The International Space Station (ISS) is a habitable artificial satellite in low Earth orbit. The ISS program is a joint project between five participating space agencies: NASA, Roscosmos, JAXA, ESA, and CSA. The ownership and use of the space station are established by intergovernmental treaties and agreements.
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. Crews live on board the ISS for extended periods of time to allow for detailed monitoring of their long-term health effects in an environment similar to that experienced during deep-space exploration missions. Training environments on Earth that simulate some aspects of living on the ISS are also used to prepare crews for future missions.
As of November 2020[update], 239 people from 19 countries have visited the ISS, spending 3,941 cumulative days there.