The U.S. Air Force’s unmanned space plane X-37 is one of the most mysterious spacecraft ever. It is known as the Orbital Test Vehicle and its primary objective is to test advanced spacecraft technologies. The program is classified, therefore it remains a secret what the tested technologies are. The X-37B was launched on 20 May 2015, this is the fourth X-37 mission and it is still in progress. The X-37B space plane has been in orbit for almost two years now and broke an orbital record when it hit 675 days in orbit around Earth on March 25th 2017.
Concept of U.S. Air Force’s X-37B in orbit [NASA Marshall Space Flight Center]
The X-37 program began as a NASA project in 1999 after which it was transferred to the U.S. Department of Defence in 2004. The X-37B spacecraft looks similar to NASA’s Space Shuttle but it is smaller, about 8.9 meters long, 2.9 meters tall with a wing span of 4.55 meters. It operates in Low Earth orbit and has an orbital time of 270 days. The design for the X-37 is based on the Space Shuttle, therefore they both have similar characteristics such as the lift-to-drag ratio. The spacecraft is taken into orbit on a rocket and lands by gliding down to Earth.
The secrecy surrounding the X-37 project initiates a lot of speculations regarding its purpose. Theories have arised about the X-37 being about the development of space-based weapons or spying operations, however none of them are confirmed or proven wrong. Some details regarding the payload have been revealed which mostly state that the payload concerns experimental materials. The X-37 is said to test for example te Hall-effect thruster. It is not yet clear when the X-37B spacecraft will end its current mission.