A secret space mission came to a close on Friday when an unpiloted Air Force space plane landed back on Earth after a record-setting 674 days in orbit. The Boeing-built X-37B Orbital Test Vehicle landed at Vandenberg Air Force Base in California, putting an end to the program’s third flight. There are no details on what the plane may have been doing during its time in space. "The landing of OTV-3 marks a hallmark event for the program," an program manager said in an Air Force release. "The mission is our longest to date and we're pleased with the incremental progress we've seen in our testing of the reusable space plane. The dedication and hard work by the entire team has made us extremely proud."