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The International Space Station (ISS) is a research facility currently being assembled in Low Earth Orbit. On-orbit construction of the station began in 1998, and is scheduled to be complete by 2011, with operations continuing until around 2015.[6] As of 2009[update], the ISS is the largest artificial satellite in Earth orbit, larger than any previous space station.[7]

The ISS programme is a joint project among the space agencies of the United States (National Aeronautics and Space Administration - NASA), Russia (Russian Federal Space Agency - RKA), Japan (Japan Aerospace Exploration Agency - JAXA), Canada (Canadian Space Agency - CSA) and ten European nations (European Space Agency - ESA).[8][b] The Brazilian Space Agency (AEB) participates through a separate contract with NASA.[9] The Italian Space Agency (ASI) similarly has separate contracts for various activities not done within the framework of ESA's ISS projects (where Italy also fully participates).[10] China has reportedly expressed interest in the project, especially if it would be able to work with the RKA, although as of 2009[update] it is not involved.[11][12]

The space station is in a Low Earth Orbit, and can be seen from Earth with the naked eye. It orbits at an altitude of approximately 350 kilometres (220 mi; 190 nmi) above the surface of the Earth,[13][14][15] travelling at an average speed of 27,724 kilometres (17,227 mi) per hour, completing 15.7 orbits per day.[13]

The ISS has been continuously staffed since the first resident crew, Expedition 1, entered the station on 2 November 2000. This has provided a permanent human presence in space for the last &0000000000000008.0000008 years, &0000000000000207.000000207 days.[16] Prior to May 2009, the station had the capacity for a crew of three. However, to fulfil an active research programme, it is to be staffed by a resident crew of six beginning with Expedition 20 (which launched May 27, 2009 and will board the station on May 29, 2009). The crew of Expedition 19 is currently aboard.[17][18]

Early crew members all came from the Russian and American space programmes until German ESA astronaut Thomas Reiter joined the Expedition 13 crew in July 2006, becoming the first crew member from another space agency. The station has been visited by astronauts from 16 different nations, and it was the destination of the first six space tourists.[19]
240px-ISS March 2009

The International Space Station as seen from the departing Space Shuttle Discovery during STS-119.

Purpose Edit

The International Space Station serves primarily as a research laboratory and is the largest ever launched into orbit.[7] The station offers 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. Long-term expedition crews conduct science daily (approximately 160 man-hours a week),[20] across a wide variety of fields, including human research, life sciences, physical sciences, and Earth observation, as well as education and technology demonstrations.[21] As of June 2006[update], 90 science investigations had been conducted on the ISS over 64 months of continuous research. In addition, there have been nine research racks and more than 7,700 kg (17,000 lb) of research equipment and facilities launched to the station. Scientific findings, in fields ranging from basic science to exploration research, are being published every month.[22]

The ISS also 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.[22]

Finally, in addition to the scientific and research aspects of the station, there are numerous opportunities for 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.[23]

Scientific research Edit

One of the main goals 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 main fields of research include biology, physics, astronomy, and meteorology.[24][25] The 2005 NASA Authorization Act designated the US segment of the International Space Station as a national laboratory with a goal to increase the utilisation of the ISS by other Federal entities and the private sector.[26]

A comparison between fire on Earth (left) and fire in a microgravity environment, such as that found on the ISS (right).One research goal is to improve the understanding of long-term space exposure on the human body. Subjects currently being studied include muscle atrophy, bone loss, and fluid shifts. The data obtained from these studies will be used to make space colonisation and lengthy space travel feasible. At the present time, current levels of bone loss and muscular atrophy would pose a significant risk of fractures and movement problems if astronauts landed on a planet following a lengthy space cruise.[27]

The effect of near-weightlessness on non-human subjects is being considered as well. Researchers are investigating the relation of the near-weightless environment of outer space to 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.[24]

Researchers are investigating the physics of fluids in microgravity, enabling them to better model the behaviour of fluids in the future. Due to 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 also hope to gain new insight regarding superconductivity.[24]

Other areas of interest include the effect of the low gravity environment on combustion, studying the efficiency of burning and control of emissions & 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.[24]

One component assisting in these various studies is the ExPRESS Logistics Carrier (ELC). Developed by NASA, there are currently 4 of these units set to be launched to the ISS. As currently envisioned, the ELCs will be delivered on two separate Space Shuttle missions. They 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. Delivery is currently scheduled for STS-129 in November 2009, and STS-133 in May 2010.[28]

The Alpha Magnetic Spectrometer (AMS), a particle physics experiment, is also scheduled to be added to the station. This device will be launched on STS-134 in 2010, and will be mounted externally on the Integrated Truss Structure. The AMS will search for various types of unusual matter by measuring cosmic rays. The experiments conducted will help researchers study the formation of the universe, and search for evidence of dark matter and antimatter.[29]

Origins Edit

Space Shuttle Atlantis docked to Mir on STS-71, during the Shuttle-Mir Programme.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. In addition, the Soviets were planning a replacement for Mir to be constructed during the 1990s called Mir-2.[30] Due to budgetary 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 canceled by the United States House of Representatives. The post-Soviet economic chaos in Russia also led to the eventual cancellation of Mir-2, with only the base block of that station, DOS-8, having been constructed.[30] Similar difficulties were being faced by the U.S. and other nations with plans for space stations. This prompted U.S. administration 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.[30]

In June 1992, U.S. 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 U.S. astronaut would board the Russian space station Mir and two Russian cosmonauts would board a space shuttle.[30]

In September 1993, U.S. Vice-president Al Gore and Russian Prime Minister Viktor Chernomyrdin announced plans for a new space station, which eventually became the International Space Station.[31] They also agreed, in preparation for this new project, that the US would be heavily involved in the Mir programme in the years ahead, as part of an agreement that later became the Shuttle-Mir Programme.[32]

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'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. Due to delays, however, the estimated completion date has been put back to 2011.[28]

Space station Edit

Assembly and structure Edit

Astronaut Ron Garan during an ISS assembly spacewalk on STS-124.The assembly of the International Space Station, a major aerospace engineering endeavour, began in November 1998. As of March 2009[update] the station is approximately 81% 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, was sent later that year in November. The year 2000 also saw the arrival of two segments of the station's Integrated Truss Structure, the Z1 and P6 trusses, providing the embryonic station with communications, guidance, electrical grounding (on Z1), and power via a pair of solar array wings, located on the P6 truss.[33]

Over the next two years the station continued to expand with a Soyuz rocket delivering the Pirs docking compartment. Space Shuttles Discovery, Atlantis, and Endeavour delivered the Destiny laboratory and Quest airlock to orbit, in addition to the station's robot arm Canadarm2, and several more segments of the truss structure.[33]

The expansion schedule was brought to an abrupt halt, however, following the destruction of the Space Shuttle Columbia on STS-107 in 2003. The resulting hiatus in the Space Shuttle programme halted station assembly until the launch of Discovery on STS-114 in 2005.[34]

A video tour of the habitable part of the ISS from January 2009.The official return to 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 last and fourth set of solar arrays.[33]

As of March 2009[update], the station consisted of ten pressurised modules and the complete Integrated Truss Structure. Awaiting launch is the final section of Kibō, the third and final American node, Tranquility, the European Robotic Arm and several Russian modules. Also awaiting launch is the Alpha Magnetic Spectrometer (AMS), which is scheduled for launch on what is currently manifested as the final space shuttle flight, STS-134, in September 2010. 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).[3][28]

Pressurised modules Edit

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

Cancelled modules Edit

Several planned pressurised modules have been cancelled, including the Centrifuge Accommodations Module,[52] for producing varying levels of artificial gravity, the Habitation Module, which was to serve as the station's living quarters (sleep stations are now spread throughout the station),[53] and several Russian modules, including two Russian Research Modules, planned to be used for general experimentation.[54]

Power supply Edit

The ISS in 2001, showing the solar panels on Zarya and Zvezda, in addition to the US P6 solar arrays.Main article: Electrical system of the International Space Station The source of electrical power for the ISS is the Sun. Light is converted into electricity through the use of solar arrays. Before assembly flight 4A (space shuttle mission STS-97, launched 30 November 2000) the only power sources were the Russian solar panels attached to the Zarya and Zvezda modules. The Russian segment of the station uses 28 volts DC, as does the space shuttle. In the remainder of the station, electricity is provided by the solar arrays attached to the truss 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.[55]

Power is stabilised and distributed at 160 volts DC before being converted to the user-required 124 volts DC. This high-voltage distribution line allows for smaller power lines, 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 segment now depends on the US-built solar arrays for power.[56]

The solar arrays normally track the Sun to maximise the amount of solar power. Each array is about 375 m² (450 yd²) in area and 58 metres (190 ft) 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, as shown in the image to the right, the beta gimbal was used for the main solar tracking. Another tracking option, the Night Glider mode, can be 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 velocity vector.[57]

Attitude control Edit

The attitude (orientation) of the station 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.[58] 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.[59] 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.[60]

Altitude control Edit

Altitude graph of ISS from 1998 through to early 2006.The ISS is maintained at an orbit from a minimum altitude of 278 km (173 mi) to a maximum of 460 km (286 mi). The normal maximum limit 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.[61] These effects vary from day-to-day, however, because of changes in the density of the outer atmosphere caused by changes in solar activity.[2] 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.[61]

Microgravity Edit

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. Due to the equivalence principle, free fall is indiscernible from a state of zero gravity, however the environment on the station is instead often described as microgravity, as it is imperfect due to 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.[62]

Life support Edit

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.[63] If required, the crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters.[64] 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.[64]

The atmosphere on board the ISS is maintained to have a composition similar to that of the Earth's atmosphere.[65] Normal air pressure on the ISS is 101.3 kPa (14.7 psi),[66] the same as at sea level on Earth.

Sightings Edit

July 2007 sighting of the International Space StationBecause 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.[67][68]

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.[67][69]

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