Top Secret Mission: Mystery Space Plane Has Been Orbiting Earth for 500 Days -- But Why?

A mystery space plane has been orbiting earth for 500 days and up to now, nobody knows what exactly is it doing up there.

IFL Science said that the 8.9-meter-long (29.3 feet) secretive vehicle by the US Air Force, was built with the help of Boeing and was launched into orbit by a United Launch Alliance Atlas V rocket.

Space.com notes that the vehicle, tagged as X-37B space plane is now on its fourth mission. It was launched May last year from Cape Canaveral in Florida to kick off the Orbital Test Vehicle-4 (OTV-4). According to the US Air force website, the X-37B Orbital Test Vehicle (OTV), is "an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S. Air Force."

Although its primary objectives state that it is intended for "reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth," details about what the plane's task is in every mission while out of orbit and when will it return are not known.

Live Science has rounded out some mindboggling theories about the vehicle and its mission. One of which is that X-37B has high-tech monitoring gear designed to thoroughly inspect on certain regions of Earth, making it a very powerful surveillance tool. The others are X-37B is taking out other satellites that are a threat to the U.S. government and is spying on Chinese space stations.

Meanwhile, Space.com says the initial OTV was launched April 22, 2010, and ended 224 days in orbit. The second OTV lasted after 468 days on orbit while the OTV-3 spent nearly 675 days in orbit, the longest so far.

In the past, all three X-37B flights landed at Vandenberg Air Force Base in California on autopilot. For the fourth OTV, none has been detailed yet.