What the Air Force isn't telling us. On 20 May 2015, the US military launched the robotic X-37B space plane into Earth's orbit to carry out its fourth top secret mission. Now, more than 500 days later, that solar-powered, unmanned drone is still up there, and with no word on how long it will remain in orbit - or what it's actually doing up there - it's spurring the same kinds of conspiracy theories that followed it through its previous missions. Is it deploying spy satellites into Earth's atmosphere? Is it interfering with the signals of other satellites? Is it part of the Pentagon's research into drone-based weaponry, or has it been keeping tabs on China's ill-fated space station this whole time? Your guess is as good as mine, but what we do know is that during its 500 days in obit, it's been carrying several payloads - a few of which have actually been identified. It's ferried a couple of prototype electric propulsion devices from the Air Force and private aerospace and defence