The military space plane X-37B has been in orbit since Sept. 7, 2017—and this week, the uncrewed, solar-powered plane broke its own record for spaceflight duration. At 6:43am Eastern time Monday, X-37B's current mission had officially lasted longer than the previous one, Orbital Test Vehicle 4 (OTV-4), which was the previous record holder at 717 days, 20 hours, and 42 minutes long. As Space.com explains, the US Air Force's robotic space plane's current mission, OTV-5, is pretty mysterious; all of its missions are classified, so it's not clear what exactly it's doing or has done on previous missions. But each of the plane's five missions has been longer than the last, leading Space.com to theorize the Air Force is testing X-37B's endurance. Other guesses have involved the testing of an EM Drive (a hypothetical fuel-less propulsion system) in space, orbital surveillance operations, or weapons research.
But the Air Force has previously denied using its space plane for any "offensive capabilities." "The primary objectives of the X-37B are twofold; reusable spacecraft technologies for America’s future in space and operating experiments which can be returned to, and examined, on Earth," reads a fact sheet from Air Force officials. "Technologies being tested in the program include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, advanced propulsion systems, advanced materials and autonomous orbital flight, reentry and landing." ScienceAlert notes that the Air Force has at least two X-37B planes, which were made by Boeing. And TechCrunch notes that the record, while impressive, is "not an overall record, as geocommunications satellites typically have life spans of five years or more." (Read more X-37B stories.)