A robotic space plane has been speeding in low Earth orbit, with some saying that it's a weapon, while others claim it to be a data-gathering mission.
The plane hits 500 days in space, and its real purpose is still a mystery, News.com.au reported.
The Boeing-built X-37B Orbital Space Vehicle, constructed in California, is one-fourth the size of the Endeavour Space Shuttle.
The Air Force craft, purely a test vehicle that will never reach production, in part aims to explore reusable space-vehicle technologies. That means the robotic vehicle can land, but no one will say when.
While intelligence gathering is widely regarded as the best explanation, no one can say for certain what it could be.
Others speculate the craft could be some sort of intelligence-gathering tool. The Secure World Foundation, a non-profit based in Broomfield, Colo., published a 2010 report on the craft. It speculated the space plane's most likely purpose was for intelligence gathering and said the vessel had a low probability of other uses, such as satellite repair.
Whatever its real purpose, the X-37B has very real capabilities. It travels low in orbit, staying around 177 to 804 kilometres above the Earth at a cruising speed of about 17,500mph (28,163kph). It's equipped with special heat-shield tiles for re-entry, which are billed by Boeing as tougher than Endeavour's.
To John Pike, director of GlobalSecurity.org, the craft's purpose is all about keeping the United States the most technologically advanced nation in the world.
