Space station

A space station is an artificial structure designed for humans to live in outer space. So far only low earth orbit (LEO) stations are implemented, also known as orbital stations. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities — instead, other vehicles are used as transport to and from the station. Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years.

Space stations are used to study the effects of long-term space flight on the human body as well as to provide platforms for greater number and length of scientific studies than available on other space vehicles. Since the ill-fated flight of Soyuz 11 to Salyut 1, all manned spaceflight duration records have been set aboard space stations. The duration record of 437.7 days was set by Valeriy Polyakov aboard Mir from 1994 to 1995. As of 2006, three astronauts have completed single missions of over a year, all aboard Mir.

Past and present space stations

(dates refer to periods when stations were inhabited by crews)

• Salyut stations (USSR, 1971-1986)
  • Salyut 1 (1971, 1 crew and 1 failed docking)
  • Salyut 2/Almaz (1973, failed shortly after launch)
  • Salyut 3/Almaz (1974, 1 crew and 1 failed docking)
  • Salyut 4 (1975-1976, 2 crews)
  • Salyut 5/Almaz (1976-1977, 2 crews and 1 failed docking)
  • Salyut 6 (1977-1981, 16 crews (5 long duration, 11 short duration) and 1 failed docking)
  • Salyut 7 (1982-1986, 10 crews (6 long duration, 4 short duration) and 1 failed docking)
• Skylab (USA, 1973-1974, 3 crews)
• Mir (USSR/Russia, 1986-1999, 28 long duration crews)
• International Space Station (ISS) (USA, Russia, Japan, Europe, Canada, 2000-ongoing, 14 long duration crews to date)

Following the controlled deorbiting of Mir in 2001, the International Space Station is the only one of these currently in orbit; it has been continuously manned since October 30, 2000. As of 9 Sept 2006, it was 44.5 metres (146 ft) in length along the core between the Destiny and Zvezda modules or 52 metres (171 ft) long when a Progress is attached, and has a mass of 183.3 metric tons (202.1 short tons).

Cancelled space stations

A second Skylab unit (Skylab B) was manufactured, as a backup article; due to the high costs of providing launch vehicles, and a desire by NASA to cease Saturn and Apollo operations in time to prepare for the Space Shuttle coming into service, it was never flown. The hull can now be seen in the National Air and Space Museum, in Washington DC, where it is a popular tourist attraction. A number of additional Salyuts were also produced, as backups or as flight articles which were later cancelled.

The International Space Station evolved from the U.S. Space Station Freedom program, which - despite being under development for ten years - was never launched; it incorporated elements of a Mir replacement station ("Mir 2") which was also never constructed. Other cancelled space station programs included the United States Air Force Manned Orbiting Laboratory project, cancelled in 1969 about a year before the first planned test flight; this was unusual in being an explicitly military project, as opposed to the Soviet Almaz program, which was heavily intertwined with - and concealed by - the contemporaneous Salyut program.

Future developments

Currently, Bigelow Aerospace is commercially developing inflatable habitat modules, derived from the earlier Transhab concept, intended to be used for space station construction and for a space prize they are funding and operating, America's Space Prize. Genesis-1 is a ⅓-scale unmanned prototype sent up to test the feasibility of inflatable structures and other systems in space.

Types of space station

"Monolithic"

Broadly speaking, the space stations so far launched have been of two types; the earlier stations, Salyut and Skylab, have been "monolithic", intended to be constructed and launched in one piece, and then manned by a crew later. As such, they generally contained all their supplies and experimental equipment when launched, and were considered "expended", and then abandoned, when these were used up.

Starting with Salyut 6 and Salyut 7, a change was seen; these were built with two docking ports, which allowed a second crew to visit, bringing a new spacecraft (for technical reasons, a Soyuz capsule cannot spend more than a few months on orbit, even powered down, safely) with them. This allowed for a crew to man the station continually. The presence of a second port also allowed Progress supply vehicles to be docked to the station, meaning that fresh supplies could be brought to aid long-duration missions. This concept was expanded on Salyut 7, which "hard docked" with a TKS tug shortly before it was abandoned; this served as a proof-of-concept for the use of modular space stations. The later Salyuts may reasonably be seen as a transition between the two groups.

Modular

The second group, Mir and the ISS, have been modular; a core unit was launched, and additional modules, generally with a specific role, were later added to that. (On Mir they were usually launched independently, whereas on the ISS most are brought by the Shuttle). This method allows for greater flexibility in operation, as well as removing the need for a single immensely powerful launch vehicle. These stations are also designed from the outset to have their supplies provided by logistical support, which allows for a longer lifetime at the cost of requiring regular support launches.

Habitability issues

These stations have various issues that limit their long-term habitability, such as very low recycling rates, high radiation levels and a lack of gravity. Some of these problems cause discomfort and long-term health effects. In the case of solar flares, most current habitats even have an acute danger of radiation poisoning. Some space habitats address these issues, and are intended for long-term occupation. Some designs might even accommodate large numbers of people, essentially "cities in space" where people would make their homes. No such design has yet been constructed, because even for a small station, the extra equipment is too expensive to place in orbit at current (2006) launch costs.

List of occupied space stations, with statistics

Crew and visitors counting is non-distinct.

• **ISS stats as of October 27, 2006.
• ***ISS stats as of October 27, 2006._{19 shuttles, 13 Soyuz, 1 Soyuz - Assembly, 22 Progress, 2 Proton and Pirs}

See also

• Lunar space elevator
• Space colonization
• Space stations in popular culture
• Space habitat

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