X-37B

The X-37B, launched in September 2017, has returned to Earth. It was the fifth acknowledged mission for the vehicle since 2010, but details of its mission are being kept under wraps.

For a secret aircraft, the X-37B wasn't very stealthy about its return to Earth. The space plane was launched in 2015.

There are many theories about what exactly the unmanned X-37B is doing as it circles the Earth on long-duration missions.