A super-secret spacecraft has been orbiting the earth for more than 500 days. No one will tell us what it’s doing up there, how long it’s staying up, or where it’s going to land.

The plot of a science fiction thriller? Nope.

The Air Force’s unmanned X-37B space plane is on its fourth secret mission which has recently passed the 500-day mark. (The third mission lasted 674 days.) One can guess it has something to do with national security or military spying. Or, if you have a conspiratorial bent, it has something to do with testing captured alien technology.

A couple of things we do know about the vehicle is that it carries a NASA advanced materials investigation and an experimental propulsion system developed by the Air Force.

Like NASA’s Space Shuttle (RIP), the X-37B is reusable, but there are only two in the fleet, at least, that’s all the Air Force will confirm. This current mission is the second flight of the second vehicle built for the Air Force by Boeing Space and Intelligence Systems in El Segundo, California.

Did I mention that the X-37B is unmanned? Well, that’s what they tell us, anyway.