The International Space Station (ISS) is a large space station that was assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United States), Roscosmos (Russia), ESA (Europe), JAXA (Japan), and CSA (Canada). The ISS is the largest space station ever built. Its primary purpose is to perform microgravity and space environment experiments. Operationally, the station is divided into two sections: the Russian Orbital Segment (ROS) assembled by Roscosmos, and the US Orbital Segment (USOS), assembled by NASA, JAXA, ESA and CSA. A striking feature of the ISS is the Integrated Truss Structure, which connects the large solar panels and radiators to the pressurized modules. The pressurized modules are specialized for research, habitation, storage, spacecraft control, and airlock functions. Visiting spacecraft dock at the station via its eight docking and berthing ports. The ISS maintains an orbit with an average altitude of 400 kilometres (250 mi) and circles the Earth in roughly 93 minutes, completing 15.5 orbits per day. The ISS programme combines two prior plans to construct crewed Earth-orbiting stations: Space Station Freedom planned by the United States, and the Mir-2 station, planned by the Soviet Union. The first ISS module was launched in 1998. Major modules have been launched by Proton and Soyuz rockets and by the Space Shuttle launch system. The first long-term residents, Expedition 1, arrived on 2 November 2000. Since then, the station has been continuously occupied for 24 years and 24 days, the longest continuous human presence in space. As of March 2024, 279 individuals from 22 countries have visited the space station. The ISS is expected to have additional modules (the Axiom Orbital Segment, for example) and will be in service until the end of 2030, after which it is planned to be de-orbited by a dedicated NASA spacecraft.