The United States is planning a new launch of its tiny, pilotless military space plane on Tuesday as part of a futuristic Air Force program that has fueled speculation over its mission.

The X-37B, which weighs five tonnes and is 29 feet (8.9 meters) long, can return material to Earth in the way of the retired shuttle Orbiter program but is designed to stay in orbit for much longer at 270 days.

The last X-37B returned in June after orbiting for 469 days in a test of endurance.

The United Launch Alliance, a joint venture between Boeing and Lockheed Martin, approved the X-37B at Cape Canaveral in Florida after finding no danger following an anomaly during a separate launch two months ago.

The company said in a statement that a Global Positioning System satellite was put into orbit as expected on October 4 but that a fuel leak took place inside the thrust chamber, triggering an investigation.

Patrick Air Force Base gave notice of a hazard from a launch between 10:45 am to 5:15 pm (1545 to 2215 GMT) on Tuesday.

Authorities have said little more about the X-37B. An Air Force fact sheet described it as "experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the US Air Force."

The secretive nature of the equipment on the X-37B has led to speculation in the media over its true nature, with some experts saying it could eventually be designed to tamper with satellites from rival nations.

China in 2007 became the first nation after the United States and the former Soviet Union to shoot down one of its own satellites, in a test seen in Washington as a sign of the rising power's ambitions in space.

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.