The ISS serves as a microgravity and space environment research laboratory in which crew members conduct experiments in biology, human biology, physics, astronomy, meteorology, and other fields. The station is suited for the testing of spacecraft systems and equipment required for missions to the Moon and Mars. The ISS maintains an orbit with an altitude of between 330 and 435 km (205 and 270 mi) by means of reboost manoeuvres using the engines of the Zvezda module or visiting spacecraft. It completes 15.54 orbits per day.
The ISS is the ninth space station to be inhabited by crews, following the Soviet and later Russian Salyut, Almaz, and Mirstations as well as Skylab from the US. The station has been continuously occupied for 16 years and 330 days since the arrival of Expedition 1 on 2 November 2000. This is the longest continuous human presence in low Earth orbit, having surpassed the previous record of 9 years and 357 days held by Mir. The station is serviced by a variety of visiting spacecraft: the Russian Soyuz and Progress, the American Dragon and Cygnus, the Japanese H-II Transfer Vehicle, and formerly the Space Shuttleand the European Automated Transfer Vehicle. It has been visited by astronauts, cosmonauts and space tourists from 17 different nations.
After the U.S. Space Shuttle programme ended in 2011, Soyuz rockets became the only provider of transport for astronauts at the International Space Station, and Dragon became the only provider of bulk cargo return to Earth (called downmass). Soyuz has very limited downmass capability.
The ISS programme is a joint project among five participating space agencies: NASA, Roscosmos, JAXA, ESA, and CSA. The ownership and use of the space station is established by intergovernmental treaties and agreements. The station is divided into two sections, the Russian Orbital Segment (ROS) and the United States Orbital Segment (USOS), which is shared by many nations. As of January 2014, the American portion of ISS is being funded until 2024. Roscosmos has endorsed the continued operation of ISS through 2024 but has proposed using elements of the Russian Orbital Segment to construct a new Russian space station called OPSEK.
On 28 March 2015, Russian sources announced that Roscosmos and NASA had agreed to collaborate on the development of a replacement for the current ISS. NASA later issued a guarded statement expressing thanks for Russia's interest in future co-operation in space exploration but fell short of confirming the Russian announcement.