The International Space Station (ISS) is an internationally developed research facility that is being assembled in low Earth orbit. On-orbit construction of the station began in 1998 and is scheduled for completion by late 2011. The station is expected to remain in operation until at least 2015, and likely 2020. With a greater mass than that of any previous space station, the ISS can be seen from Earth with the naked eye, and, as of 2010, is by far the largest artificial satellite orbiting Earth. The ISS serves as a research laboratory that has a microgravity environment in which crews conduct experiments in biology, human biology, physics, astronomy and meteorology. The station has a unique environment for the testing of the spacecraft systems that will be required for missions to the Moon and Mars. The ISS is operated by Expedition crews, with the station programme maintaining an uninterrupted human presence in space since the launch of Expedition 1 on 31 October 2000, a total of 9 years and 198 days. The programme is thus approaching the current record, set aboard Mir, of 3,644 days (8 days short of 10 years), with the ISS expected to take the record on 23 October 2010. As of 17 March 2010), the crew of Expedition 23 is aboard.

The ISS is a synthesis of several space station projects that includes the American Freedom, the Soviet/Russian Mir-2, the European Columbus and the Japanese Kibō. Budget constraints led to the merger of these projects into a single multi-national programme. The ISS project began in 1994 with the Shuttle-Mir programme, and the first module of the station, Zarya, was launched in 1998 by Russia. Assembly continues, as pressurised modules, external trusses and other components are launched by American space shuttles, Russian Proton rockets and Russian Soyuz rockets. As of February 2010, the station consisted of 13 pressurised modules and an extensive integrated truss structure (ITS). Power is provided by 16 solar arrays mounted on the external truss, in addition to four smaller arrays on the Russian modules. The station is maintained at an orbit between 278 km (173 mi) and 460 km (286 mi) altitude, and travels at an average speed of 27,724 km/h (17,227 mph), completing 15.7 orbits per day.

Operated as a joint project between the five participant space agencies, the station's sections are controlled by mission control centres on the ground operated by the American National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), the Russian Federal Space Agency (RKA), the Japan Aerospace Exploration Agency (JAXA) and the Canadian Space Agency (CSA). The ownership and use of the space station is established in intergovernmental treaties and agreements that allow the Russian Federation to retain full ownership of its own modules, with the remainder of the station allocated between the other international partners. The cost of the station has been estimated by ESA as €100 billion over 30 years, and, although estimates range from 35 billion dollars to 160 billion dollars, the ISS is believed to be the most expensive object ever constructed. The financing, research capabilities and technical design of the ISS programme have been criticised because of the high cost. The station is serviced by Soyuz spacecraft, Progress spacecraft, space shuttles, the Automated Transfer Vehicle and the H-II Transfer Vehicle, and has been visited by astronauts and cosmonauts from 15 different nations.