Shrouded by darkness, the military’s miniature space shuttle — a unmanned robotic craft — returned early Friday from a trial run in orbit that spanned 224 days.

The Orbital Test Vehicle, also known as the X-37B, touched down at 1:16 a.m. PST at California’s Vandenberg Air Force Base, becoming the first U.S. vehicle to make an autonomous runway landing from space. The former Soviet Union’s Buran space shuttle accomplished the feat in 1988, following the sole spaceflight of the Soviet shuttle program.

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The military won’t say what the X-37B was doing during its seven-plus months in space, but officials were satisfied enough to reiterate their intention to launch a second X-37B vehicle in the spring of 2011.

Before the X-37B’s launch on April 22, the program manager at the time said the primary purpose of the flight was to test the vehicle as a platform for experiments. It is not known if the space plane carried anything in its small cargo hold.

“We are very pleased that the program completed all the on-orbit objectives for the first mission,” program manager Lt. Col. Troy Giese said in a statement.

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Although the X-37B has landed, its mission is not yet over.

“Probably the most important demonstration is on the ground,” Gary Payton, who served as undersecretary of the Air Force for Space Programs, told reporters before the launch. “Once we get the bird back, see what it really takes to turn this bird around and get it ready to go fly again.”

The vehicle expected to fly in the spring is a second spacecraft, but the idea is to cut down the time needed for servicing the space planes from months to days — and at a fraction of the cost NASA pays to get its shuttles ready for flight.

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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.

Rather than hydrogen-oxygen fuel cells like the space shuttle orbiters, the X-37B is powered by gallium arsenide solar cells with lithium-ion batteries. It is designed to stay in orbit for up to 270 days, deorbit itself and land autonomously on a runway. NASA’s space shuttles can stay in space for up to about three weeks.

Image: The X-37B shortly after landing at Vandenberg AFB early Friday morning. Credit: U.S. Air Force.