The International Space Station (ISS) is an internationally-developed research facility, which is being assembled in low Earth orbit and is the largest space station ever constructed. On-orbit construction of the station began in 1998 and is scheduled for completion by 2012. The station is expected to remain in operation until at least 2020, and potentially to 2028. Like many artificial satellites, the ISS can be seen from Earth with the naked eye. 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, and has been continuously staffed since 2 November 2000—an uninterrupted human presence in space for the past &000000000000001000000010 years and &0000000000000238000000238 days. As of June 2011[update], the crew of Expedition 28 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 program, 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 November 2009[update], the station consisted of 11 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 (17,227 mi) per hour, 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 Russian Federal Space Agency (RKA), the Japan Aerospace Exploration Agency (JAXA), the Canadian Space Agency (CSA), and the European Space Agency (ESA). 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 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. The cost of the station has been estimated by ESA as €100 billion over 30 years, although other estimates range from 35 billion dollars to 160 billion dollars. The financing, research capabilities and technical design of the ISS program have been criticised because of the high cost.