On 20 May 2015, the US Air Force launched a solar-powered, unmanned space plane into Earth's orbit to carry out its fourth top secret mission, and now, almost two years later, we're still no closer to figuring out what it's doing up there.

The robotic X-37B space plane has now entered its 678th day in orbit, breaking its own record for the number of consecutive days spent in space, and the government remains tight-lipped on when it might be brought back down to Earth.

"The landing date will be determined based on the completion of the program's on-orbit demonstrations and objectives for this mission," Captain AnnMarie Annicelli, an Air Force spokeswoman, told Space.com.

Exactly what those mission objects are is anyone's guess, and there have been many in recent years - including spying on the Middle East, space bombing (which isn't even technically feasible), and keeping tabs on China's ill-fated space station.

The space-bomber theory has been more or less debunked, with former chief scientist of the US Air Force, Mark Lewis, telling Popular Science that the amount of thrust needed to keep changing its position in orbit would be tremendous.