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The International Space Station (ISS) is an internationally developed research facility currently being assembled in Low Earth Orbit. On-orbit construction of the station began in 1998 and is scheduled to be completed by 2011, with operations continuing until at least 2015. The station can be seen from the Earth with the naked eye, and, , is the largest artificial satellite in Earth orbit, with a mass larger than that of any previous space station. The ISS serves as a long-term research laboratory in space, with experiments in fields including biology, human biology, physics, astronomy and meteorology being carried out daily in the station's microgravity environment. The station also provides a safe testing location for efficient, reliable spacecraft systems that will be required for long-duration missions to the Moon and Mars. The ISS and its experiments are operated by long-duration Expedition crews, with the station being continuously staffed since the first resident crew, Expedition 1, arrived on 2 November 2000. This has provided an uninterrupted human presence in space for the last . , the crew of Expedition 22 is aboard.

The station represents a union of several space station projects including the American Space Station Freedom, the Soviet/Russian Mir-2, the European Columbus and the Japanese Kibō. Budget issues with each station, however, led to the separate projects being merged into a single multi-national space station. The ISS project began in 1994 with the Shuttle-Mir programme, and the first module of the station, Zarya, was launched in 1998 by Russia. Assembly has been ongoing ever since, with a complex of pressurised modules, external trusses and other components being launched by American Space Shuttles, Russian Proton rockets and Russian Soyuz rockets. , the station consists of eleven pressurised modules and an extensive Integrated Truss Structure (ITS). Power is provided by sixteen large solar arrays mounted on the external truss, in addition to four smaller arrays on Russian modules. 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 kilometres (17,227 mi) per hour, completing 15.7 orbits per day.

The ISS is operated as a joint project between the American National Aeronautics and Space Administrationmarker (NASA), the Russian Federal Space Agency (RKA), the Japan Aerospace Exploration Agency (JAXA), the Canadian Space Agency (CSA), and the European Space Agencymarker (ESA). Ownership and utilisation of the space station is set out via several intergovernmental treaties and agreements, with the Russian Federationmarker retaining full ownership of its own modules, and the rest of the station being allocated between the other international partners. The cost of the station project has been estimated by ESA as €100 billion over a course of 30 years, although cost estimates vary between 35 billion dollars and 160 billion dollars, making the ISS the most expensive object ever constructed. This large cost has meant that the ISS programme has been the target of various criticisms over its financing, research capabilities and technical design.

The various sections of the station are controlled by several mission control centres on the ground, including MCC-Hmarker, TsUP, Col-CC, ATV-CC, JEM-CC, HTV-CC and MSS-CC. The station is serviced by a wide variety of manned and unmanned spacecraft, including the Soyuz spacecraft, Progress spacecraft, Space Shuttle, Automated Transfer Vehicle, and H-II Transfer Vehicle, and has been visited by astronauts and cosmonauts from 15 different nations.

Purpose

The International Space Station serves primarily as a research laboratory, offering an advantage over spacecraft such as NASA's Space Shuttle because it is a long-term platform in the space environment, allowing long-duration studies to be performed, both on specific experiments and on the human crews that operate them. The presence of a permanent crew means that the station offers benefits over unmanned spacecraft as experiments can be monitored, replenished, repaired or replaced as required by the crew, as can other components of the spacecraft itself. Scientists on the ground have swift access to their data and can modify experiments or launch new ones as and when required; benefits generally unavailable on specialised unmanned spacecraft.

Crews flying long-term expeditions lasting several months, conduct scientific experiments each day (approximately 160 man-hours a week) across many fields, including human research (space medicine), life sciences, physical sciences and Earth observation. As of the conclusion of Expedition 15, 138 major science investigations had been conducted on the ISS since the launch of Zarya in 1998. Scientific findings, in fields ranging from basic science to exploration research, are being published every month.

The ISS provides a testing location for efficient, reliable spacecraft systems that will be required for long-duration missions to the Moon and Mars, allowing for equipment to be evaluated in the relatively safe location of Low Earth Orbit. This provides experience in maintaining, repairing, and replacing systems on-orbit, which will be essential in operating spacecraft further from Earth. This aspect of ISS operations reduces mission risks, and advances the capabilities of interplanetary spacecraft.

Part of the crew's mission is educational outreach and international cooperation. The crews of the ISS provide educational opportunities for students back home on Earth, including student-developed experiments, educational demonstrations, student participation in classroom versions of ISS experiments, NASA investigator experiments, and ISS engineering activities. The ISS programme itself, and the international cooperation that it represents, allows 14 nations to live and work together in space, providing important lessons that can be taken forward into future multi-national missions.

Scientific research



A main goal of the ISS is to provide a place to conduct experiments that require one or more of the unusual conditions present on the station. The primary fields of research include biology, physics, astronomy, and meteorology. The 2005 NASA Authorization Act designated the American segment of the International Space Station as a national laboratory with a goal to increase the use of the ISS by other Federal entities and the private sector.

One research goal is to improve the understanding 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 following a lengthy space cruise.

A variety of large scale medical studies are being 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 to diagnose and potentially treat hundreds of medical conditions in space. Usually, there is no physician onboard the International Space Station and diagnosis of medical conditions is challenging. This study's techniques are now being applied to cover professional and Olympic sports injuries as well as ultrasound scans performed by non-expert operators in populations such as medical and high school students. 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 often rare.

Researchers are investigating the effect of the station's near-weightless environment on the evolution, development and growth, and the 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.

The physics of fluids in microgravity is being investigated, enabling researchers to better model the behaviour of fluids in the future. Because of the ability to almost completely combine fluids in microgravity, physicists are interested in investigating the combinations of fluids that will not normally mix well on Earth. In addition, by examining reactions that are slowed down by low gravity and temperatures, scientists hope to gain new insight regarding superconductivity.

Materials science is an important part of the research activity aboard the station, with the goal of reaping economic benefits by improving techniques used on the ground. Experiments are intended to provide a better understanding of the relationship between processing, structure, and properties so the conditions required on Earth to achieve desired materials properties can be reliably predicted.

Other areas of interest include the effect of the low gravity environment on combustion, studying the efficiency of burning and control of emissions and pollutants. These findings may improve our understanding of energy production, and in turn have an economic and environmental impact. There are also plans to use 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.

Origins



Originating during the Cold War, the International Space Station represents a union of several space station projects from various nations. During the early 1980s, NASA had planned to launch a modular space station called Freedom as a counterpart to the Soviet Salyut and Mir space stations. Whilst the Soviets were planning to construct Mir-2 in the 1990s as a replacement for Mir. Because of budget and design constraints, however, Freedom never progressed past mock-ups and minor component tests.

With the fall of the Soviet Union ending the Cold War and Space Race, Freedom was nearly cancelled by the United States House of Representatives. The post-Soviet economic chaos in Russia also led to the cancellation of Mir-2, though only after its base block, DOS-8, had been constructed. Similar budgetary difficulties were being faced by other nations with space station projects, prompting American government officials to start negotiations with partners in Europe, Russia, Japan, and Canada in the early 1990s to begin a collaborative, multi-national, space station project.

In June 1992, American President George H. W. Bush and Russian president Boris Yeltsin agreed to cooperate on space exploration by signing the Agreement between the United States of America and the Russian Federation Concerning Cooperation in the Exploration and Use of Outer Space for Peaceful Purposes. This agreement called for setting up a short, joint space programme, during which one American astronaut would board the Russian space station Mir and two Russian cosmonauts would board a Space Shuttle.

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. They also agreed, in preparation for this new project, that the United States would be heavily involved in the Mir programme in the years ahead, as part of an agreement that later included Space Shuttle orbiters docking with Mir.

The ISS programme was planned to combine the proposed space stations of all participating space agencies, including Freedom, Mir-2 (with DOS-8 later becoming Zvezda), ESA'smarker Columbus, and the Japanese Kibō laboratory. When the first module, Zarya, was launched in 1998, the station was expected to be completed by 2003. Because of delays, however, the estimated completion date has been put back to 2011.

Space station

Assembly and structure



The assembly of the International Space Station, a major endeavour in space architecture, began in November 1998. the station was 82.8% complete.

The first segment of the ISS, Zarya, was launched into orbit on 20 November 1998 on a Russian Proton rocket, followed two weeks later by the first of three node modules, Unity, launched aboard STS-88. This bare 2-module core of the ISS remained unmanned for the next one and a half years until the Russian module Zvezda was added in July 2000, allowing a maximum crew of three people to occupy the ISS continuously. The first resident crew, Expedition 1, arrived in November 2000, midway between STS-92 and STS-97, which added of two segments of the station's Integrated Truss Structure, the Z1 and P6 trusses. These components provided the embryonic station with communications, guidance, electrical grounding (on Z1) and power via a pair of solar array wings located on the P6 truss.

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

The expansion schedule was interrupted by the destruction of the 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.

The official resumption of assembly was marked by the arrival of Atlantis, flying STS-115, delivering the station's second set of solar arrays. These were later followed by several more truss segments and a third set of arrays on STS-116, STS-117, and STS-118. This major expansion of the station's power generating capabilities meant that more pressurised modules could be accommodated, and as a result 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 marked the completion of the Integrated Truss Structure with the installation of the fourth and final set of solar arrays, while the final section of Kibō was delivered by Endeavour in July 2009 on STS-127.

, the station consisted of ten pressurised modules and the complete Integrated Truss Structure. Awaiting launch is the third and final American node, Tranquillity, a Permanent Logistics Module, the European Robotic Arm, several Russian modules and the Alpha Magnetic Spectrometer (AMS). 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).


Pressurised modules

When completed, the ISS will consist of sixteen pressurised modules with a combined volume of around 1,000 m³. These modules include laboratories, docking compartments, airlocks, nodes and living quarters. Eleven of these components are already in orbit, with the remaining five awaiting launch. Each module was or will be launched either by the Space Shuttle, Proton rocket or Soyuz rocket.

Module Assembly mission Launch date Launch system Nation Isolated View
Zarya (FGB) 1A/R 20 November 1998 Proton-K Russia (Builder)

USA (Financier)
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.
Unity (Node 1) 2A 4 December 1998 Space Shuttle , STS-88 USA
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.
Zvezda (Service Module) 1R 12 July 2000 Proton-K Russia
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.
Destiny (US Laboratory) 5A 7 February 2001 Space Shuttle , STS-98 USA
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 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.
Quest (Joint Airlock) 7A 12 July 2001 Space Shuttle Atlantis, STS-104 USA
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.
Pirs (Docking Compartment) 4R 14 September 2001 Soyuz-U, Progress M-SO1 Russia
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.
Harmony (Node 2) 10A 23 October 2007 Space Shuttle , STS-120 Europe (Builder)

USA (Operator)
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.
Columbus (European Laboratory) 1E 7 February 2008 Space Shuttle Atlantis, STS-122 Europe
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 (JEM–ELM) 1J/A 11 March 2008 Space Shuttle Endeavour, STS-123 Japan
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 and an unpressurised section to serve external payloads.
Kibō Pressurised Module (JEM–PM) 1J 31 May 2008 Space Shuttle Discovery, STS-124 Japan
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 also mounted on the PM.
Poisk

(Mini-Research Module 2)
5R 10 November 2009 Soyuz-U, Progress M-MRM2 Russia
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.


Scheduled to be launched

Module Assembly mission Launch date Launch system Nation Isolated View
Tranquillity

(Node 3)
20A NET 4 February 2010 Space Shuttle Endeavour, STS-130 Europe (Builder)

USA (Operator)
The third and last of the station's US nodes, Tranquillity will contain 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 NET 4 February 2010 Space Shuttle Endeavour, STS-130 Europe (Builder)

USA (Operator)
The Cupola is an observatory module that will provide 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 will come equipped with robotic workstations for operating the SSRMS and shutters to prevent its windows from being damaged by micrometeorites.
Rassvet

(Mini-Research Module 1)
ULF4 NET 14 May 2010 Space Shuttle Atlantis, STS-132 Russia
MRM1 will be used for docking and cargo storage aboard the station.
Leonardo

(PMM)
ULF5 NET 16 September 2010 Space Shuttle Discovery, STS-133 Europe (Builder)

USA (Operator)
The PMM will house spare parts and supplies, allowing longer times between resupply missions and freeing space in other modules, particularly Columbus. Leonardo will be the MPLM used for this purpose.
Nauka

(Multipurpose Laboratory Module)
3R c. December 2011 Proton-M Russia
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.


Cancelled modules

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



Unpressurised elements

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

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 search for types of unusual matter by measuring cosmic rays, enabling study of the formation of the universe, and will search for evidence of dark matter and antimatter.

The ITS also serves as a base for the station's main remote manipulator system (RMS), the Mobile Servicing System (MSS), consisting of the Mobile Base System, the Canadarm2, and the Special Purpose Dexterous Manipulator. The MBS rolls along a rails built into some of the ITS segments to allow the arm to reach all parts of the US segment of the station. The MSS is due to be augmented by an Orbiter Boom Sensor System, due to be installed on STS-133, to increase its reach.

Two other remote manipulator systems are also 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, and the JEM RMS, which services the JEM Exposed Facility,, was launched on STS-124 attached to the JEM Pressurised Module. In addition to these robotic arms, two Russian Strela cargo cranes, used for transferring parts and spacewalking cosmonauts around the exterior of the Russian Orbital Segment, are also present on station.

The station in its complete form will also feature 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 on the station's exterior. Four ExPRESS Logistics Carriers (ELCs), which 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, with ELCs 1 & 2 delivered on STS-129 in November 2009, and ELCs 3 & 4 scheduled for delivery on STS-134 in July 2010 and STS-133 in September 2010. The JEM Exposed Facility also allows experiments to be exposed directly to space, serving as an external 'porch' for the Japanese Experiment Module complex, as does the European Columbus laboratory, which provides mounting sites for experiments such as the European Technology Exposure Facility and the Atomic Clock Ensemble in Space.

Power supply

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, with sunlight being converted into electricity through the use of 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.

Power is stabilised and distributed at 160 volts DC before being converted to the user-required 124 volts DC. This high-voltage distribution allows the use of electrical cables with a smaller diameter, thus reducing weight. Power can be shared between the two segments of the station using converters. This 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.

Since the station is often not in direct sunlight, it relies on rechargeable nickel-hydrogen batteries to provide continuous power during the "eclipse" part of the orbit (35 minutes of every 90 minute orbit). The batteries ensure that the station is never without power to sustain life-support systems and experiments. During the sunlit part of the orbit, the batteries are recharged. The batteries have a working life of 6.5 years which means that they must be replaced multiple times during the expected 20-year life of the station.

The US solar arrays normally track the Sun to maximise the amount of solar power. Each array is about 375 m2 (450 yd2) in area and 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. Until the main truss structure arrived, the arrays were in a temporary position perpendicular to the final orientation. In this configuration, the beta gimbal was used for the main solar tracking. Another tracking option, the Night Glider mode, is used to reduce the effects of drag produced by the tenuous upper atmosphere, through which the station flies, by orienting the solar arrays edgewise to the direction of travel when the panels are in the Earth's shadow.

Orbit control

Graph showing the changing altitude of the ISS from November 1998 until January 2006
The ISS is maintained at an orbit with a minimum altitude of 278 km (173 mi) to a maximum of 460 km (286 mi), and travels at an average speed of 27,724 kilometres (17,227 mi) per hour, completing 15.7 orbits per day. The normal maximum altitude is 425 km (264 mi) to allow Soyuz rendezvous missions. As the ISS constantly loses altitude because of slight atmospheric drag, it needs to be boosted to a higher altitude several times each year. These effects vary from day to day, however, because of changes in the density of the outer atmosphere caused by changes in solar activity. This reboost can be performed by 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) to be boosted several kilometres higher.

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. This technology could, in the future, allow station-keeping to be done much more economically than at present.

The station's navigational position is established using the US Global Positioning System and Russian GLONASS system, with the attitude (orientation) of the station monitored by a set of sun, star and horizon sensors on Zvezda. Attitude is maintained by either of two mechanisms; normally, a system using several 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—a situation whereby a CMG exceeds its operational range or cannot track a series of rapid movements—it can lose its ability to control station attitude. In this event, the Russian attitude control system is designed to take over automatically, using thrusters to maintain station attitude, allowing the CMG system to desaturate. This scenario has only occurred once, during Expedition 10. When a space shuttle is docked to the station, it can also be used to maintain station attitude. This procedure was used during STS-117 as the S3/S4 truss was being installed.

Communications

The various communications systems used by the ISS.
Radio communications are essential to the operation of the ISS, providing telemetry and scientific data links between the station and Mission Control Centres spread over the globe. Radio links are also used during rendezvous and docking procedures and for audio & video communication between crewmembers, flight controllers and family members. As a result, the ISS is equipped with a number of internal and external communication systems, used for different purposes.

The first set of communications equipment to be launched was the Russian Regul VHF radio system, which transmits telemetry and other data from the Russian Orbital Segment (ROS) to the TsUP in Moscowmarker via a network of ground receiving stations and the orbiting Luch and Molniya systems via the large Lira antenna mounted to Zvezda. Communications between modules are via analogue copper wire connections.

The US Orbital Segment (USOS), meanwhile, 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, allowing for almost continuous real-time communications with MCC-Hmarker in Houstonmarker. The system can also be used to transmit data from the ROS, using a permanent telephone line between MCC-H and the TsUP. Data channels for the Canadarm2, European Columbus laboratory and Japanese Kibō modules are also routed via the S band and Ku band systems, although the European Data Relay Satellite System and a similar Japanese system will eventually complement the TDRSS in this role. Communications between modules are carried on an internal digital wireless network.

UHF radio is used by astronauts and cosmonauts conducting EVAs outside the station, with US spacewalkers linked to the USOS and Russian spacewalkers able to communicate with both the ROS and directly to the TsUP when in range of a ground station, although this system is prone to interference from ground-based radio such as Air Traffic Control transmissions. UHF is also employed by other spacecraft docking or undocking to 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. Automated spacecraft such as the HTV and ATV are also fitted with other communications equipment, with the ATV making use of a LASER attached to the spacecraft and a small set of mirrors attached to Zvezda, known as the Proximity Communications Equipment, to accurately dock to the station, and the HTV making its approach assisted by a GPS system mounted to Kibō.

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. The environment on the station is, however, often described as microgravity, as the weightlessness is imperfect because of four effects:

  • 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. This is also called the tidal force.


Life support

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 elements such as atmospheric pressure, fire detection and suppression, oxygen levels, 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. This process includes recycling fluid from the sink, shower, toilet, and condensation from the air. The Elektron system aboard Zvezda and a similar oxygen generation system in Destiny generate oxygen aboard the station. If required, the crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters. 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.

The atmosphere on board the ISS is maintained with a composition similar to that of the Earth's atmosphere. Normal air pressure on the ISS is 101.3 kPa (14.7 psi); 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 fire, which was responsible for the deaths of the Apollo 1 crew.

Sightings

A July 2007 sighting of the International Space Station


Because of the size of the International Space Station (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 brief periods of time, ranging from two to five minutes.

In order 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 2000 km of the observing 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, this will cause the station to suddenly fade and disappear as it moves further from the dusk, going from west to east. In the reverse situation, it may suddenly appear in the sky as it approaches the dawn.

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

Politics, utilisation and financing

Legal aspects

The ISS is a joint project among the space agencies of multiple nations. These consist of the US National Aeronautics and Space Administrationmarker (NASA), the Russian Federal Space Agency (RKA), the Japan Aerospace Exploration Agency (JAXA), the Canadian Space Agency (CSA) and the European Space Agencymarker (ESA).

As a multinational project, the legal and financial aspects of the ISS are complex. Issues of concern include the ownership of modules, station utilisation by participating nations, and responsibilities for station resupply. The main legal document establishing obligations and rights between the ISS partners is 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, Switzerland). This set the stage for a second layer of agreements, 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. Use of the Russian Orbital Segment is also negotiated at this level.

In addition to these main intergovernmental agreements, Brazil has a contract with NASA to supply hardware. In return, NASA will fly one Brazilian to the station during 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. China has reportedly expressed interest in the project, especially if it would be able to work with the RKA. However, China is not involved because of US objections.

Utilisation rights

Allocation of American segment hardware between nations
ISS crew time utilisation by partner nations from March 2000 to September 2009


The Russian part of the station is operated and controlled by the Russian Federationmarker's space agency and provides Russia with the right to nearly one-half of the crew time for the ISS (at a permanent crew level of six, on the average two to three crew member spaces are permanently allocated to Russia). The allocation of crew time (three to four crew members of the total permanent ISS 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.
  • Kibō: 51% for the JAXA, 46.7% for NASA, and 2.3% for CSA.
  • Destiny: 97.7% for NASA and 2.3% for CSA.
  • 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.


Costs

The most cited figures of an overall cost estimate for the ISS range from 35 billion to 100 billion dollar. ESA, the only agency actually stating potential overall costs, estimates 100 billion for the entire station over a period of 30 years. Giving a precise cost estimate for the ISS is not straightforward, as it is difficult to determine which costs should actually be attributed to the ISS programme, or how the Russian contribution should be measured.

Criticism

The International Space Station has been the target of varied criticism over the years. Critics 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 here on Earth, or just tax savings. Some critics, such as Robert L. Park, argue that very little scientific research was convincingly planned for the ISS in the first place, and that the primary feature of a space-based laboratory, its microgravity environment, can be studied less expensively with a "vomit comet".

The research capabilities of the ISS have been criticised, particularly following the cancellation of the ambitious Centrifuge Accommodations Modulemarker, which, alongside other equipment cancellations, means research done on the ISS is generally limited to experiments which do not require any specialized apparatus. For example, in the first half of 2007, ISS research dealt primarily with human biological responses to being in space, covering topics like kidney stones, circadian rhythm, and the effects of cosmic rays on the nervous system. Other criticisms hinge on the technical design of the ISS, including the necessity for a high level of maintenance to be carried out by the crew, particularly via risky EVAs, and the high inclination of the station's orbit, which leads to a higher cost for US-based launches to the station.

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 tangible benefits to people on Earth. 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. 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.

End of mission & deorbit plans

NASA currently plans to deorbit the ISS in the first quarter of 2016. However, the plan for ending the ISS programme in 2015, as determined by former President George W. Bush has been increasingly questioned. Current NASA administrator Charles Bolden has stated that he thinks the lifetime of the International Space Station will be extended beyond 2016, the year that international funding for the station is currently set to end. The Augustine Commission, which is currently reviewing NASA's human space flight programme has voiced a strong recommendation to extend the ISS programme to at least 2020 in their summary report issued on 8 September 2009. In particular, Leroy Chiao, a former space station commander and space shuttle astronaut who sat on the advisory panel, is quoted in a CNN interview as stating, “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 are awaiting a decision from the Obama administration on the future direction of the ISS in particular and the human spaceflight programme in general.

NASA has the responsibility for deorbiting 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 using a modified European Automated Transfer Vehicle or a specially constructed deorbit vehicle.

A 2009-report stated that RKK Energia is looking at ways to remove some Russian modules from the station 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 that would be removed from the current ISS include the Multipurpose Laboratory Module (MLM), which is currently scheduled to be launched at the end of 2011, and other Russian modules which are currently planned to be attached to the MLM until 2015, which however have not yet received funding. 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 quotes an unnamed Russian engineer stated that experience from Mir indicates that, except for micrometeorite damage, a thirty year life should be possible, given that the Russian modules have been built with on-orbit refurbishment in mind.

Life on board

Sleeping in space

The station provides crew quarters for each member of permanent Expedition crews, with two 'sleep stations' in the Russian Orbital Segment and four spread around the rest of the station, which will eventually be moved into Tranquillity when it is added to the station. The American quarters are private, approximately person-sized soundproof booth modules. A crewmember can sleep in them in a tethered sleeping bag, relax, 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. Visiting crews, who have no allocated sleep module, simply attach a sleeping bag to an available space on a wall—it is perfectly possible to sleep floating freely through the station, but this is generally avoided because of the possibility of drifting through the station and bumping into sensitive equipment. However, it is important that crew accommodations are well ventilated, as warm air does not rise in space. This means that astronauts can end up waking up gasping for air as they have become oxygen deprived because of a bubble of their own exhaled carbon dioxide forming around their heads.

Hygiene

Since the removal of the Habitation module from the station's design, the ISS has not featured a shower. 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.

There are two space toilets on the ISS, both of Russian design, located in Zvezda and Harmony. 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. A lever operates a powerful fan and a suction hole slides open: the air stream carries the waste neatly away. Solid waste is collected in individual micro-perforated bags which are stored in an aluminium container. Full containers are transferred to Progress spacecraft for disposal. 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.

Food & drink

Most of the food eaten by station crews is frozen, refrigerated or canned, with menus selected by the astronauts, with the help of a dietitian, prior to their arrival on the station. As the sense of taste is reduced in orbit because of fluid shifting to the head, spicy food is a favourite of many crews. Each crewmember has their food packages identified by means of a coloured label, 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. Drinks are provided dehydrated in powder form, and are mixed with water—some of which is recycled from the crew's urine—before consumption. Drinks and soups are sipped from plastic bags with straws, whilst 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 clogging up the station's air filters and other equipment.

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.



To prevent some of the adverse physiological effects associated with weightlessness, the station is equipped with two treadmills (including the COLBERT) and a stationary bicycle, with each astronaut spending at least two hours per day exercising using the equipment. The treadmills are intended to help astronauts stay fit, preventing loss of bone density and muscle atrophy that otherwise comes with space travel. Astronauts use bungee cords to strap themselves to the treadmill in order to remain in contact with the equipment while in microgravity. 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.

Crew schedule

The time zone used on board the ISS is Coordinated Universal Time (UTC, sometimes informally 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. 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.

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 on the ground 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, when the daily schedule is complete. In general, the crew works 10 hours per day on a weekday, and 5 hours on Saturdays, with the rest of the time being their own for relaxation, games or work catch-up.

Station operations

Expeditions

Each permanent station crew is given a sequential Expedition number: , , and so on. 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 from 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, however, several expeditions, such as Expedition 16, have consisted of up to six individual astronauts or cosmonauts, with individuals being flown up and down to the station on individual flights.

On 27 May 2009, the current expedition to ISS Expedition 20 began. Expedition 20 is 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 will not be permanently occupied by six crew members during all 52 weeks of the year. For example, when the Expedition 20 crew (Roman Romanenko, Frank De Winne and Bob Thirsk) returns to Earth in November 2009, for a period of about two weeks only two crew members (Jeff Williams and Max Surayev) will be aboard. This will increase to five in early December, when another Soyuz, carrying Oleg Kotov, Timothy Creamer and Soichi Noguchi arrives. It will then decrease to three when Williams and Surayev depart in March 2010, returning to six in April 2010 with the launch of the next three-person Soyuz.

The International Space Station is the most-visited spacecraft in the history of space flight. , it has had 266 non-distinct visitors comprising 185 individual people. Mir had 137 non-distinct visitors.

Visiting spacecraft



Spacecraft from four different space agencies visit the International Space Station, 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. Space shuttle missions provide resupply missions, assembly and logistics flights, and crew rotation. , there have been 20 Soyuz, 35 Progress, 1 ATV, 1 HTV and 31 Space Shuttle flights to the station. Expeditions require, on average, 2,722 kg of supplies, and , crews had consumed a total of around 19,000 meals. Soyuz crew rotation flights and Progress resupply flights visit the station on average two and three times respectively each year, with the ATV and HTV planned to visit annually from 2010 onwards.

, there are three spacecraft docked with the ISS:




Mission control centres

As an international project, the components of the ISS are operated and monitored by their respective space agencies at control centres across the globe, including:
Space centres involved with the ISS programme


Safety aspects

Orbital debris

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, deliberate insertion of small needles, and many other objects. These objects, in addition to natural micrometeoroids, 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. Micrometeoroids also pose a risk to spacewalking astronauts, as such objects could puncture their spacesuit and cause it to depressurise, exposing them directly to the space environment.

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 in order to avoid the debris. DAMs are not uncommon, taking place if computational models show the debris will approach within a certain threat distance.

However, 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 the station in the event it was damaged by the debris. This partial station evacuation has occurred twice, on 6 April 2003 and 13 March 2009.

Radiation

Without the protection of the Earth's atmosphere, astronauts are exposed to high levels of radiation through a steady flux of cosmic rays. Station crews are subjected to about 1 millisievert of radiation per day, which is about the same as someone would get from natural sources on Earth in a whole year, with three months' exposure roughly equaling 10% of the increased cancer risk caused by regular smoking. This results in a high risk of astronauts developing cancer. High levels of radiation can create 'chromosomal aberrations' in blood lymphocytes. These cells are heavily involved in the immune system and so any damage may contribute to the lowered immunity experienced by astronauts. Over time immunodeficiency results in the rapid 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.

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 in order to make long-duration human spaceflight further into the Solar System a possibility.

References



External links

Official International Space Station webpages of the participating space agencies:


Interactive/Multimedia:



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