The US Air Force's super-secretive X-37B space plane has spent 200 days in orbit as it carries out a classified mission for the American military.

The unmanned X-37B launched on May 20 on the back of a United Launch Alliance Atlas V rocket from Cape Canaveral Air Force Station in Florida. The space plane is the fourth spacecraft of its kind for the US Air Force.

The mission, called Orbital Test Vehicle-4 (OTV-4), marks the second flight of the X-37B, built for the Air Force by Boeing Space and Intelligence Systems.

Amateur satellite trackers have spotted the Air Force's secret mini space shuttle as it orbits around the Earth.

The reusable space plane looks like a miniature version of NASA's now-retired space shuttle orbiter. The X-37B is 29 feet long and 9.5 feet tall, and has a wingspan of nearly 15 feet. It has a payload bay about the size of a pickup truck bed.

While the overall duties of the space plane remain secretive, it was announced that it carries a NASA advanced materials experiment and an experimental propulsion system developed by the Air Force.

Source: Sputnik News