An anonymous reader quotes a report from Space.com: The fifth mystery mission of the U.S. Air Force's X-37B space plane is now underway. A SpaceX Falcon 9 rocket carrying the robotic X-37B lifted off today (Sept. 7) at 10 a.m. EDT (1400 GMT) from historic Launch Complex 39A at NASA's Kennedy Space Center (KSC) in Florida. About 2.5 minutes into the flight, the Falcon 9's two stages separated. While the second stage continued hauling the X-37B to orbit, the first stage maneuvered its way back to Earth, eventually pulling off a vertical touchdown at Landing Zone 1, a SpaceX facility at Cape Canaveral Air Force Station, which is next door to KSC. The Air Force is known to possess two X-37Bs, both of which were built by Boeing. The uncrewed vehicles look like NASA's now-retired space shuttle orbiters, but are much smaller; each X-37B is 29 feet (8.8 meters) long and 9.6 feet (2.9 m) tall, with a payload bay the size of a pickup truck bed. For comparison, the space shuttles were 122 feet (37 m) long, with 78-foot (24 m) wingspans. Like the space shuttle, the X-37B launches vertically and comes to back to Earth horizontally, in a runway landing. Together, the two X-37Bs have completed four space missions, each of which has set a new duration standard for the program. Exactly what the X-37B did during those four missions, or what it will do during the newly launched OTV-5, is a mystery; most X-37B payloads and activities are classified.