After spending more than a year orbiting Earth on a mystery mission, the U.S. Air Force’s X-37B space plane is due to return to Earth any day now, local media reported Wednesday.   The X-37B spacecraft’s landing window opened on June 11 and runs through June 18, with Friday being the next opportunity. U.S. Air Force officials say landing day for the unmanned X-37B space plane is imminent, and could occur on Friday.   But ultimately landing date and time depend on weather at its intended landing site at Vandenberg Air Force Base in California, technical considerations and other factors, said Air Force Lt. Austin Fallin, a Vandenberg spokesman.   The reusable space plane is on its second-ever space mission known as the Orbital Test Vehicle 2 or OTV-2. The OTV-2 mission and its predecessor were both launched from Cape Canaveral Air Force Station in Florida. U.S. Air Force officials have said they intend to launch the OTV-1 vehicle on another test flight soon, possibly as soon as this fall.   The X-37B space planes are built in tight secrecy by Boeing and look much like NASA’s retired space shuttles, only much smaller. Each X-37B spacecraft is about 8.8 meters long and 4.5 meters wide.   Industry analysts said the spacecraft could be a precursor to an orbiting weapon, capable of dropping bombs or disabling foreign satellites as it circles the globe.   The first X-37B space plane, OTV-1, launched in 2010 and spent 225 days in space. The estimated mission length for the space planes is about 270 days.