Space Shuttle Discovery (Orbiter Vehicle Designation: OV-103) is one of the three currently operational orbiters (Space Shuttle orbiter) in the Space Shuttle fleet of NASA, the space agency of the United States. (The other two are Atlantis (Space Shuttle Atlantis) and Endeavour (Space Shuttle 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 (International Space Station)) assembly missions.