The U.S. Air Force's robotic X-37B space plane finally returned to Earth Saturday (June 16), wrapping up a mysterious mission that lasted more than year in orbit. The unmanned X-37B spacecraft, also known as Orbital Test Vehicle-2 (OTV-2), glided back to Earth on autopilot, touching down at California's Vandenberg Air Force Base at 5:48 a.m. PDT. The landing brought to an end the X-37B program's second-ever spaceflight, a mission that lasted more than 15 months with objectives that remain shrouded in secrecy. The X-37B stayed in orbit for 469 days this time, more than doubling the 225 days its sister ship, OTV-1, spent in space last year on the program's maiden flight. Officials at Vandenberg said the spacecraft conducted on-orbit experiments" during its mission. The space plane was designed to stay aloft for 270 days, but the Air Force kept it flying well beyond that milestone. Exactly what the spacecraft, which is built by Boeing's Phantom Works division, was doing up there for so long is a secret. The details of the X-37B's mission ... are classified, as is its payload. This secrecy has led to some speculation, especially online and abroad, that the X-37B could be a space weapon of some sort - perhaps a sophisticated satellite-killer. Some experts also suspect that the vehicle may be an orbital spy platform.