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This undated photo released by the U.S. air force shows the X-37B spacecraft. It launched into space on Thursday. ((U.S. air force/Associated Press))

A mysterious unmanned U.S. military spacecraft, known as the X-37B Orbital Test Vehicle, has been launched on its maiden voyage.

It was launched off an Atlas Five rocket shortly before 8 p.m. ET Thursday from Cape Canaveral, Fla., the U.S. air force said in a statement issued the same night.

Mystery has swirled as the force has released little information about the aircraft, which is reusable and similar to a small space shuttle.

Likened to the size of a sports car with an equivalent trunk capacity, the X-37B has two angled tail fins rather than a single vertical stabilizer. It weighs 5,000 kilograms, is about three metres tall, nine metres long and has a wingspan of about 4.5 metres.

The force has only released a general description of the aircraft's mission objectives: to test the guidance, navigation, control, thermal protection and autonomous operation in orbit, re-entry and landing.

The X-37B can stay in orbit for up to 270 days before gliding to a runway landing, which is expected to be at the Vandenberg Air Force Base northwest of Los Angeles, the force said in its statement.

The plane is being used as a test platform for unspecified experiments, the force said.

"This launch helps ensure that our war fighters will be provided the capabilities they need in the future," Col. Andre Lovett, a launch official and vice-commander of the air force's 45th Space Wing, said in the statement.

The force called the launch a success, but gave no further details of the mission's progress or length.

Experts have speculated the aircraft is intended to speed up development of combat-support and weapons systems.

The X-37B project has been in the works since the 1990s. It began with NASA, then the U.S. Pentagon's research and development arm, and is now in the hands of the Air Force Rapid Capabilities Office.

It was built by Boeing Co.'s Phantom Works.

With files from The Associated Press