WASHINGTON  - The United States is planning a new launch of its tiny, pilotless military space plane 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 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.