but the exact number of missions has not been determined , '' a statement said . The X-37B spacecraft is 29 feet , 3 inches long and 9 feet , 6 inches high with a wingspan of 14 feet and 11 inches . It weighs about 5.5 tons . It is lifted into space by United Launch Alliance 's Atlas V rockets . CNN 's Brad Lendon contributed to this report .
The X-37B Orbital Test Vehicle lands at Vandenberg Air Force Base . (Image credit: U.S. Air Force) The military has spoken only in generalities about the spacecraft . Theorists speculate it 's a space bomber or a spy plane against China space station .