The US Air Force’s X-37B, an unmanned space plane, will reach a milestone on April 25 by being in space for 500 days.

The X-37B was launched on December 11, 2012. It launched on OTV-3 in Cape Canaveral, Florida, and has broken its own record of spending 469 days in space.

However, little is known about the mission of the X-37B. What we know is that it is an unmanned space plane, and that two models exist. It flew its first test mission in 2010. In 2005 and 2006, NASA performed tests with an early variant, called the X-37A.

Because of its classified mission, no one knows when and where the X-37B will land. Its previous landing was held at California’s Vandenberg Air Force Base, and was only announced after landing.

The X-37B is also smaller than other space planes, measuring only 8.8 meters in length, with a maximum weight of 4,990 kilograms. The space plane is solar powered.

The X-37B is the lightest and smallest space plane, and is the second to land on its own, after Russia’s Buran, which performed one mission and after one orbit, landed at the Baikonur Cosmodrome in November, 1988.

The X-37B is part of a global fascination for a “secret space race” that has overshadowed other space programs from countries around the world. An example is the Corona mission, which was executed from 1959 to 1972, and was declassified 23 years later.

Experts have suggested that the X-37B is used to test new technologies. Others have speculated that it is used to watch over the Chinese spacecraft Tiangong-1, after news of X-37B’s frequent passes over North Korea and China. However, others have refuted this theory because the X-37B and Tiangong-1 are on different orbits.

Spectators can view the X-37B tonight, as the space plane is sitting in an orbit of 392 by 296 kilometers. This makes it visible “from latitudes 55 degrees north to south.”

X-37B Space Plane

Image via Wikimedia Commons