CAPE CANAVERAL - A prototype miniature space shuttle blasted off aboard an unmanned Atlas 5 rocket from Cape Canaveral Air Force Station on Saturday for a demonstration run that could last as long as nine months.

The experimental vehicle, known as the Orbital Test Vehicle, or OTV, lifted off at 5:46 pm EST (2246 GMT). It is the second ship to be put in space under the US military's X-37B program.

The vehicles are smaller versions of NASA's space shuttle orbiters -- 29 feet (8.8 metres) long, 14 feet (4.3 metres) across. The one-third scale spaceships are solar powered, unlike the space shuttles, and are not designed to carry people.

Like OTV-1, which returned from a 224-day mission on December 3, what OTV-2 will do in orbit, as well as any cargo or experiments that are aboard are classified.

They are intended to test technologies and processes for low-cost, quick-turnaround, reusable space vehicles, as well as serve as orbital testbeds for instruments that could be incorporated into future satellites.

Once operational, the X-37B could be used for a variety of missions including reconnaissance, in-space service and repair of satellites, deploying and retrieving spacecraft, and demonstrating new technologies, the Air Force said.

OTV-1 returned from flight in good condition, paving the way for launch of its sister ship with few modifications. A more detailed inspection and analysis of OTV-1 will be undertaken as part of its refurbishment.

OTV-1 has not yet been scheduled for a second launch, but the Air Force anticipates it will return to orbit.

No significant changes were made to OTV-2 as a result of the OTV-1 flight.

Minor tweaks include a reduction in the vehicle's main landing gear tire pressure by about 15 percent to help avoid repeating the blown tire that OTV-1 experienced upon touchdown at Vandenberg Air Force Base on December 3.

The reduced pressure should better accommodate imperfections in Vandenberg's 15,000-foot-long (4,572 metres) runway, the Air Force said.

The vehicles were built by Boeing.