The X-37B space plane has spent 500 days orbiting the Earth on its fourth mission – and we still don’t have a clue what it’s doing up there.
The vehicle is an experimental and secretive project by the US Air Force. It’s fourth mission, referred to as OTV-4, was launched on May 20, 2015 from Cape Canaveral in Florida.
Its secrecy has led to both speculation and paranoid pondering.
Some have suggested secret surveillance. Theories range from spying on China's space station, other satellites, or even back home on Earth, namely around the Middle East. Other more outlandish ideas suggest it could be a “space bomber” or a weapon to aggressively destroy rival satellites. Live Science has a nice round-up of these “imaginative” theories.
Nevertheless, here’s what we do know about it. Described as the Air Force's "newest and most advanced" re-entry spacecraft, it was constructed with the help of Boeing and launched into orbit by a United Launch Alliance Atlas V rocket.
A launch on May 20, 2015, believed to be for the X-37B. James McCloskey/Flickr (CC BY-SA- 2.0)
This is the fourth mission for the 8.9-meter-long (29.3 feet) plane, all of which have successfully landed back on Earth. The third of these missions also broke the 500-day orbit mark, eventually landing on October 17, 2014, after 674 days in orbit.
The US Air Force says the space plane is on a “test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the US Air Force.”
It also says that the mission is being used to test out high-end technology, such as navigation and control systems, as well as more practical concerns like the durability of high-temperature materials. The payloads and other activities of the X-37B mission remain classified.
However, speaking to Space.com in April last year about the space plane's fourth mission, Air Force spokesperson Captain Chris Hoyler said: "The X-37B is designed for an on-orbit duration of 270 days. Longer missions have been demonstrated. As with previous missions, the actual duration will depend on test objectives, on-orbit vehicle performance, and conditions at the landing facility."
There's no word yet on when this mission is set to land, although we have asked the US Air Force for details. We'll let you know if we hear anything.