The X-37B Orbital Test Vehicle’s third mission (OTV-3) came to an end on October 17 when the unmanned craft landed safely and autonomously at Vandenberg Air Force Base in southern California after 675 days in orbit. Altogether, the three missions have totaled 1367 days, and a fourth mission is expected to launch from Cape Canaveral in Florida sometime in 2015. The project, headed by Air Force Space Command (AFSPC) and Defense Advanced Research Projects Agency (DARPA), is classified. As such, no one really knows what OTV-3 has been doing up there for nearly 2 years.

The Air Force has been fairly vague about the mission. The spaceplane was said to be testing “advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, and autonomous orbital flight, reentry and landing.”

The Boeing X-37B is a modified version of the NASA’s X-37 design. The unmanned craft intended for low Earth orbit looks like a smaller version of the space shuttle at only 29-feet-long with a 15-foot wingspan. Like the space shuttle, it is launched vertically using Atlas V rockets, though it is able to land horizontally on a runway, like a plane. Solar panels charge the spaceplane’s lithium ion batteries once it has left Earth’s atmosphere. Though it was designed to remain in orbit for 270 days, it has far exceeded those expectations.

The Air Force claims that the objectives of the X-37B are to develop “reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth.” However, it doesn’t seem likely that it will be used to retrieve satellites. Its cargo area is rather small at about 7 feet by 4 feet, so it is not clear what it could be carrying or what its payload might have been during OTV-3.

The latitudes at which it was observed while in orbit rules out any chance it could have been doing reconnaissance on Russia. If it were doing data collection, it was likely targeting the Middle East, Southeast Asia, or Northern Africa—none of which would be particularly surprising.

The secrecy of the mission and the fact that DARPA and the Air Force are at the helm have caught the attention of conspiracy theorists who love to speculate. Some have asserted that the X-37B was used for deploying bombs, interfering with satellites of other countries, spying on China’s upcoming space station, among other theories. However, many of these are not practical for the X-37B’s design.

It’s hard to tell exactly when the X-37B’s purpose will be revealed. Unless AFSPC decides to release that information, the documents could remain classified for a long time. According to Executive Order 13526, all documents are to be unclassified after a period of 25 years. However, under extenuating circumstances, the documents describing the mission could remain classified indefinitely.