A secret space plane operated by the United States Space Force (USSF) has landed back on Earth after spending a record 908 days in orbit. But what it was doing above our heads remains shrouded in mystery. The uncrewed X-37B space plane touched down at NASA's Kennedy Space Station on Nov. 12 at 5:22 a.m. ET, concluding the sixth mission that it and another identical vehicle have completed since the first flight in 2010. Details on its activities during the record-smashing trip are sparse, but officials claim it was conducting a number of scientific experiments at around 249 miles (400...