The US Air Force has two X-37B space planes, and since 2010 each of them has flown two missions into outer space. Those flights have ranged in length from 224 to 717 days. The X-37B, which is autonomous and looks something like a miniature version of NASA's space shuttle, launches on top of a rocket and orbits the Earth before returning and landing on a runway.

For the first four missions, the Boeing-developed space plane has launched on top of an Atlas V rocket, the military's go-to vehicle manufactured by United Launch Alliance. However, on Tuesday during a meeting of the US Senate Armed Services Committee, Air Force Secretary Heather Wilson revealed that the upcoming fifth mission of the X-37B will be launched into space by a Falcon 9 rocket. That SpaceX launch is tentatively scheduled for August.

Wilson testified that the emergence of the commercial space industry has proven a boon for the US military. "The benefit we're seeing now is competition," she said. "There are some very exciting things happening in commercial space that bring the opportunity for assured access to space at a very competitive price."

The admission came in response to a question from Senator Martin Heinrich (D-N.M.), who asked about the military's capability for a rapid response to military space needs. After picking up a model of the X-37B and showing it to committee members, Wilson said the addition of companies like SpaceX, as well as other launch firms, was expanding the capacity of the military and significantly lowering costs.

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.

Some military advisers have begun urging the Air Force to embrace the commercial space industry more completely. Recently, the Air Force's Air University conducted a study that found that developments by SpaceX, Blue Origin, and other companies, such as Vulcan Aerospace and Virgin Galactic, had given the United States a definitive edge over global competitors in this new area of reusable rocket technology. However, the report warned, other countries, such as China, could copy these ideas and surpass the United States if strategic government investments are not made.