In 2012, the Air Force sent an unmanned robot plane into space. It is scheduled to return today, completing its third voyage into space, one that began way back in the simpler days of 2012. The X-37B (not to be confused with the U.S. Navy’s terrestrial experimental unmanned X-47B airplane) is a known space plane with a secret purpose.
One early guess as to the function of the X-37B? It might ferry future troops into battle, taking them very high and then sending them back to Earth very fast. The fast transport mission doesn't match the most striking feature of the X-37B's performance, however, which is just how long it can stay in space. The space plane's first major trip into orbit lasted for seven months, and the plane's second kept it in space for 15 months. When it returns to Earth today, its third long trip into space, it will have been 22 months since it left in December 2012.
Long, unmanned trips aren't the stuff of a troop transport. Instead, they're the sort of things drones do. The U.S. Air Force's fact sheet on the X-37B lists the official purpose of the craft as "reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth." Speculationabounds that there's an intelligence purpose to the missions, but the secrecy around the program means whatever the X-37B does besides test orbital re-entry will remain secret for some time.
Welcome back to Earth, secret space robot.
Update 3:10 10/14/2014: The X-37B has not landed yet. The airspace around the Vandenberg Air Force Base, where the plane is expected to landed, is restricted to only official use through the 17th, and the airspace is closed from 8am to 5pm Wednesday. Expect the X-37B back on Earth not today, but soon.