A space station is a Spacecraft built to fly in a permanent, fixed 'Low Earth Orbit'. For example the International Space Station orbits at an altitude of about 350km. At this altitude there is still a small amount of atmosphere, sufficient to cause drag and slow the station down. As it slows the station loses altitude and slows even more. Eventually it would fall out of orbit and be destroyed.
Most stations are therefore equipped with rocket motors that can be fired periodically to boost the speed of the station and keep it in the correct orbit. Alternatively, visiting Spacecraft can dock to the rear of the station and use their motors to boost the station's speed.
A station is also designed to accommodate a crew who operate and maintain it. The crew must be kept supplied with food, water, oxygen and maintenance items. For long duration missions station crews are replaced every four to six months because of the long term physical and mental effects of space flight on them.
Originally space stations were envisaged as transfer facilities. Earth to orbit shuttles would deliver people to the station and they would then be transferred to lunar or inter-planetary craft for travel throughout the solar system.
For example, in Arthur C. Clarke's "2001 A Space Odyssey", space planes transported people from Earth to a large wheel shaped space station. People were then transferred to shuttles which flew from the station to the moon.
Unfortunately space technology has not progressed to this stage yet. The space stations built so far have been designed with the primary purpose of research.
All materials and personnel used to build, operate and maintain space stations are transported from the Earth by dedicated spacecraft missions. Each mission is carefully planned to optimise pay load capacity and ensure materials are delivered as required.
Most station spacecraft use rocket launch vehicles (launchers) to reach orbit and then use their own engines to rendezvous with the stations. The U.S. Space Shuttle is the exception, using its own engines, with the help of booster rockets, to reach orbit.
This article gives an outline of the many different spacecraft and launchers which have been used to build, operate and maintain the stations. It also includes a summary of the spacecraft missions to the stations. Links are given to "rdataspace" articles with technical details of the spacecraft and launch vehicles.
Early space stations consisted of a single module which was launched, un-occupied, and used on-board engines to attain orbit. A combination automation and remote control was used to place it in the correct orbit.
Crews were then transported to the station on specialized spacecraft based on general orbital piloted craft. These craft remained docked to the station for return to earth. For long duration missions they were replaced periodically with fresh
craft.
The first space station, the Soviet Salyut-1, was an example of a single module station.
Modular stations contain a number of habitable modules and support structures which are assembled in orbit. This method is used when the design of the station is too large to be built on Earth and launched into orbit.
Building in orbit also allows the components to be lighter because they are not subject to the full force of gravity found on Earth.
The Russian Mir space station and International space station are examples of the largest modular stations built to date. Mir's first module was launched in 1986 and the whole station was destroyed, by de-orbiting, in 2001.
The first module of the International Space Station, ISS, was launched in 1998 and is still operational today. It will remain in orbit as long as it is practical to be maintained and updated. Like satellites, these stations were not designed to return to Earth or to leave there designated range of orbits.
