A US spaceplane returned safe and sound yesterday after a two-year mission somewhere up there, reports Space.com. And what the unmanned X-37B Orbital Test Vehicle was doing for all that time is, as they say, classified. The shuttle-like craft came in without a hitch at Vandenberg Air Force Base in California, completing its third—and longest—secret mission. Another is planned for next year. Rumors abound about what the spacecraft is up to, with everything from launching satellites, to spying on China's space lab, to killing satellites being floated, reports CNET and the BBC.
But the latter site thinks the most credible explanation is more mundane: The spaceplane might be testing technology that will be used in future space missions. "If you consider how expensive a satellite mission is—several hundred million dollars—you'd like to be sure that any innovations are going to work straight out of the box." Testing prototypes aboard the X-37B, then, could save expensive mistakes down the line. (Click to read about a 13-year-old who hopes to be the first person on Mars.)