The US has something very mysterious in orbit.

Top Secret

Dutch astronomer and satellite tracker Ralf Vanderberg just spotted something extraordinary and rare: the U.S. Air Force's top-secret X-37B space plane.

Also known as the Orbital Test Vehicle, the military craft looks like a scaled-down version of NASA's Space Shuttle and measures just 29 feet long with a wingspan of 15 feet. The craft spotted by Vanderberg is the fifth iteration, launched into space atop a SpaceX Falcon 9 in 2017.

"It is really a small object, even at only 300 kilometers [186 miles] altitude, so don't expect the detail level of ground-based images of the real space shuttle," Vandebergh told "We can recognize a bit of the nose, payload bay and tail of this mini-shuttle, with even a sign of some smaller detail."

Space-Based Mission

Details surrounding the mission are extremely sparse. The Air Force squadron responsible for the mission oversees space-based demonstration operations, as well as gathering info on objects flying at high altitudes over Earth, according to

The X-37 project began in 1999 and was taken over from NASA by the U.S. Department of Defense in 2004. An earlier version, called the X-37A, completed a test flight attached to the underbelly of California Spaceport Scaled Composites' White Knight carrier plane in 2005.

Though we still don't know what the X-37 is doing up there, the episode is a startling reminder of the power of citizen science — even in the age of the militarization of space.

READ MORE: Skywatcher Captures Rare Image of Mysterious X-37B Military Space Plane [Live Science]

More on X-37B: SpaceX Just Launched Its “Secret Mission” for the U.S. Air Force

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