The United States Air Force’s experimental X-37B unmanned spaceplane returned to Earth on May 7, 2017, after a record-breaking 718-day stay in orbit: However, little is publicly known about the craft’s secret mission.

Initially created by NASA in 1999, the Boeing-built X-37 was transferred to DARPA in 2004, of whom placed the project under its current classified status. The USAF announced that they would develop their own X-37 in 2006, now designated X-37B; originally intended to be launched by NASA’s Space Shuttle Orbiter, it is now launched on the tip of an Atlas V rocket.

According to the USAF’s website, they are using the miniature spaceplane to develop, "advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, advanced propulsion systems and autonomous orbital flight, reentry and landing" technologies.

This most recent mission, designated OTV-4, was initially launched on May 20, 2015, and continued for 717 days and 20 hours, ending with a successful landing at the Kennedy Space Center’s Shuttle Landing Facility. Little is known about what happened during the heavily-classified mission, except that tests were performed on an ion engine built by Aerojet Rocketdyne, and testing of long-term effects on various materials was conducted.

The Air Force stresses that their spaceplane isn’t being used for any underhanded shenanigans. However, the non-profit group Secure World Foundation, who promotes the peaceful exploration of space, says that the heavily-classified nature of the X-37B’s mission suggests that intelligence-related hardware was installed on board. In the end, the USAF has released few details regarding the experimental spacecraft’s mission. While the X-37B would be nearly useless as a space-based weapons platform for attacking targets on the ground, it could easily carry surveillance or reconnaissance equipment, or be used as a platform for the development of potential anti-satellite technologies.