The U.S. Air Force has launched an experimental robotic space plane on a secret mission. The pilotless X-37B Orbital Test Vehicle lifted off from California yesterday aboard an Atlas 5 rocket. Air Force officials have kept details of the craft secret, but said the mission is to test new outer space technologies. The mystery has fueled speculation that the X-37B is some kind of weapon or spy device which has been denied by Air Force officials. The craft is about 9 meters long and looks like a miniature space shuttle. It is designed to stay in orbit for up to 270 days, and operates completely independently. This is the second mission for the X-37B, which was first launched in April last year and landed in December. It was originally developed by the U.S. space agency NASA, but was later taken over by the military.