Secret military mini-shuttle headed back to Earth

The X-37B Orbital Test Vehicle in the encapsulation cell at the Astrotech facility in April 2010, in Titusville, Florida.

CAPE CANAVERAL, Florida (Reuters) - A miniature robotic space shuttle launched from Cape Canaveral in April has completed a nine-month classified mission for the military and will be headed for a landing as early as Friday, Air Force officials said on Tuesday.

The vehicle, known as the Orbital Test Vehicle or X-37B, is expected to land at Vandenberg Air Force Base in California between Friday and Monday, depending on weather and technical considerations, the Air Force said in a statement.

The project, started by NASA in the late 1990s and later adopted by the military, is intended to test technologies for a next-generation space shuttle.

The military is looking at the space plane as a way to test new equipment, sensors and material in space, with the intention of incorporating successful technologies into satellites and other operational systems.

The Air Force imposed a news blackout on the X-37B's activities while in orbit, though it was tracked by amateur satellite-watchers throughout its nine-month mission.

The X-37B looks like a space shuttle orbiter, with a similar shape and payload bay for cargo and experiments.

But it measures 29 feet, 3 inches in length and has a 15-foot (4.5-meter) wing span, compared to the 122-foot (37-meter) orbiters with wing spans of 78 feet.

Unlike NASA's space shuttles, which can stay in orbit about two weeks, X-37B is designed to spend as long as nine months in space, then land autonomously on a runway.

The Air Force plans to fly its second X-37B vehicle this spring. The vehicles were built by Boeing Co's advanced research lab, Phantom Works.

(Editing by Kevin Gray and Todd Eastham)