The Pentagon's secretive X-37B Orbital Test Vehicle broke a new flight duration record on 25 August, having spent 720 days in space over the course of its fifth mission called OTV-5.
The current mission kicked off on 7 September 2017. The X-37B was sent into orbit for the first time via a SpaceX Falcon 9 rocket instead of an Atlas V, which uses Russian RD-180 engines. It's unclear when the OTV-5’s mission will end, but the OTV-6 is expected to be launched in December 2019. It's possible that the OTV-5 will still be in orbit then since the Pentagon has two flight-capable X-37Bs.
Information about the OTV-5 mission is scarce since all OTV projects are shrouded in secrecy, but the US Air Force announced that among other things the X-37B launched in 2017 would be conducting oscillating heat pipe technology performance tests. Oscillating heat pipe technology is used to divert excessive heat from certain part of mechanisms or electronics.
X-37B is essentially an unmanned autonomous reusable shuttle, launched vertically and can land on an airfield. Its maiden flight in 2010 was the first flight of unmanned shuttle since the 1988 flight of the Soviet Buran shuttle, a project that was soon abandoned due to dissolution of the USSR.