During a meeting of the US Senate Armed Services Committee, Air Force Secretary Heather Wilson revealed that the fifth mission of the secretive X-37B will launch in August using a Falcon 9 rocket from SpaceX.
The X-37B was designed by Boeing and is about one quarter the size of the now retired Space Shuttle. It's a reusable unmanned spacecraft and just like the Space Shuttle it launches on top of a rocket and lands on a runway.
There are two X-37B experimental space planes and each has flown two missions. The four mission were launched on top of an Atlas V rocket from United Launch Alliance (ULA) and the US Air Force is excited about the addition of companies like SpaceX to expand the capacity of the military while also significantly lowering launch costs.
What remains is mystery is the purpose of the X-37B. The Air Force is keeping its lips shut tight about what the X-37B does during its long-duration orbits. The general assumption is that the Air Force is testing new space tech and/or new space surveillance systems.
The US military has not said what the X-37B has been doing up in space for years at a time, beyond furthering "operations development for reusable space vehicle technologies." In addition to testing surveillance technology, some experts think the Air Force may be working on equipment that could be incorporated into a human-rated version of the vehicle that could carry a flight crew. Among the applications contemplated for the X-37B would be the recovery of satellites for repair on Earth.