Space Shuttle Discovery (Orbiter Vehicle Designation: OV-103) is one of the three currently operational orbiters in the Space Shuttle fleet of NASA, the space agency of the Unite States. (The other two are Atlantis and Endeavour.) When first flown in 1984, Discovery became the third operational orbiter, and is now the oldest orbiter in service. Discovery has performed both research and International Space Station (ISS) assembly missions.