The United States Air Force's secretive X-37B space plane has been circling Earth for more than 10 months, and there's no telling when it might come down. Because it is an experimental vehicle, they kind of want to see what its limits are," said Brian Weeden, a technical adviser with the Secure World Foundation and a former orbital analyst with the Air Force. The X-37B looks a lot like NASA's now-retired space shuttle, only much smaller. The unmanned vehicle is about 29 feet long by 15 feet wide (8.8 by 4.5 meters), with a payload bay the size of a pickup truck bed. For comparison, two entire X-37Bs could fit inside the payload bay of a space shuttle. Just what the X-37B does for so long while circling our planet remains a mystery, because the space plane's payloads and missions are classified. Partly as a result of the secrecy, some concern has been raised - particularly by Russia and China - that the X-37B might be a space weapon of some sort. But the Air Force has repeatedly denied that charge, claiming that the vehicle's chief task is testing out new technologies for future satellites. The spacecraft is flying repeatedly over the stretch of Earth from 43 degrees north latitude to 43 degrees south latitude. The space plane may be observing the Middle East and Afghanistan with some brand-new spy gear, perhaps instruments optimized to observe in wavelengths beyond the visible-light spectrum.