Up in the sky, and soon landing on Earth, a mystery space plane _ but why? US military mum

It looks like a mini space shuttle, can fly itself for millions of miles and goes by the name X-37B.

But what is it?

Maybe a secret space weapon. Or something to rocket next-generation spy gear into orbit. How about a decoy to keep China's military guessing.

The purpose of the Air Force's space plane is classified, only fueling speculation about why it has been circling Earth for nearly two years on this, the program's third mission.

The plane is expected to land as soon as Wednesday at Vandenberg Air Force Base, about 130 miles northwest of Los Angeles.

The military isn't saying much, other than that crews at Vandenberg are well-trained for when the X-37B does touch down.