The U.S. Air Force’s unmanned X-37B Orbital Test Vehicle (OTV) has been orbiting Earth for over 500 days, but its exact purpose still remains a mystery to us.

This robotic space plane is not dissimilar to NASA’s Space Shuttle in appearance, but it’s considerably smaller at around 9 meters in length. It also launches vertically on the back of a rocket and then returns by landing horizontally much like the Space Shuttle. It does not require an onboard pilot.

X-37B is solar-powered and can therefore remain in orbit for extended periods, and so far there are no hints as to when it will be returning to Earth. It’s also reported to have only made one or two maneuvers whilst in orbit, suggesting it’s got lots of fuel left. It zips around Earth at around 17,500 miles per hour, maintaining a low orbit of up to 500 miles above ground.

Although 500 days may seem like a long time, former orbital analyst with the Air Force Brian Weeden told Space.com “It’s common for satellites, especially typical national security satellites that are well-designed, to be in orbit for years to decades.”

Supposedly X-37B is a secret project, yet pages of details have been released on it by the U.S. Air Force. According to their website “The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America’s future in space and operating experiments which can be returned to, and examined, on Earth.” While this is probably true, not everyone is convinced that the story ends there, since the Air Force won’t give details on its orbital activities. Many are beginning to wonder what the purpose of this secretive space plane is.

A few different ideas have cropped up about what X-37B might be doing, but of course they are only speculations. Amateur satellite spotters that have been keeping an eye on it have suggested that at the moment it is orbiting Earth at a pretty low altitude of around 218 miles above Earth. It’s also cruising between 43.5 degrees north latitude and 43.5 degrees south latitude, which has led some to believe that it’s focusing on the Middle East. This, coupled with its reported low orbit, has caused ideas to be thrown around that it could be spying on this area. Some conspiracy theorists also believe that it could be a space weapon, but this idea has been strongly refuted by military officials.

Steven Aftergood, senior research analyst at the Federation of American Scientists, thinks that the publicity of this secret plane is puzzling. “Generally speaking, there are a couple of ways that the secrecy of the program could be justified: if disclosure enabled adversaries to defeat the program, or to diminish its effectiveness, or if it made it possible for others to replicate the technology.”

As it stands, there is still a big question mark over the precise purpose of this enigmatic space plane, and it seems unlikely that we will find out more any time soon.