Air Force personnel in &34'self-contained atmospheric protective ensemble suits&34' perform initial post-orbit checks on the X-37B space plane early today on a runway at Vandenberg Air Force Base.

The X-37B, an unmanned U.S. Air Force space plane whose mystery mission set off a round of speculation over the spring and summer, returned to Earth early this morning after its maiden flight lasted 220 days in orbit.

The space plane landed at 1:16 a.m. PT today at Vandenberg Air Force Base in California, officially making it the U.S.'s first unmanned vehicle to return from space and land on its own, according to Boeing, which designed the craft. Launched in April from Cape Canaveral by an Atlas 5 rocket, the X-37B was designed to stay in orbit for a maximum of 270 days, or about nine months.

"We are very pleased that the program completed all the on-orbit objectives for the first mission," Lt. Col. Troy Giese, the X-37B program manager from the Air Force Rapid Capabilities Office, said in a statement today.

Also known as the Orbital Test Vehicle 1, the X-37B is comparable to--but much smaller than--NASA's space shuttles, which have served for three decades as the pre-eminent reusable spacecraft able to return to Earth from orbit. With the shuttles due to go into retirement early next year, the goal behind the X-37B is to demonstrate the feasibility of unmanned and reusable space vehicles for use in future missions.

The X-37B Orbital Test Vehicle on the runway, at an unspecified earlier date.

More specifically, the Air Force's stated purpose for this first flight was to see how the craft's guidance, navigation, and other unmanned systems would handle the orbit and landing. But a statement from the Air Force in April at the time of the launch date hinted at other uses.

"If these technologies on the vehicle prove to be as good as we estimate, it will make our access to space more responsive, perhaps cheaper, and push us in the vector toward being able to react to warfighter needs more quickly," Gary Payton, the Air Force deputy undersecretary for space programs, said in a statement at that time.

The Air Force has kept mum about any potential military uses of the X-37B, along with whatever might have been in the space plane's payload bay during the test flight. Some experts and analysts believe the craft is designed to help push the development of combat and weapons systems, while others say it could serve as an unmanned orbital spy platform.

A second vehicle, dubbed the Orbital Test Vehicle 2, is being readied for launch in the spring.

"This marks a new era in space exploration, and we look forward to the launch of the second vehicle in 2011," Paul Rusnock, Boeing's vice president of experimental systems and program director for the X-37B, said in a statement today. "By combining the best of aircraft and spacecraft into an affordable, responsive unmanned vehicle, Boeing has delivered an unprecedented capability to the RCO [the Air Force Rapid Capabilities Office]."

Weighing 11,000 pounds, measuring 29 feet long, and sporting a wingspan of less than 15 feet, the X-37B resembles a smaller version of the space shuttle. That's more than just a coincidence since the craft was originally a NASA project before the Air Force took it over.