Few details are being released about the space plane's mission, other than it will be used for "fine tuning" the aircraft's performance. The first X-37B launched last April.
Citing the classified nature of its role, the Air Force has refused to talk about the space plane's exact mission while in space; it also hasn't released budget details. The secrecy surrounding the project has spawned speculation ranging from the possibility of a weaponized space bomber to a sophisticated spy plane.
The Air Force has denied that the X-37B is designed as a space weapon, describing it simply as "an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S. Air Force."
Its unique ability, according to the Air Force, is that it can carry a payload, such as a satellite, into space and then return it to Earth for testing.
Often compared to a scaled-down version of the NASA space shuttle, the X-37B is built by Boeing's secretive Phantom Works division. Though now operated by the Air Force, it originated as a NASA program.
The X-37B is boosted into space using an expendable rocket but once in orbit, it is designed to stay for up to 270 days and then return to Earth, landing like an airplane.
The Air Force has not said how long this X-37B will orbit the Earth, but the previous one stayed in space for seven months. It is scheduled to land at Vandenberg Air Force Base in California.