The International Space Station is a large artificial satellite designed to be habitable by humans, which orbits the Earth. Since the first mission in 1998, the ISS has been built slowly over the years, with additional sections being added on during subsequent missions. By November 2000 the station was ready for permanent inhabitants, and has since (as of 2014) been continuously occupied by changing crews from the various countries involved in maintaining it.
Solar powered, the International Space Station is used both as a laboratory and as a space port for space vehicles from many countries. It is 239 feet long, 356 feet wide and 66 feet tall, which gives it more habitable space than an average six-bedroom house. Its unique zero-gravity environment allows for long-term research experiments that would be impossible to perform on short duration space flights.
Many different scientific questions are investigated on the ISS. These include an analysis of how negligible gravity affects the growth of organisms and organic crystals, as well as an investigation into the behaviors of fluids in such an environment. For example, some fluids, which would quickly separate in Earth's gravity, mix completely when placed in minimal gravity conditions.
Aside from research, the ISS is also an important base for space exploration. Spacecraft systems use the station as a test area for evaluating if they are able to handle other more arduous future journeys, such as missions to Mars.