The United States on Tuesday launched for the third time its futuristic X-37B spacecraft, a small and pilotless plane that experts believe could open a new realm of espionage.

The 29-foot (8.9-meter) long vehicle blasted off aboard an Atlas V rocket at 1:03 pm (1803 GMT) from Cape Canaveral in Florida on a mission about which the US Air Force has offered minimal details.

The United Launch Alliance, a joint venture between Boeing and Lockheed Martin, streamed live footage on its website of the blastoff. It said the mission would support "space experimentation."

The US military has described the X-37B program as a way to demonstrate "technologies for a reliable, reusable, unmanned space test platform" following the retirement of NASA's shuttle Orbiter program.

The secretive nature of the equipment on the X-37B has led to speculation in the media over the missions' aims, with some experts believing that the US Air Force is looking at new way of spying.

Space experts believe that the small vehicle, with its ability to return to Earth and head back up, could be part of a state-of-the-art espionage program or may potentially be used to interfere with rival nations' satellites.

China has shown a growing interest in space. In 2007, China became the third nation after the United States and former Soviet Union to shoot down one of its own satellites in a test viewed in Washington as a warning.

It is the second mission of the original X-37B which went into space in 2010 in the program's inaugural flight and stayed in orbit for more than half a year.

A second X-37B vehicle returned to Earth in June after orbiting for 469 days in an endurance test that went far beyond the aircraft's intended flight span of 270 days.

The test had been put on hold after a fuel leak in the thrust chamber during a separate, successful launch on October 4 of a Global Positioning System satellite.

The X-37B project was launched by the space agency NASA in 1999 before being adopted by the Defense Advanced Research Projects Agency, which designs new technologies for the US military.