A space station, also known as an orbital station or an orbital space station, is a spacecraft capable of supporting crewmembers, which is designed to remain in space (most commonly as an artificial satellite in low Earth orbit) for an extended period of time and for other spacecraft to dock. A space station is distinguished from other spacecraft used for human spaceflight by lack of major propulsion or landing systems. Instead, other vehicles transport people and cargo to and from the station. As of April 2018[update], two space stations are in Earth orbit: the International Space Station (operational and permanently inhabited), and China's Tiangong-2 (operational but not permanently inhabited). Previous stations include the Almaz and Salyut series, Skylab, Mir, and most recently Tiangong-1.
Today's space stations are research platforms, 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. Each crew member stays aboard the station for weeks or months, but rarely more than a year. Since the ill-fated flight of Soyuz 11 to Salyut 1, all human spaceflight duration records have been set aboard space stations. The duration record for a single spaceflight is 437.7 days, set by Valeriy Polyakov aboard Mir from 1994 to 1995. As of 2016[update], four cosmonauts have completed single missions of over a year, all aboard Mir. Space stations have also been used for both military and civilian purposes. The last military-use space station was Salyut 5, which was used by the Almaz program of the Soviet Union in 1976 and 1977.
Space stations have been envisaged since at least as early as 1869 when Edward Everett Hale wrote "The Brick Moon". The first to give serious consideration to space stations were Konstantin Tsiolkovsky in the early 20th century and Hermann Oberth about two decades later. In 1929 Herman Potočnik's The Problem of Space Travel was published, the first to envision a "rotating wheel" space station to create artificial gravity.
During the Second World War, German scientists researched the theoretical concept of an orbital weapon based on a space station. Pursuing Oberth's idea of a space-based weapon, the so-called "sun gun" was a concept of a space station orbiting Earth at a height of 8,200 kilometres (5,100 mi), with a weapon that was to utilize the sun's energy.
In 1951, in Collier's Weekly, Wernher von Braun published his design for a rotating wheel space station, which referenced Potočnik's idea – however these concepts would never leave the concept stage during the 20th century.
During the same time as von Braun pursued Potočnik's ideas, the Soviet design bureaus – chiefly Vladimir Chelomey's OKB-52 – were pursuing Tsiolkovsky's ideas for space stations. The work by OKB-52 would lead to the Almaz programme and (together with OKB-1) to the first space station: Salyut 1. The developed hardware laid the ground for the Salyut and Mir space stations, and is even today a considerable part of the ISS space station.
The first space station was Salyut 1, which was launched by the Soviet Union on April 19, 1971. Like all the early space stations, it was "monolithic", intended to be constructed and launched in one piece, and then inhabited by a crew later. As such, monolithic stations generally contained all their supplies and experimental equipment when launched, and were considered "expended", and then abandoned, when these were used up.
The earlier Soviet stations were all designated "Salyut", but among these there were two distinct types: civilian and military. The military stations, Salyut 2, Salyut 3, and Salyut 5, were also known as Almaz stations.
The civilian stations Salyut 6 and Salyut 7 were built with two docking ports, which allowed a second crew to visit, bringing a new spacecraft with them; the Soyuz ferry could spend 90 days in space, after which point it needed to be replaced by a fresh Soyuz spacecraft. This allowed for a crew to man the station continually. Skylab was also equipped with two docking ports, like second-generation stations, but the extra port was never utilized. The presence of a second port on the new stations 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.
Unlike previous stations, the Soviet space station Mir had a modular design; a core unit was launched, and additional modules, generally with a specific role, were later added to that. This method allows for greater flexibility in operation, as well as removing the need for a single immensely powerful launch vehicle. Modular 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.
USOS modules were brought to the station by the Space Shuttle and manually attached to the ISS by crews during EVAs. Connections are made manually for electrical power, data, propulsion and cooling fluids. This results in a single piece which is not designed for disassembly.
The Russian Orbital Segment's modules are able to launch, fly and dock themselves without human intervention using Proton rockets. Connections are automatically made for power, data and propulsion fluids and gases. The Russian approach would hypothetically allow the assembly of space stations orbiting other worlds in preparation for human missions.
Russian modular or "second-generation" space stations differ from "monolithic" single-piece stations by allowing reconfiguration of the station to suit changing needs. According to a 2009 report, RKK Energia considered removing some modules of the ROS when the end of mission is reached for the ISS to reuse them as parts of a new station, known as the Orbital Piloted Assembly and Experiment Complex. However, in September 2017 the head of Roscosmos said that the technical feasibility of separating the station to form OPSEK had been studied, and there were now no plans to separate the Russian segment from the ISS.
China's first space laboratory, Tiangong-1 was launched in September 2011. The uncrewed Shenzhou 8 then successfully performed an automatic rendezvous and docking in November 2011. The crewed Shenzhou 9 then docked with Tiangong-1 in June 2012, the crewed Shenzhou 10 in 2013. Tiangong-2 was launched in September 2016; a planned Tiangong-3 was merged with Tiangong-2 and therefore not ordered.
In May 2017, China informed the United Nations Office for Outer Space Affairs that Tiangong-1's altitude was decaying and that it would soon reenter the atmosphere and break up. The reentry was projected to occur in late March or early April 2018. According to the China Manned Space Engineering Office, Tiangong-1 reentered over the South Pacific Ocean, northwest of Tahiti, on 2 April 2018 at 00:15 UTC.
The space station environment presents a variety of challenges to human habitability, including short-term problems such as the limited supplies of air, water and food and the need to manage waste heat, and long-term ones such as weightlessness and relatively high levels of ionizing radiation. These conditions can create long-term health problems for space-station inhabitants, including muscle atrophy, bone deterioration, balance disorders, eyesight disorders, and elevated risk of cancer.
Future space habitats may attempt to address these issues, and could be designed for occupation beyond the weeks or months that current missions typically last. Possible solutions include the creation of artificial gravity by a rotating structure, the inclusion of radiation shielding, and the development of on-site agricultural ecosystems. Some designs might even accommodate large numbers of people, becoming essentially "cities in space" where people would reside semi-permanently. For now, no space station suitable for long-term human residence has ever been built, since the current launch costs for even a small station are not economically or politically viable.
A space station is a complex system with many interrelated subsystems, including structure, electrical power, thermal control, attitude determination and control, orbital navigation and propulsion, automation and robotics, computing and communications, environmental and life support, crew facilities, and crew and cargo transportation.
Despite an expanding array of molecular approaches for detecting microorganisms, rapid and robust means of assessing the differential viability of the microbial cells, as a function of phylogenetic lineage, remain elusive. Molds that develop aboard space stations can produce acids that degrade metal, glass and rubber.
The Soviet space stations came in two types, the civilian Durable Orbital Station (DOS), and the military Almaz stations.
Dates refer to periods when stations were inhabited by crews.
UPDATE: #JFSCC confirmed #Tiangong1 reentered the atmosphere over the southern Pacific Ocean at ~5:16 p.m. (PST) April 1. For details see http://www.space-track.org @US_Stratcom @usairforce @AFSpaceCC @30thSpaceWing @PeteAFB @SpaceTrackOrg