The U.S. Air Force's X-37B space plane blasted into Earth orbit today, kicking off the robotic vehicle's clandestine fourth mission — as well as the first flight of a tiny solar-sailing spacecraft. br br "We are excited about our fourth X-37B mission," Randy Walden, director of the Air Force Rapid Capabilities Office, said in a statement late last month. "With the demonstrated success of the first three missions, we're able to shift our focus from initial checkouts of the vehicle to testing of experimental payloads." br br A United Launch Alliance Atlas V rocket launched the AFSPC-5 mission for the U.S. Air Force on May 20, 2015, from Space Launch Complex (SLC)-41 at Cape Canaveral Air Force Station, Florida. The mission is the 4th flight of the mysterious X-37B space plane, also known as the Orbital Test Vehicle. See photos from the mission, called OTV-4, here. At Left: The X-37B space plane is seen after landing at Vandenberg Air Force Base in California to end its third mission in October 2014.