One of the world's best - and worst - kept military secret has returned home after a marathon mission.

The X-37B robotic 'mini shuttle' landed at California's Vandenberg Air Force Base on Saturday morning, after 15 months in orbit - 199 days longer than its batteries and solar arrays were supposed to last.

The craft, which is 8.9 metres long and 4.5 metres wide, has a payload bay roughly the size of a pickup truck. However, what if anything was on board is classified.

Another mission, the third for the two X-37B craft, is due to lift off over the autumn.

The military, which took over the program from Nasa, says it is using them to learn how to quickly and inexpensively refurbish reusable spaceships, and to test new satellite technology.

However, some have speculated the craft may have been used to spy on China's Tiangong-1 space lab, or North Korean missile development.