International Space Station

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International Space Station
A planform view of the ISS backdropped by the limb of the Earth. In view are the station's four large, gold-coloured solar array wings, two on either side of the station, mounted to a central truss structure. Further along the truss are six large, white radiators, three next to each pair of arrays. In between the solar arrays and radiators is a cluster of pressurised modules arranged in an elongated T shape, also attached to the truss. A set of blue solar arrays are mounted to the module at the aft end of the cluster.
The flags of the United States, United Kingdom, France, Denmark, Spain, Italy, The Netherlands, Sweden, Canada, Germany, Switzerland, Belgium, Brazil, Japan, Norway, and Russia.
The International Space Station on 19 February 2010 as seen from the departing Space Shuttle Endeavour during STS-130.
A silhouette of the ISS shown orbiting the Earth, contained within it, a blue shield with the words 'International Space Station' at the top.
ISS Insignia
Station statistics
NSSDC ID: 1998-067A
Call sign: Alpha
Crew: 6
Launch: 1998–2011
Launch pad: KSC LC-39,
Baikonur LC-1/5 & LC-81/23
Mass: 344,378 kg (759,220 lb)
Length: 73 m (240 ft)
from PMA-2 to Zvezda
Width: 108.5 m (356 ft)
along truss, arrays extended
Height: c. 20 m (c. 66 ft)
nadir–zenith, arrays forward–aft
Living volume: c. 373 m3
(c. 13,200 cu ft)
Atmospheric pressure:
101.3 kPa (29.91 inHg) (1 atm)
Perigee: 336 km altitude (181 nmi)
Apogee: 346 km altitude (189 nmi)
Orbit inclination: 51.6419 degrees
Average speed: 7,706.6 m/s
(27,743.8 km/h, 17,239.2 mph)
Orbital period: c. 91 minutes
Days in orbit: 4157
(8 April 2010)
Days occupied: 3446
(8 April 2010)
Number of orbits: c. 65606
(8 April 2010)
Orbital decay: 2 km/month
Statistics as of 27 November 2009
(unless noted otherwise)
References: [1][2][3][4][5][6]
The components of the ISS in an exploded diagram, with modules on-orbit highlighted in orange, and those still awaiting launch in blue or pink.
Station elements as of 12 February 2010 (2010 -02-12)
(exploded view)

The International Space Station (ISS) is an internationally developed research facility that is being assembled in low Earth orbit. On-orbit construction of the station began in 1998 and is scheduled for completion by 2011. The station is expected to remain in operation until at least 2015, and likely 2020.[7][8] With a greater mass than that of any previous space station, the ISS can be seen from the Earth with the naked eye,[9] and, as of 2010, is the largest artificial satellite orbiting the Earth.[10] The ISS serves as a research laboratory that has a microgravity environment in which crews conduct experiments in biology, human biology, physics, astronomy and meteorology.[11][12][13] The station has a unique environment for the testing of the spacecraft systems that will be required for missions to the Moon and Mars.[14] The ISS is operated by Expedition crews, and has been continuously staffed since 2 November 2000, meaning the ISS programme has maintained an uninterrupted human presence in space for the past &0000000000000009.0000009 years and &0000000000000159.000000159 days, which is approaching the current record, set aboard Mir, of 9 years and 257 days.[15] As of 18 March 2010 (2010 -03-18), the crew of Expedition 23 is aboard.[16]

The ISS is a synthesis of several space station projects that includes the American Freedom, the Soviet/Russian Mir-2, the European Columbus and the Japanese Kibō.[17][18] Budget constraints led to the merger of these projects into a single multi-national programme.[17] The ISS project began in 1994 with the Shuttle-Mir programme,[19] and the first module of the station, Zarya, was launched in 1998 by Russia.[17] Assembly continues, as pressurised modules, external trusses and other components are launched by American space shuttles, Russian Proton rockets and Russian Soyuz rockets.[18] As of February 2010, the station consisted of 13 pressurised modules and an extensive integrated truss structure (ITS). Power is provided by 16 solar arrays mounted on the external truss, in addition to four smaller arrays on the Russian modules.[20] The station is maintained at an orbit between 278 km (173 mi) and 460 km (286 mi) altitude, and travels at an average speed of 27,724 km/h (17,227 mph), completing 15.7 orbits per day.[21]

Operated as a joint project between the five participant space agencies, the station's sections are controlled by mission control centres on the ground operated by the American National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), the Russian Federal Space Agency (RKA), the Japan Aerospace Exploration Agency (JAXA) and the Canadian Space Agency (CSA).[22][23] The ownership and use of the space station is established in intergovernmental treaties and agreements[24] that allow the Russian Federation to retain full ownership of its own modules,[25] with the remainder of the station allocated between the other international partners.[24] The cost of the station has been estimated by ESA as €100 billion over 30 years,[26] and, although estimates range from 35 billion dollars to 160 billion dollars, the ISS is believed to be the most expensive object ever constructed.[27] The financing, research capabilities and technical design of the ISS programme have been criticised because of the high cost.[28][29] The station is serviced by Soyuz spacecraft, Progress spacecraft, space shuttles, the Automated Transfer Vehicle and the H-II Transfer Vehicle,[23] and has been visited by astronauts and cosmonauts from 15 different nations.[10]


[edit] Purpose

The International Space Station (ISS) is an internationally developed satellite currently being assembled in Low Earth Orbit. Primarily a research laboratory, the ISS offers an advantage over spacecraft such as NASA's Space Shuttle because it is a long-term platform in the space environment, where extended studies are conducted.[10][30] The presence of a permanent crew affords the ability to monitor, replenish, repair, and replace experiments and components of the spacecraft itself. Scientists on Earth have swift access to the crew's data and can modify experiments or launch new ones, benefits generally unavailable on specialised unmanned spacecraft.[30]

Crews, who fly expeditions of several months' duration, conduct scientific experiments each day (approximately 160 man-hours a week).[11][31] As of the conclusion of Expedition 15, 138 major science investigations had been conducted on the ISS.[32] Scientific findings, in fields from basic science to exploration research, are published every month.[14]

The ISS provides a location in the relative safety of Low Earth Orbit to test spacecraft systems that will be required for long-duration missions to the Moon and Mars. This provides experience in the maintenance, repair, and replacement of systems on-orbit, which will be essential in operating spacecraft further from Earth. Mission risks are reduced, and the capabilities of interplanetary spacecraft are advanced.[14]

Part of the crew's mission is educational outreach and international cooperation. The crew of the ISS provide opportunities for students on Earth by running student-developed experiments, making educational demonstrations, and allowing for student participation in classroom versions of ISS experiments, NASA investigator experiments, and ISS engineering activities. The ISS programme itself, with the international cooperation that it represents, allows 14 nations to live and work together in space, providing lessons for future multi-national missions.[23][33]

[edit] Scientific research

A man wearing a blue polo shirt reached into a large machine. The machine has a large windows at the front with two holes in it for access, and is full of scientific apparatus. Transient space station hardware is visible in the background.
Expedition 8 Commander and Science Officer Michael Foale conducts an inspection of the Microgravity Science Glovebox.
Side by side images of a candle flame (left) and a glowing translucent blue hemisphere of flame (right).
A comparison between fire on Earth (left) and fire in a microgravity environment (right), such as aboard the ISS

The ISS provides a platform to conduct experiments that require one or more of the unusual conditions present on the station. The primary fields of research include human research, space medicine, life sciences, physical sciences, astronomy and meteorology.[11][12][13] The 2005 NASA Authorization Act designated the American segment of the International Space Station as a national laboratory with the goal of increasing the use of the ISS by other federal agencies and the private sector.[34]

Research on the ISS improves knowledge about the effects of long-term space exposure on the human body. Subjects currently under study include muscle atrophy, bone loss, and fluid shift. The data will be used to determine whether space colonisation and lengthy human spaceflight are feasible. As of 2006, data on bone loss and muscular atrophy suggest that there would be a significant risk of fractures and movement problems if astronauts landed on a planet after a lengthy interplanetary cruise (such as the six-month journey time required to fly to Mars).[35][36] Large scale medical studies are conducted aboard the ISS via the National Space and Biomedical Research Institute (NSBRI). Prominent among these is the Advanced Diagnostic Ultrasound in Microgravity study in which astronauts (including former ISS Commanders Leroy Chiao and Gennady Padalka) perform ultrasound scans under the guidance of remote experts. The study considers the diagnosis and treatment of medical conditions in space. Usually, there is no physician onboard the ISS and diagnosis of medical conditions is a challenge. It is anticipated that remotely guided ultrasound scans will have application on Earth in emergency and rural care situations where access to a trained physician is difficult.[37][38][39]

Researchers are investigating the effect of the station's near-weightless environment on the evolution, development, growth and internal processes of plants and animals. In response to some of this data, NASA wants to investigate microgravity's effects on the growth of three-dimensional, human-like tissues, and the unusual protein crystals that can be formed in space.[12]

The investigation of the physics of fluids in microgravity will allow researchers to model the behaviour of fluids better. Because fluids can be almost completely combined in microgravity, physicists investigate fluids that do not mix well on Earth. In addition, an examination of reactions that are slowed by low gravity and temperatures will give scientists a deeper understanding of superconductivity.[12]

The study of materials science is an important ISS research activity, with the objective of reaping economic benefits through the improvement of techniques used on the ground.[40] Other areas of interest include the effect of the low gravity environment on combustion, through the study of the efficiency of burning and control of emissions and pollutants. These findings may improve our knowledge about energy production, and lead to economic and environmental benefits. Future plans are for the researchers aboard the ISS to examine aerosols, ozone, water vapour, and oxides in Earth's atmosphere, as well as cosmic rays, cosmic dust, antimatter, and dark matter in the universe.[12]

[edit] Origins

A cluster of cylindrical modules with projecting feathery solar arrays and a space shuttle docked to the lower module. In the background is the blackness of space, and, in the lower right corner, Earth.
Space Shuttle Atlantis docked to Mir on STS-71, during the Shuttle-Mir Programme

The International Space Station represents a union of several national space station projects that originated during the Cold War. In the early 1980s, NASA planned to launch a modular space station called Freedom as a counterpart to the Soviet Salyut and Mir space stations, while the Soviets were planning to construct Mir-2 in the 1990s as a replacement for Mir.[17] Because of budget and design constraints, Freedom never progressed past mock-ups and minor component tests.

With the fall of the Soviet Union and the end of the Space Race, Freedom was nearly cancelled by the United States House of Representatives. The post-Soviet economic chaos in Russia led to the cancellation of Mir-2, though only after its base block, DOS-8, had been constructed.[17] Similar budgetary difficulties were faced by other nations with space station projects, which prompted the American government to negotiate with European states, Russia, Japan, and Canada in the early 1990s to begin a collaborative project.[17]

In June 1992 American president George H. W. Bush and Russian president Boris Yeltsin agreed to cooperate on space exploration. The resulting Agreement between the United States of America and the Russian Federation Concerning Cooperation in the Exploration and Use of Outer Space for Peaceful Purposes called for a short, joint space programme, with one American astronaut deployed to the Russian space station Mir and two Russian cosmonauts deployed to a Space Shuttle.[17]

In September 1993, American Vice-President Al Gore, Jr., and Russian Prime Minister Viktor Chernomyrdin announced plans for a new space station, which eventually became the International Space Station.[41] They also agreed, in preparation for this new project, that the United States would be heavily involved in the Mir programme as part of an agreement that later included Space Shuttle orbiters docking with Mir.[19]

According to the plan, the ISS programme would combine the proposed space stations of all participant agencies: NASA's Freedom, the RSA's Mir-2 (with DOS-8 later becoming Zvezda), ESA's Columbus, and the Japanese Kibō laboratory. When the first module, Zarya, was launched in 1998, the station was expected to be completed by 2003. Delays have led to a revised estimated completion date of 2011.[42]

[edit] Station structure

[edit] Assembly

An astronaut uses a screwdriver to activate a docking port on an ISS module.
Astronaut Ron Garan during an STS-124 ISS assembly spacewalk
A video touring the interior of the space station. Beginning at the forward end of Node 2, the tour shows PMA-2, the Japanese Experiment Module, the Columbus and Destiny laboratories, followed by Node 1 and the Quest airlock. The tour then proceeds through PMA-1 and into the Russian segment, visiting the FGB, a docked Soyuz spacecraft, Docking Compartment, Service Module and two Progress spacecraft.
Expedition 18 commander Michael Fincke's video tour of the habitable part of the ISS from January 2009

The assembly of the International Space Station, a major endeavour in space architecture, began in November 1998.[2] Astronauts install each element using spacewalks. By 27 November 2009, they had completed 136, totalling 849 hours of extra-vehicular activity (EVA), all devoted to assembly and maintenance of the station. Twenty-eight of these spacewalks originated from the airlocks of docked Space Shuttles; the remaining 108 were launched from the station.[1]

The first segment of the ISS, Zarya, was launched on 20 November 1998 on a Russian Proton rocket, followed two weeks later by Unity—the first of three node modules—which was launched aboard Space Shuttle flight STS-88. This bare two-module core of the ISS remained unmanned for the next one-and-a-half years. In July 2000 the Russian module Zvezda was added, allowing a maximum crew of three to occupy the ISS continuously. The first resident crew, Expedition 1, arrived in November 2000 on Soyuz TM-31, midway between the flights of STS-92 and STS-97. These two Space Shuttle flights each added segments of the station's Integrated Truss Structure, which provided the embryonic station with communications, guidance, electrical grounding (on Z1), and power via solar arrays located on the P6 truss.[43]

Over the next two years the station continued to expand. A Soyuz-U rocket delivered the Pirs docking compartment. The Space Shuttles Discovery, Atlantis, and Endeavour delivered the Destiny laboratory and Quest airlock, in addition to the station's main robot arm, the Canadarm2, and several more segments of the Integrated Truss Structure.[43]

The expansion schedule was interrupted by the destruction of the Space Shuttle Columbia on STS-107 in 2003, with the resulting hiatus in the Space Shuttle programme halting station assembly until the launch of Discovery on STS-114 in 2005.[44]

The official resumption of assembly was marked by the arrival of Atlantis, flying STS-115, which delivered the station's second set of solar arrays. Several more truss segments and a third set of arrays were delivered on STS-116, STS-117, and STS-118. As a result of the major expansion of the station's power-generating capabilities, more pressurised modules could be accommodated, and the Harmony node and Columbus European laboratory were added. These were followed shortly after by the first two components of Kibō, the Japanese Experiment Module. In March 2009, STS-119 completed the Integrated Truss Structure with the installation of the fourth and final set of solar arrays. The final section of Kibō was delivered in July 2009 on STS-127, and the third node, Tranquillity, in February 2010 during STS-130 by the Space Shuttle Endeavour, alongside the Cupola.[43]

As of February 2010, the station consisted of thirteen pressurised modules and the complete Integrated Truss Structure. Still to be launched is the Pressurised Multipurpose Module Leonardo, the European Robotic Arm, two Russian modules and a number of external components, including the Alpha Magnetic Spectrometer (AMS-02). Assembly is expected to be completed by 2011, by which point the station will have a mass in excess of 400 metric tons (440 short tons).[2][42]

[edit] Pressurised modules

When completed, the ISS will consist of sixteen pressurised modules with a combined volume of around 1,000 cubic metres (35,000 cu ft). These modules include laboratories, docking compartments, airlocks, nodes and living quarters. Thirteen of these components are already in orbit, with the remaining three awaiting launch. Each module was or will be launched either by the Space Shuttle, Proton rocket or Soyuz rocket.[43]

Module Assembly mission Launch date Launch system Nation Isolated View

(lit. 'dawn')
1A/R 20 November 1998 Proton-K Russia (Builder)
USA (Financier)
A lone module floats against the blackness of space. The module consists of a stepped cylinder with a flattened cone at one end and a spherical docking compartment at the other. Two blue solar arrays project from the sides of the module. [45]
The first component of the ISS to be launched, Zarya provided electrical power, storage, propulsion, and guidance during initial assembly. The module now serves as a storage compartment, both inside the pressurised section and in the externally mounted fuel tanks.
(Node 1)
2A 4 December 1998 Space Shuttle Endeavour, STS-88 USA A module floats against the blackness of space. The module is a metallic cylinder, with two large, white circles visible on it. A black cone is visible at either end of the module. [46]
The first node module, connecting the American section of the station to the Russian section (via PMA-1), and providing berthing locations for the Z1 truss, Quest airlock, Destiny laboratory and Tranquillity node.
(lit. 'star')
(Service Module)
1R 12 July 2000 Proton-K Russia A module consisting of a stepped-cylinder main compartment with a spherical docking compartment at one end. Two blue solar arrays project from the module, with Earth and space in the background. [47]
The station's service module, which provides the main living quarters for resident crews, environmental systems and attitude & orbit control. The module also provides docking locations for Soyuz spacecraft, Progress spacecraft and the Automated Transfer Vehicle, and its addition rendered the ISS permanently habitable for the first time.
(US Laboratory)
5A 7 February 2001 Space Shuttle Atlantis, STS-98 USA A module consisting of a long, metallic cylinder, floats against the blackness of space suspended by the ISS robotic arm. The module has a highly flattened cone at each end, and pieces of ISS and space shuttle hardware are visible to the right of the image. [48]
The primary research facility for US payloads aboard the ISS, Destiny is intended for general experiments. The module houses 24 International Standard Payload Racks, some of which are used for environmental systems and crew daily living equipment, and features a 51-centimetre (20 in) optically perfect window, the largest such window ever produced for use in space. Destiny also serves as the mounting point for most of the station's Integrated Truss Structure.
(Joint Airlock)
7A 12 July 2001 Space Shuttle Atlantis, STS-104 USA A module suspended in space by the ISS robotic arm. In view are the module's two compartments, the short, wide equipment lock to the left of the image, and the long, narrow crew lock to the left. The Earth and blackness of space are visible in the background, with the blurred corner of another module visible in the foreground, at top-right. [49]
The primary airlock for the ISS, Quest hosts spacewalks with both US EMU and Russian Orlan spacesuits. Quest consists of two segments; the equipment lock, that stores spacesuits and equipment, and the crew lock, from which astronauts can exit into space.
(lit. 'pier')
(Docking Compartment)
4R 14 September 2001 Soyuz-U, Progress M-SO1 Russia A small, cylindrical module, covered in white insulation with docking equipment at one end. In the background are some other modules and some blue solar arrays. [50]
Pirs provides the ISS with additional docking ports for Soyuz and Progress spacecraft, and allows egress and ingress for spacewalks by cosmonauts using Russian Orlan spacesuits, in addition to providing storage space for these spacesuits.
(Node 2)
10A 23 October 2007 Space Shuttle Discovery, STS-120 Europe (Builder)
USA (Operator)
A module shown with Earth in the background. The module, a metallic cylinder, is pictured at the end of the ISS robotic arm, with the blackness of space and some other ISS components visible at the bottom of the image. A black cone is attached to one end of the module, and a large white circle is also visible. [51]
The second of the station's node modules, Harmony is the utility hub of the ISS. The module contains four racks that provide electrical power, bus electronic data, and acts as a central connecting point for several other components via its six Common Berthing Mechanisms (CBMs). The European Columbus and Japanese Kibō laboratories are permanently berthed to the module, and American Space Shuttle Orbiters dock with the ISS via PMA-2, attached to Harmony's forward port. In addition, the module serves as a berthing port for the Multi-Purpose Logistics Modules during shuttle logistics flights.
(European Laboratory)
1E 7 February 2008 Space Shuttle Atlantis, STS-122 Europe A module seen through a space shuttle window. The module is a metallic cylinder with flattened cones at each end, with a large white circle visible on the end facing the camera. In the background is the wing of a space shuttle, some other ISS hardware and the blackness of space. [52][53]
The primary research facility for European payloads aboard the ISS, Columbus provides a generic laboratory as well as facilities specifically designed for biology, biomedical research and fluid physics. Several mounting locations are affixed to the exterior of the module, which provide power and data to external experiments such as the European Technology Exposure Facility (EuTEF), Solar Monitoring Observatory, Materials International Space Station Experiment, and Atomic Clock Ensemble in Space. A number of expansions are planned for the module to study quantum physics and cosmology.
Kibō Experiment Logistics Module
(lit. 'hope' and 'wish' JEM–ELM)
1J/A 11 March 2008 Space Shuttle Endeavour, STS-123 Japan A module consisting of a short, metallic cylinder with a flattened cone at one end. A number of gold-coloured handrails are visible on the module, along with other pieces of ISS hardware in the background. [54]
Part of the Kibō Japanese Experiment Module laboratory, the ELM provides storage and transportation facilities to the laboratory with a pressurised section to serve internal payloads.
Kibō Pressurised Module
1J 31 May 2008 Space Shuttle Discovery, STS-124 Japan A module consisting of a long, metallic cylinder. The module has a robotic arm attached to the end of the cylinder facing the camera, along with an airlock and several covered windows. On the right-hand side of the module is a Japanese flag. A space shuttle and other ISS hardware is visible in the background, with the blackness of space as the backdrop. [54][55]
Part of the Kibō Japanese Experiment Module laboratory, the PM is the core module of Kibō to which the ELM and Exposed Facility are berthed. The laboratory is the largest single ISS module and contains a total of 23 racks, including 10 experiment racks. The module is used to carry out research in space medicine, biology, Earth observations, materials production, biotechnology, and communications research. The PM also serves as the mounting location for an external platform, the Exposed Facility (EF), that allows payloads to be directly exposed to the harsh space environment. The EF is serviced by the module's own robotic arm, the JEM–RMS, which is mounted on the PM.
(lit. 'search')
(Mini-Research Module 2)
5R 10 November 2009 Soyuz-U, Progress M-MRM2 Russia A squat cylindrical module, covered in white insulation, with a small porthole and the Russian word for "search" visible. Attached to the module is another cylindrical module, covered in brown insulation. A folded solar array and a third module, covered in white insulation, is visible at the top of the image. [56][57]
One of the Russian ISS components, MRM2 will be used for docking of Soyuz and Progress ships, as an airlock for spacewalks and as an interface for scientific experiments.
(Node 3)
20A 8 February 2010 Space Shuttle Endeavour, STS-130 Europe (Builder)
USA (Operator)
A module shown against a backdrop of the Earth, held by a white robotic arm. The module is a large metallic cylinder, with a white circle visible on the side facing the camera. A short, conical module covered in white insulation is visible at one end of it. [58][59]
The third and last of the station's US nodes, Tranquillity contains an advanced life support system to recycle waste water for crew use and generate oxygen for the crew to breathe. The node also provides four berthing locations for more attached pressurised modules or crew transportation vehicles, in addition to the permanent berthing location for the station's Cupola.
Cupola 20A 8 February 2010 Space Shuttle Endeavour, STS-130 Europe (Builder)
USA (Operator)
A small, squat module with three of seven windows visible, seen against the backdrop of space. Open shutters are visible next to each window, and an astronaut can be seen inside the module through the windows. [60]
The Cupola is an observatory module that provides ISS crew members with a direct view of robotic operations and docked spacecraft, as well as an observation point for watching the Earth. The module comes equipped with robotic workstations for operating the SSRMS and shutters to protect its windows from damage caused by micrometeorites.

[edit] Scheduled to be launched

Module Assembly mission Launch date Launch system Nation Isolated View
(lit. 'dawn')
(Mini-Research Module 1)
ULF4 NET 14 May 2010 Space Shuttle Atlantis, STS-132 Russia A small, cylindrical module, covered in white insulation, is held by a crane. A number of struts project from the module, and a small cylinder and a folded radiator are attached to its top. Russian labels and patches are visible on the module's sides. [42]
MRM1 will be used for docking and cargo storage aboard the station.
(Pressurised Multipurpose Module)
ULF5 NET 16 September 2010 Space Shuttle Discovery, STS-133 Europe (Builder)
USA (Operator)
A silver, cylindrical module, with the NASA logo and a number of Italian symbols placed upon it, seen attached to another module on the edge of the image at left. The module has a yellow and silver attachment at each corner, and the image is backdropped by the Earth, with a white robotic arm visible in the foreground. [61][62][63]
The Leonardo PMM will house spare parts and supplies, allowing longer times between resupply missions and freeing space in other modules, particularly Columbus. The PMM was created by converting the Leonardo Multi-Purpose Logistics Module into a module that could be permanently attached to the station. The arrival of the module will mark the completion of the US Orbital Segment.
(lit. 'science')
(Multipurpose Laboratory Module)
3R c. December 2011 Proton-M Russia A computer-generated image of a module. The module is a stepped cylinder covered in white insulation, with a spherical compartment and airlock at one end. Two blue solar arrays project from the module, as does a robotic arm. Several other pieces of ISS hardware, faded to highlight the module, are visible in the background. [42][64]
The MLM will be Russia's primary research module as part of the ISS and will be used for general microgravity experiments, docking, and cargo logistics. The module provides a crew work and rest area, and will be equipped with a backup attitude control system that can be used to control the station's attitude. Based on the current assembly schedule, the arrival of Nauka will complete construction of the Russian Orbital Segment and it will be the last major component added to the station.

[edit] Cancelled modules

A small, stubby spaceplane, coloured black on its underside and white on its topside, descending against a cloudy sky. The words "United States" and the NASA logo are visible on its sides.
The prototype X-38 lifting body, the cancelled ISS Crew Return Vehicle

Several modules planned for the station have been cancelled over the course of the ISS programme, whether for budgetary reasons, because the modules becoming unnecessary, or following a redesign of the station after the 2003 Columbia disaster. The cancelled modules include:

[edit] Unpressurised elements

An astronaut, dressed in a white spacesuit, attached to the end of a long, jointed robotic arm covered in white insulation. The Earth's horizon and the blackness of space serves as a backdrop.
Astronaut Stephen K. Robinson anchored to the end of Canadarm2 during STS-114

In addition to the pressurised modules, the ISS features a large number of external components. The largest component is the Integrated Truss Structure (ITS), to which the station's main solar arrays and thermal radiators are mounted.[20] The ITS consists of ten separate segments forming a structure 108.5 m (356 ft) long.[2]

The Alpha Magnetic Spectrometer (AMS), a particle physics experiment, is scheduled to be launched on STS-134 in 2010, and will be mounted externally on the ITS. The AMS will measure cosmic rays and look for evidence of dark matter and antimatter.[71]

The ITS serves as a base for the main remote manipulator system called the Mobile Servicing System (MSS). This consists of the Mobile Base System (MBS), the Canadarm2, and the Special Purpose Dexterous Manipulator. The MBS rolls along rails built into some of the ITS segments to allow the arm to reach all parts of the US segment of the station.[72] The MSS is due to have its reach increased by an Orbiter Boom Sensor System, due to be installed by STS-133.[73]

Two other remote manipulator systems are present in the station's final configuration. The European Robotic Arm, which will service the Russian Orbital Segment, will be launched alongside the Multipurpose Laboratory Module;[74] the JEM RMS, which services the JEM Exposed Facility,[75] was launched on STS-124 and is attached to the JEM Pressurised Module. In addition to these robotic arms, there are two Russian Strela cargo cranes used for moving spacewalking cosmonauts and parts around the exterior of the Russian Orbital Segment.[76]

The station in its complete form will have several smaller external components, such as the three External Stowage Platforms (ESPs), launched on STS-102, STS-114 and STS-118, which are used for storage of spare parts. Four ExPRESS Logistics Carriers (ELCs) will allow experiments to be deployed and conducted in the vacuum of space, and will provide the necessary electricity and computing to process experimental data locally. ELCs 1 and 2 were delivered on STS-129 in November 2009, and ELCs 3 and 4 are scheduled for delivery on STS-134 in July 2010 and STS-133 in September 2010.[42][77] There are also two exposure facilities mounted directly to laboratory modules, the JEM Exposed Facility, which serves as an external 'porch' for the Japanese Experiment Module complex,[78] and a facility on the European Columbus laboratory, which provides power and data connections for experiments such as the European Technology Exposure Facility[79][80] and the Atomic Clock Ensemble in Space.[81]

[edit] Power supply

The ISS shown orbiting the Earth, with the blackness of space behind. In view are one of the large orange solar array wings at the top, a cluster of pressurised modules below, and four smaller, blue solar arrays projecting from the modules.
The ISS in 2001, showing the solar arrays on Zarya and Zvezda, in addition to the US P6 solar arrays

The source of electrical power for the ISS is the sun. Sunlight is converted into electricity by solar arrays. The Russian segment of the station uses 28 volts DC (partly provided by four solar arrays mounted directly to Zarya and Zvezda), as does the space shuttle, but in the remainder of the station, electricity provided by the US solar arrays is distributed at a voltage ranging from 130 to 180 volts DC. These arrays are arranged as four pairs of wings, and each pair is capable of generating nearly 32.8 kW of DC power.[20]

Power is stabilised and distributed at 160 volts DC, and then is converted to the user-required 124 volts DC. The high-voltage distribution allows the use of small-diameter electrical cables and consequently reduces weight. Power can be shared between the two segments of the station using converters. The power sharing feature has become essential since the cancellation of the Russian Science Power Platform because the Russian Orbital Segment now depends on the US-built solar arrays for power.[82]

Since the station is often not in direct sunlight, it relies on rechargeable nickel-hydrogen batteries to provide continuous power for the 35 minutes of every 90 minute orbit during which it is eclipsed by the Earth. During the sunlit part of the orbit, the batteries are recharged. The batteries have a working life of 6.5 years, and they are expected to be replaced multiple times during the anticipated 20-year life of the station.[83]

The US solar arrays normally track the sun to maximise power generation. Each array is about 375 m2 (450 yd2) in area and 58 metres (63 yd) long. In the complete configuration, the solar arrays track the sun in each orbit by rotating the alpha gimbal, while the beta gimbal adjusts for the angle of the sun from the orbital plane. Another tracking option, the Night Glider mode, which aligns the solar arrays edgewise to the direction of travel when the panels are in the Earth's shadow, is used to reduce the effects of the drag produced by the tenuous upper atmosphere, through which the station flies.[84]

[edit] Orbit control

The graph has a vaguely sawtoothed shape, with a deep valley in 2000 and a gentle descent in the average from 2003 onwards, picking up again after mid-2007. See adjacent text for details.
Graph showing the changing altitude of the ISS from November 1998 until January 2009

The ISS is maintained in a near circular orbit with a minimum mean altitude of 278 km (173 mi) and a maximum of 460 km (286 mi). It travels at an average speed of 27,724 kilometres (17,227 mi) per hour, and completes 15.7 orbits per day.[21] The normal maximum altitude is 425 km (264 mi) to allow Soyuz rendezvous missions. As the ISS constantly loses altitude because of a slight atmospheric drag, it needs to be boosted to a higher altitude several times each year.[30][85] This boost can be performed by either the station's two main engines on the Zvezda service module, a docked space shuttle, a Progress resupply vessel, or by ESA's ATV. It takes approximately two orbits (three hours) for the boost to a higher altitude to be completed.[85]

In December 2008 NASA signed an agreement with the Ad Astra Rocket Company which may result in the testing on the ISS of a VASIMR plasma propulsion engine.[86] This technology could allow station-keeping to be done more economically than at present.[87][88]

[edit] Attitude (orientation) control

The station's navigational position and velocity, or state vector, is independently established using the US Global Positioning System (GPS) and a combination of state vector updates from Russian Ground Sites and the Russian GLONASS system. The attitude (orientation) of the station is independently determined by a set of sun, star and horizon sensors on Zvezda and the US GPS with antennas on the S0 truss and a receiver processor in the US lab. The attitude knowledge is propagated between updates by rate sensors.[23] Attitude control is maintained by either of two mechanisms; normally, a system of four control moment gyroscopes (CMGs) keeps the station oriented, with Destiny forward of Unity, the P truss on the port side, and Pirs on the Earth-facing (nadir) side. When the CMG system becomes 'saturated'—when the set of CMGs exceed their operational range or cannot track a series of rapid movements—they can lose their ability to control station attitude.[89] In this event, the Russian attitude control system is designed to provide desaturating thruster firings, or take over in an automated fashion. while under Russian thruster control, thrusters are used to maintain station attitude while the CMG system can be reset for control. This Automatic attitude control safing has only occurred once, during Expedition 10.[90] When a space shuttle is docked to the station, it can also be used to maintain station attitude. This occurs during portions of every mated shuttle ISS mission. Shuttle control was used exclusively during STS-117 as the S3/S4 truss was installed.[91]

[edit] Communications

Diagram showing communications links between the ISS and other elements. See adjacent text for details.
The communications systems used by the ISS
* Luch satellite not currently in use.

Radio communications provide telemetry and scientific data links between the station and Mission Control Centres. Radio links are also used during rendezvous and docking procedures and for audio and video communication between crewmembers, flight controllers and family members. As a result, the ISS is equipped with internal and external communication systems used for different purposes.[92]

The Russian Orbital Segment communicates directly with the ground via the Lira antenna mounted to Zvezda.[23][93] The Lira antenna also has the capability to use the Luch data relay satellite system.[23] This system, used for communications with Mir, fell into disrepair during the 1990s, and as a result is no longer in use.[17][94][95] However, two new Luch satellites—Luch-5A and Luch-5B—are planned for launch in 2011 to restore the operational capability of the system.[96] The US Orbital Segment (USOS) makes use of two separate radio links mounted in the Z1 truss structure: the S band (used for audio) and Ku band (used for audio, video and data) systems. These transmissions are routed via the US Tracking and Data Relay Satellite System (TDRSS) in geostationary orbit, which allows for almost continuous real-time communications with NASA's Mission Control Centre (MCC-H) in Houston.[18][23][92] Data channels for the Canadarm2, European Columbus laboratory and Japanese Kibō modules are routed via the S band and Ku band systems, although the European Data Relay Satellite System and similar Japanese system will eventually complement the TDRSS in this role.[18][97] Communications between modules are carried on an internal digital wireless network.[98]

UHF radio is used by astronauts and cosmonauts conducting EVAs. UHF is employed by other spacecraft that dock to or undock from the station, such as Soyuz, Progress, HTV, ATV and the Space Shuttle (except the shuttle also makes use of the S band and Ku band systems via TDRSS), to receive commands from Mission Control and ISS crewmembers.[23] Automated spacecraft are fitted with their own communications equipment; the ATV uses a laser attached to the spacecraft and equipment attached to Zvezda, known as the Proximity Communications Equipment, to accurately dock to the station.[99][100]

[edit] Microgravity

At the station's orbital altitude, the gravity from the Earth is 88% of that at sea level. The state of weightlessness is caused by the constant free fall of the ISS, which, because of the equivalence principle, is indiscernible from a state of zero gravity.[101] The environment on the station is, however, often described as microgravity, as the weightlessness is imperfect. This is caused by four separate effects:[102]

  • The drag resulting from the residual atmosphere.
  • Vibratory acceleration caused by mechanical systems and the crew on board the ISS.
  • Orbital corrections by the on-board gyroscopes or thrusters.
  • The spatial separation from the real centre of mass of the ISS. Any part of the ISS not at the exact centre of mass will tend to follow its own orbit. However, as each point is physically part of the station, this is impossible, and so each component is subject to small accelerations from the forces which keep them attached to the station as it orbits.[102] This is also called the tidal force.

[edit] Life support

A flowchart diagram showing the components of the ISS life support system. See adjacent text for details.
The interactions between the components of the ISS Environmental Control and Life Support System (ECLSS)

The ISS Environmental Control and Life Support System (ECLSS) provides or controls atmospheric pressure, fire detection and suppression, oxygen levels, waste management and water supply. The highest priority for the ECLSS is the ISS atmosphere, but the system also collects, processes, and stores waste and water produced and used by the crew—a process that recycles fluid from the sink, shower, toilet, and condensation from the air. The Elektron system aboard Zvezda and a similar system in Destiny generate oxygen aboard the station.[103] The crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters.[104] Carbon dioxide is removed from the air by the Vozdukh system in Zvezda. Other by-products of human metabolism, such as methane from the intestines and ammonia from sweat, are removed by activated charcoal filters.[104]

The atmosphere on board the ISS is similar to the Earth's.[105] Normal air pressure on the ISS is 101.3 kPa (14.7 psi);[106] the same as at sea level on Earth. An Earth-like atmosphere offers benefits for crew comfort, and is much safer than the alternative, a pure oxygen atmosphere, because of the increased risk of a fire such as that responsible for the deaths of the Apollo 1 crew.[107]

[edit] Sightings

A streak of light in a starry sky over some houses.
A July 2007 sighting of the International Space Station.
An image of a gibbous Moon, with a small white dot visible over the part of the Moon that is in shadow. A black sky forms the backdrop for the image.
The ISS flew across the face of the Moon over the Kennedy Space Centre in Florida approximately 15 minutes before the launch of Space Shuttle Discovery on STS-131.

Because of the size of the ISS (about that of an American football field) and the large reflective area offered by its solar panels, ground based observation of the station is possible with the naked eye if the observer is in the right location at the right time. In many cases, the station is one of the brightest naked-eye objects in the sky, although it is visible only for periods ranging from two to five minutes.[9]

To view the station, the following conditions need to be fulfilled, assuming the weather is clear: The station must be above the observer's horizon, and it must pass within about 2,000 kilometres (1,200 mi) of the observation site (the closer the better). It must be dark enough at the observer's location for stars to be visible, and the station must be in sunlight rather than in the Earth's shadow. It is common for the third condition to begin or end during what would otherwise be a good viewing opportunity. In the evening, as the station moves further from the dusk, going from west to east it will appear to suddenly fade and disappear. In the reverse situation, it may suddenly appear in the sky as it approaches the dawn.[9][108]

The station has now become bright enough to be seen during the day under certain conditions.[109]

[edit] Politics, utilisation and financing

[edit] Legal aspects

A world map highlighting Belgium, Denmark, France, Germany, Italy, Netherlands, Norway, Spain, Sweden and Switzerland in red and Brazil in pink. See adjacent text for details.
     Primary contributing nations     NASA contracted nations

The ISS is a joint project of several space agencies: the US National Aeronautics and Space Administration (NASA), the Russian Federal Space Agency (RKA), the Japan Aerospace Exploration Agency (JAXA), the Canadian Space Agency (CSA) and the European Space Agency (ESA).[22]

As a multinational project, the legal and financial aspects are complex. Issues of concern include the ownership of modules, station utilisation by participant nations, and responsibilities for station resupply. Obligations and rights are established by the Space Station Intergovernmental Agreement (IGA). This international treaty was signed on 28 January 1998 by the primary nations involved in the Space Station project; the United States of America, Russian Federation, Japan, Canada and ten member states of the European Space Agency (Belgium, Denmark, France, Germany, Italy, The Netherlands, Norway, Spain, Sweden and Switzerland).[24] A second layer of agreements was then achieved, called Memoranda of Understanding (MOU), between NASA and ESA, CSA, RKA and JAXA. These agreements are then further split, such as for the contractual obligations between nations, and trading of partners' rights and obligations.[24] Use of the Russian Orbital Segment is also negotiated at this level.[25]

In addition to these main intergovernmental agreements, Brazil joins as a bilateral partner of the United States by a contract with NASA to supply hardware[110]. In return, NASA will provide Brazil with access to its ISS facilities on-orbit, as well as a flight opportunity for one Brazilian astronaut during the course of the ISS programme. Italy has a similar contract with NASA to provide comparable services, although Italy also takes part in the programme directly via its membership in ESA.[111] China has reportedly expressed interest in the project, especially if it would be able to work with the RKA. However, as of 2009 China is not involved because of US objections.[112][113]

[edit] Utilisation rights

Four pie charts indicating how each part of the American segment of the ISS is allocated. See adjacent text for details.
Allocation of American segment hardware utilisation between nations

The Russian part of the station is operated and controlled by the Russian Federation's space agency and provides Russia with the right to nearly one-half of the crew time for the ISS. The allocation of remaining crew time (three to four crew members of the total permanent crew of six) and hardware within the other sections of the station has been assigned as follows:

  • Columbus: 51% for the ESA, 46.7% for NASA, and 2.3% for CSA.[24]
  • Kibō: 51% for the JAXA, 46.7% for NASA, and 2.3% for CSA.[97]
  • Destiny: 97.7% for NASA and 2.3% for CSA.[114]
  • Crew time, electrical power and rights to purchase supporting services (such as data upload and download and communications) are divided 76.6% for NASA, 12.8% for JAXA, 8.3% for ESA, and 2.3% for CSA.[24][97][114]

[edit] Costs

The cost estimates for the ISS range from 35 billion to 160 billion dollars.[27] ESA, the one agency which actually presents potential overall costs, estimates 100 billion for the entire station over 30 years.[26] A precise cost estimate for the ISS is unclear, as it is difficult to determine which costs should be attributed to the ISS programme, or how the Russian contribution should be measured.[27]

[edit] Criticism

Critics of the ISS contend that the time and money spent on the ISS could be better spent on other projects—whether they be robotic spacecraft missions, space exploration, investigations of problems on Earth, or just tax savings.[28][29] Some critics, such as Robert L. Park, argue that little scientific research was convincingly planned for the ISS, and that the primary feature of a space-based laboratory, its microgravity environment, can be studied less expensively with a "vomit comet".[28][115][116]

The research capabilities of the ISS have been criticised, particularly following the cancellation of the ambitious Centrifuge Accommodations Module, which, alongside other equipment cancellations, means research done on the ISS is generally limited to experiments which do not require any specialised apparatus. For example, in the first half of 2007, ISS research dealt primarily with human biological responses to living and working in space, covering topics like kidney stones, circadian rhythm, and the effects of cosmic rays on the nervous system.[117][118][119] Other criticisms hinge on the technical design of the ISS, including the high inclination of the station's orbit, which leads to a higher cost for US-based launches to the station.[120]

In response to some of these comments, advocates of manned space exploration say that criticism of the ISS project is short-sighted, and that manned space research and exploration have produced billions of dollars' worth of benefits. NASA has estimated that the indirect economic return from spin-offs of human space exploration has been many times the initial public investment, although this estimate is based on the Apollo programme and was made in the 1970s.[121] A review of the claims by the Federation of American Scientists argued that NASA's rate of return from spin-offs is actually very low, except for aeronautics work that has led to aircraft sales.[122]

[edit] End of mission and deorbit plans

NASA planned to deorbit the ISS in the first quarter of 2016.[123] However, the plan to end the ISS programme in 2015, as determined in 2004 by then-President George W. Bush, has been rejected by the current Obama administration. With the new budget announced on 1 February 2010, the administration aims to extend the lifetime through 2020. Current NASA administrator Charles Bolden has stated the lifetime of the ISS will be extended beyond 2016, the year that international funding for the station is planned to end.[124] The Augustine Commission, which reviewed NASA's human space flight program, recommended in its final report of 23 October 2009 the extension of the ISS programme to at least 2020.[125] In particular, Leroy Chiao, a former space station commander and space shuttle astronaut who sat on the advisory panel, stated in a CNN interview: “You've got all of these different countries working together on this common project in space. And if we go ahead and stop [...] it's going to break up that framework. The different countries around the world will lose confidence in the US as a leader in space exploration." NASA officials received confirmation from the Obama administration on the future direction of the ISS in particular and the human spaceflight programme in general on 1 February 2010, with a budget proposing an extension to the ISS programme until at least 2020,[8][126] with talks between ISS partners suggesting that the station could conceivably remain operational until 2028.[127]

NASA has the responsibility to deorbit the ISS. Although Zvezda has a propulsion system used for station-keeping, it is not powerful enough for a controlled deorbit. Options for controlled deorbit of the ISS include the use of a modified European Automated Transfer Vehicle or a specially constructed deorbit vehicle.[128][129]

According to a 2009 report, RKK Energia is considering methods to remove from the station some modules of the Russian Orbital Segment when the end of mission is reached and use them as a basis for a new station, known as the Orbital Piloted Assembly and Experiment Complex. The modules under consideration for removal from the current ISS include the Multipurpose Laboratory Module (MLM), currently scheduled to be launched at the end of 2011, with other Russian modules which are currently planned to be attached to the MLM until 2015, although still currently unfunded. Neither the MLM nor any additional modules attached to it would have reached the end of their useful lives in 2016 or 2020. The report presents a statement from an unnamed Russian engineer who believes that, based on the experience from Mir, a thirty-year life should be possible, except for micrometeorite damage, because the Russian modules have been built with on-orbit refurbishment in mind.[130]

[edit] Life on board

[edit] Crew schedule

The time zone used on board the ISS is Coordinated Universal Time (UTC, also called GMT). The windows are covered at night hours to give the impression of darkness because the station experiences 16 sunrises and sunsets a day. During visiting space shuttle missions, the ISS crew will mostly follow the shuttle's Mission Elapsed Time (MET), which is a flexible time zone based on the launch time of the shuttle mission.[131][132] Because the sleeping periods between the UTC time zone and the MET usually differ, the ISS crew often has to adjust its sleeping pattern before the space shuttle arrives and after it leaves to shift from one time zone to the other in a practice known as sleep shifting.[133]

A typical day for the crew begins with a wake-up at 06:00, followed by post-sleep activities and a morning inspection of the station. The crew then eats breakfast and takes part in a daily planning conference with Mission Control before starting work at around 08:10. The first scheduled exercise of the day follows, after which the crew continues work until 13:05. Following a one-hour lunch break, the afternoon consists of more exercise and work before the crew carries out its pre-sleep activities beginning at 19:30, including dinner and a crew conference. The scheduled sleep period begins at 21:30. In general, the crew works 10 hours per day on a weekday, and 5 hours on Saturdays, with the rest of the time their own for relaxation, games or work catch-up.[134]

[edit] Sleeping in space

An astronaut emerges from a small sleeping compartment surrounded by equipment.
Astronaut Peggy Whitson in the doorway of a sleeping rack in the Destiny laboratory

The station provides crew quarters for each member of permanent Expedition crews, with two 'sleep stations' in the Russian Orbital Segment and four more, due to be installed in Tranquillity, currently spread around the US Orbital Segment. The American quarters are private, approximately person-sized soundproof booths. A crewmember can sleep in them in a tethered sleeping bag, listen to music, use a laptop, and store personal effects in a large drawer or in nets attached to the module's walls. The module also provides a reading lamp, a shelf and a desktop.[135][136][137] Visiting crews have no allocated sleep module, and attach a sleeping bag to an available space on a wall—it is possible to sleep floating freely through the station, but this is generally avoided because of the possibility of bumping into sensitive equipment.[138] It is important that crew accommodations are well ventilated. Otherwise, astronauts can wake up oxygen deprived and gasping for air, because a bubble of their own exhaled carbon dioxide has formed around their heads.[137]

[edit] Hygiene

The ISS does not feature a shower, although it was planned as part of the now cancelled Habitation Module. Instead, crewmembers wash using a water jet and wet wipes, with soap dispensed from a toothpaste tube-like container. Crews are also provided with rinseless shampoo and edible toothpaste to save water.[138]

There are two space toilets on the ISS, both of Russian design, located in Zvezda and Destiny.[135] These Waste and Hygiene Compartments use a fan-driven suction system similar to the Space Shuttle Waste Collection System. Astronauts first fasten themselves to the toilet seat, which is equipped with spring-loaded restraining bars to ensure a good seal.[137] A lever operates a powerful fan and a suction hole slides open: the air stream carries the waste away. Solid waste is collected in individual bags which are stored in an aluminium container. Full containers are transferred to Progress spacecraft for disposal.[135][139] Liquid waste is evacuated by a hose connected to the front of the toilet, with anatomically correct “urine funnel adapters” attached to the tube so both men and women can use the same toilet. Waste is collected and transferred to the Water Recovery System, where it is recycled back into drinking water.[136]

[edit] Food and drink

Thirteen astronauts seated around a table covered in open cans of food strapped down to the table. In the background a selection of equipment is visible, as well as the salmon-coloured walls of the Unity node.
The crews of STS-127 and Expedition 20 enjoy a meal inside Unity.

Most of the food eaten by station crews is frozen, refrigerated or canned. Menus are prepared by the astronauts, with the help of a dietitian, before the astronauts' flight to the station.[136] As the sense of taste is reduced in orbit because of fluid shifting to the head, spicy food is a favourite of many crews.[137] Each crewmember has individual food packages and cooks them using the onboard galley, which features two food warmers, a refrigerator, and a water dispenser that provides both heated and unheated water.[135] Drinks are provided in dehydrated powder form, and are mixed with water before consumption.[135][136] Drinks and soups are sipped from plastic bags with straws, while solid food is eaten with a knife and fork, which are attached to a tray with magnets to prevent them floating away. Any food which does float away, including crumbs, must be collected to prevent it from clogging up the station's air filters and other equipment.[136]

[edit] Exercise

The most significant adverse effects of long-term weightlessness are muscle atrophy and deterioration of the skeleton, or spaceflight osteopenia. Other significant effects include fluid redistribution, a slowing of the cardiovascular system, decreased production of red blood cells, balance disorders, and a weakening of the immune system. Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, excess flatulence, and puffiness of the face. These effects begin to reverse quickly upon return to the Earth.[35]

A woman wearing a blue t-shirt and shorts, seen running on a treadmill to which she is attached by white strapping. Floating equipment and cabinets are visible in the background.
Astronaut Sunita "Suni" Williams is attached to the TVIS treadmill with bungee cords aboard the International Space Station

To prevent some of these adverse physiological effects, the station is equipped with two treadmills (including the COLBERT) and a stationary bicycle; each astronaut spends at least two hours per day exercising on the equipment.[135][137] Astronauts use bungee cords to strap themselves to the treadmill.[140] Researchers believe that exercise is a good countermeasure for the bone and muscle density loss that occurs when humans live for a long time without gravity.[141]

[edit] Station operations

[edit] Expeditions

Each permanent station crew is given a sequential expedition number. Expeditions have an average duration of half a year, and they commence following the official handover of the station from one Expedition commander to another. Expeditions 1 through 6 consisted of three person crews, but the Columbia accident led to a reduction to two crew members for Expeditions 7 to 12. Expedition 13 saw the restoration of the station crew to three, and the station has been permanently staffed as such since. While only three crew members are permanently on the station, several expeditions, such as Expedition 16, have consisted of up to six astronauts or cosmonauts, who are flown to and from the station on separate flights.[142][143]

On 27 May 2009, Expedition 20 began. Expedition 20 was the first ISS crew of six. Before the expansion of the living volume and capabilities from STS-115 the station could only host a crew of three. Expedition 20's crew was lifted to the station in two separate Soyuz-TMA flights launched at two different times (each Soyuz-TMA can hold only three people): Soyuz TMA-14 on 26 March 2009 and Soyuz TMA-15 on 27 May 2009. However, the station would not be permanently occupied by six crew members all year. For example, when the Expedition 20 crew (Roman Romanenko, Frank De Winne and Bob Thirsk) returned to Earth in November 2009, for a period of about two weeks only two crew members (Jeff Williams and Max Surayev) were aboard. This increased to five in early December, when Oleg Kotov, Timothy Creamer and Soichi Noguchi arrived on Soyuz TMA-17. It decreased to three when Williams and Surayev departed in March 2010, and finally returned to six in April 2010 with the arrival of Soyuz TMA-18, carrying Aleksandr Skvortsov, Mikhail Korniyenko and Tracy Caldwell Dyson.[142][143]

The International Space Station is the most-visited spacecraft in the history of space flight. As of 24 November 2009 (2009 -11-24), it had had 266 visitors (185 different people).[10] Mir had 137 visitors (104 different people).[17]

[edit] Visiting spacecraft

A space shuttle, with its payload bay full of equipment, seen orbiting over a cloudy sky above a mountainous region of the Earth.
The Space Shuttle Endeavour approaching the ISS during STS-118

Spacecraft from four different space agencies visit the ISS, serving a variety of purposes. The Automated Transfer Vehicle from the European Space Agency, the Russian Roskosmos Progress spacecraft and the H-II Transfer Vehicle from the Japan Aerospace Exploration Agency have provided resupply services to the station. In addition, Russia supplies a Soyuz spacecraft used for crew rotation and emergency evacuation, which is replaced every six months. Finally, the US services the ISS through its Space Shuttle programme, providing resupply missions, assembly and logistics flights, and crew rotation. As of 27 November 2009 (2009 -11-27), there have been 20 Soyuz, 35 Progress, 1 ATV, 1 HTV and 31 Space Shuttle flights to the station.[1] Expeditions require, on average, 2,722 kg of supplies, and as of 27 November 2009 (2009 -11-27), crews had consumed a total of around 19,000 meals.[1] Soyuz crew rotation flights and Progress resupply flights visit the station on average two and three times respectively each year,[144] with the ATV and HTV planned to visit annually from 2010 onwards.

As of 8 April 2010 (2010 -04-08), there are six spacecraft docked with the ISS:[144]

[edit] Mission control centres

A world map highlighting the locations of space centres. See adjacent text for details.
Space centres involved with the ISS programme

The components of the ISS are operated and monitored by their respective space agencies at control centres across the globe, including:

[edit] Safety aspects

[edit] Orbital debris

An image of a flat metallic structure with a half-inch hole punched through it. A ruler is visible to demonstrate scale.
The entry hole in Space Shuttle Endeavour's radiator panel caused by space debris during STS-118

At the low altitudes at which the ISS orbits there is a variety of space debris, consisting of everything from entire spent rocket stages and defunct satellites, to explosion fragments, paint flakes, slag from solid rocket motors, coolant released by RORSAT nuclear powered satellites, small needles, and many other objects.[150] These objects, in addition to natural micrometeoroids,[151] pose a threat to the station as they have the ability to puncture the pressurised modules and cause damage to other parts of the station.[152][153] Micrometeoroids also pose a risk to spacewalking astronauts, as such objects could puncture their spacesuits, causing them to depressurise.[154]

Space debris objects are tracked remotely from the ground, and the station crew can be notified of any objects with sufficient size to cause damage on impact. This allows for a Debris Avoidance Manoeuvre (DAM) to be conducted, which uses thrusters on the Russian Orbital Segment to alter the station's orbital altitude, avoiding the debris. DAMs are not uncommon, taking place if computational models show the debris will approach within a certain threat distance.[152] Eight DAMs had been performed prior to March 2009,[155] the first seven between October 1999 and May 2003.[156] Usually the orbit is raised by one or two kilometres by means of an increase in orbital velocity of the order of 1 m/s. Unusually there was a lowering of 1.7 km [156] on 27 August 2008, the first such lowering for 8 years.[157] [158] There were two DAMs in 2009 : 22 March and 17 July.[159] If a threat from orbital debris is identified too late for a DAM to be safely conducted, the station crew close all the hatches aboard the station and retreat into their Soyuz spacecraft, so that they would be able to evacuate in the event it was damaged by the debris. This partial station evacuation has occurred twice, on 6 April 2003 and 13 March 2009.[152]

[edit] Radiation

Without the protection of the Earth's atmosphere, astronauts are exposed to higher levels of radiation from a steady flux of cosmic rays. The station's crews are exposed to about 1 millisievert of radiation each day, which is about the same as someone would get from natural sources on Earth in a year.[160] This results in a higher risk of astronauts' developing cancer. High levels of radiation can cause damage to the chromosomes of lymphocytes. These cells are central to the immune system and so any damage to them could contribute to the lowered immunity experienced by astronauts. Over time lowered immunity results in the spread of infection between crew members, especially in such confined areas. Radiation has also been linked to a higher incidence of cataracts in astronauts. Protective shielding and protective drugs may lower the risks to an acceptable level, but data is scarce and longer-term exposure will result in greater risks.[35]

Despite efforts to improve radiation shielding on the ISS compared to previous stations such as Mir, radiation levels within the station have not been vastly reduced, and it is thought that further technological advancement will be required to make long-duration human spaceflight further into the Solar System a possibility.[160]

[edit] References

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[edit] External links

[edit] Official International Space Station webpages of the participating space agencies

[edit] Interactive and multimedia

[edit] Experiments and science