An unmanned Air Force space plane that spent seven months in orbit is set to return to Earth.

The X-37B Orbital Test Vehicle is scheduled to land at Vandenberg Air Force Base northwest of Los Angeles sometime between Friday and Monday, depending on the weather and other factors.

The Air Force Space Command said in a statement Tuesday the base has begun preparations for the landing.

The X-37B resembles a small space shuttle. Since it launched in April, space enthusiasts have speculated about its ultimate purpose. The Air Force has said the space plane was to serve as a test platform for unspecified experiments.

Hundreds of millions of dollars have been spent on the X-37 program, but the current total hasn't been released.