International Space Station

International Space Station

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. As of 2009, the ISS is the largest artificial satellite in Earth orbit, larger than any previous space station.

The ISS programme is a joint project among the space agencies of the United States (NASA), Russia (RKA), Japan (JAXA), Canada (CSA) and ten European nations through the European Space Agency. The Brazilian Space Agency (AEB) participates through a separate contract with NASA. The Italian Space Agency similarly has separate contracts for various activities not done within the framework of ESA's ISS projects (where Italy also fully participates). China has reportedly expressed interest in the project, especially if it would be able to work with the RKA, although as of 2009 it is not involved.

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 km (190 nautical miles) above the surface of the Earth, travelling at an average speed of 27,700 kilometres (17,210 mi) per hour, completing 15.7 orbits per day.

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, &0000000000000150.000000150 days. At present, the station has the capacity for a crew of three. However, to fulfil an active research programme, it will be staffed by a resident crew of six beginning with Expedition 20. The crews of Expedition 18 and Expedition 19 are currently aboard.

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.

The International Space Station serves primarily as a research laboratory and is the largest ever launched into orbit. 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), across a wide variety of fields, including human research, life sciences, physical sciences, and Earth observation, as well as education and technology demonstrations. As of June 2006, 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, from fields ranging from basic science to exploration research, are being published every month.

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 developed 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.

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 any multi-national missions in the future.

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. 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.

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 an extraterrestrial planet following a lengthy space cruise.

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 outerspace to evolution, development and growth, and the internal processes of plants and animals. In response to some of this data, NASA has indicated a desire 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.

Researchers are also attempting to gain a better understanding of 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 concerning states of matter, specifically in regards to superconductivity.

Other areas of interest include the effect of the low gravity environment on combustion, studying the efficiency of burning and the creation of by-products from certain materials. 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.

One component assisting in these various studies is the ExPRESS Logistics Carrier. 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.

The station is also anticipating a particle physics experiment, called the Alpha Magnetic Spectrometer (AMS). 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.

With origins in the Cold War, the International Space Station represents a union of several space station projects from various nations. During the early 1980s, NASA 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. 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, it was nearly cancelled 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.

Similar difficulties were being faced by the US, Russia, and other nations with plans for space stations. This prompted US 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. In June 1992, American president George H. W. Bush and Russian president Boris Yeltsin agreed to join hands in 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. The agreement called for setting up a short joint space programme, during which one US 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 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 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.

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 eventual delays, however, the estimated completion date has been pushed to 2011.

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 time spent running experiments by the crew, power from the solar panel structure, and rights to purchase supporting services (such as data upload & download and communications) are divided at 76.6% for NASA, 12.8% for JAXA, 8.3% for ESA, and 2.3% for CSA.

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 also has a separate contract with NASA to provide similar services, although Italy also takes part in the programme directly via its membership in the ESA.

The most cited figure of an overall cost estimate for the ISS ranges from 35 billion to 100 billion USD. The 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.

Former NASA Administrator Michael D. Griffin says the International Space Station has a role to play as NASA moves forward with a new focus for the manned space programme, which is to go out beyond Earth orbit for purposes of human exploration and scientific discovery. "The International Space Station is now a stepping stone on the way, rather than being the end of the line," Griffin said. Griffin has said that station crews will not only continue to learn how to live and work in space, but also will learn how to build hardware that can survive and function for the years required to make the round-trip voyage from Earth to Mars.

Despite this view, however, in an internal e-mail leaked to the press on 18 August 2008 from Griffin to NASA managers, Griffin apparently communicated his belief that the current US administration had made no viable plan for US crews to participate in the ISS beyond 2011, and that the Office of Management and Budget (OMB) and Office of Science and Technology Policy (OSTP) were actually seeking its demise. The e-mail appeared to suggest that Griffin believed the only reasonable solution was to extend the operation of the space shuttle beyond 2010, but noted that Executive Policy (i.e. the White House) was firm that there will be no extension of the space shuttle retirement date, and thus no US capability to launch crews into orbit until the Ares I/Orion system becomes operational in 2014, at the earliest. He did not see purchase of Russian launches for NASA crews as politically viable following the 2008 South Ossetia war, and hoped the incoming Barack Obama administration would resolve the issue in 2009 by extending space shuttle operations beyond 2010.

On 15 October 2008, President Bush signed the NASA Authorization Act of 2008, giving NASA funding for one additional mission to "deliver science experiments to the station". The Act allows for an additional space shuttle flight, STS-134, to the ISS to install the Alpha Magnetic Spectrometer, which was previously cancelled.

President Barack Obama has supported the continued operation of the station, and supported the NASA Authorization Act of 2008. Obama's plan for space exploration includes finishing the station and completion of the Orion spacecraft programme.

The assembly of the International Space Station, a major aerospace engineering endeavour, began in November 1998. As of March 2009 the station is approximately 81% complete.

The first segment of the ISS, the Zarya FGB, 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.

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.

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.

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.

As of March 2009, the station consisted of ten pressurised modules and the complete Integrated Truss Structure. Awaiting launch is the final section of Kibō, the American Node 3, and the European Robotic Arm, in addition to several Russian modules. Also awaiting launch is the Alpha Magnetic Spectrometer, which is scheduled for 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).

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.

Several planned pressurised modules have been cancelled, including the Centrifuge Accommodations Module, 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),, and several Russian modules, including two Russian Research Modules, planned to be used for general experimentation.

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.

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.

The solar array normally tracks the Sun to maximise the amount of solar power. The 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.

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. 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.

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. 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.

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 the 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 to have 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.

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.

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 that stars are 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.

NASA, ESA and the independent Heavens-Above provide data on opportunities for viewing the ISS on their web pages.

All permanent station crews are named Expedition 1, Expedition 2, and so on. Expeditions have an average duration of half a year and are often considered synonymous with "Increments." However, Increments are distinguished from Expeditions as the programme planning period for activities that are to occur during a particular Expedition's residence on ISS. The start of both an Expedition and an Increment is defined by the departure of the previous Expedition crew on a Soyuz spacecraft. The definition of the Increment is in flux in preparation for 6-person crews that will be broken up into 3-person crews which overlap in their 6-month missions on ISS. The current expedition to ISS is Expedition 18.

The International Space Station is the most-visited spacecraft in the history of space flight. As of 11 April 2008 (2008 -04-11), it has had 213 non-distinct visitors, and 167 distinct visitors. Mir had 137 non-distinct visitors.

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 breakfasts 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.

Spacecraft from three different space agencies visit the International Space Station, serving a variety of purposes. The Automated Transfer Vehicle from the European Space Agency has provided resupply services to the station. Also serving the station in this capacity is the Russian Roskosmos Progress spacecraft. In addition, Russia also supplies a Soyuz spacecraft, used for crew rotation and emergency evacuation, which is replaced every six months. Finally, the United States services the ISS through its Space Shuttle programme. Space shuttle missions provide resupply missions, assembly and logistics flights, and crew rotation.

Throughout the remainder of the station's operating life, a variety of spacecraft by various ISS program members are planned with the intent to service the ISS. Currently under construction and planned for operation in 2009, is the Japanese H-II Transfer Vehicle (HTV), which is intended as a resupply vehicle for the JAXA Kibō modules. Still in initial funding stages is the Russian Kliper spacecraft, which, if it comes to fruition in 2012 as planned, is intended as a replacement of the Soyuz spacecraft. Being designed at this moment is the American Orion spacecraft, with plans to launch starting from 2014 as another resupply spacecraft and provide crew rotation. In hopes of bridging the gap between the Space Shuttle and Orion, NASA has started the Commercial Orbital Transportation Services program to develop commercial spacecraft services dedicated to the station.

As of 2008, six space tourists have visited the ISS, each paying around US $25 million. The tourists, or Spaceflight participants, were launched and returned via Russian crew rotation missions on Soyuz spacecraft. In addition, the ISS was the location for the first space wedding, during which Russian cosmonaut Yuri Malenchenko, flying Expedition 7, married Ekaterina Dmitrieva, who was in Texas at the time. The last space tourist flight to the ISS will take place in April 2009. After that, the station will be upgraded to a 6-person permanent crew, meaning that no more Soyuz seats will be available to Space Adventures, the company which runs the visits.

During an EVA in Expedition 14, a special golf ball equipped with a tracking device was hit from the station and sent into its own low Earth orbit. The stunt was paid for by a Canadian golf equipment manufacturer.

Japanese scientists and origami masters propose to launch a flotilla of paper planes from the ISS in early 2009. The mission will take place during STS-127. Around 30 planes will make the descent, each gliding downward over what is expected to be the course of several months. If one of the planes survives to Earth, it will have made the longest flight ever by a paper plane, traversing some 400 km (250 mi), and will have demonstrated the feasibility of slow-speed, low-friction atmospheric reentry. A prototype of the origami aeroplane passed a durability test in a wind tunnel in March 2008, and Japan's space agency adopted it for feasibility studies.

The Space Shuttle Columbia disaster on 1 February 2003 resulted in a two-and-a-half-year suspension of the US Space Shuttle programme. Another one-year suspension following STS-114 (because of continued foam shedding on the external tank) led to some uncertainty about the future of the International Space Station. All crew exchanges between February 2003 and July 2006 were carried out using the Russian Soyuz spacecraft; a STS-114 visit in July 2005 was purely logistical. Starting with Expedition 7, caretaker crews of just two astronauts were launched, in contrast to the previously launched crews of three. Because the ISS had not been visited by a space shuttle for over three years, more waste had accumulated than anticipated, which temporarily hindered station operations in 2004. Automated Progress transports and the STS-114 mission were able to eliminate this waste build-up.

On 18 September 2006, the Expedition 13 crew activated a smoke alarm in the Russian segment of the International Space Station when fumes from one of the three oxygen generators triggered momentary fear about a possible fire. The crew initially reported smoke in the cabin, as well as a smell. The alarm was later found to be caused by a leak of potassium hydroxide from an oxygen vent. The associated equipment was turned off, and officials said there was no fire and the crew was not in any danger.

The station's ventilation system was shut down to prevent the spread of smoke or contaminants through the rest of the complex. A charcoal air filter was put in place to scrub the atmosphere of any lingering potassium hydroxide fumes. The space station's programme manager said the crew never donned gas masks, but as a precaution put on surgical gloves and masks to prevent contact with any contaminants.

On 2 November 2006, the payload brought by the Russian Progress M-58 allowed the crew to repair the Elektron using spare parts.

On 14 June 2007, during Expedition 15 and flight day 7 of STS-117's visit to ISS, a computer malfunction on the Russian segments at 06:30 UTC left the station without thrusters, oxygen generation, carbon dioxide scrubber, and other environmental control systems, causing the temperature on the station to rise. A successful restart of the computers resulted in a false fire alarm that woke the crew at 11:43 UTC.

By June 15, the primary Russian computers were back online, and communicating with the US side of the station by bypassing a circuit, but secondary systems remained offline. NASA reported that without the computer that controls the oxygen levels, the station had 56 days of oxygen available.

By the afternoon of June 16, ISS Programme Manager Michael Suffredini confirmed that all six computers governing command and navigation systems for Russian segments of the station, including two thought to have failed, were back online and would be tested over several days. The cooling system was the first system brought back online. Troubleshooting of the failure by the ISS crew found that the root cause was condensation inside the electrical connectors, which led to a short-circuit that triggered the power off command to all three of the redundant processing units. This was initially a concern because the European Space Agency uses the same computer systems, supplied by EADS Astrium Space Transportation, for the Columbus laboratory module and the Automated Transfer Vehicle. Once the cause of the malfunction was understood, plans were implemented to avoid the problem in the future.

On 30 October 2007, during Expedition 16 and flight day 7 of STS-120's visit to ISS, following the repositioning of the P6 truss segment, ISS and Space Shuttle Discovery crew members began the deployment of the two solar arrays on the truss. The first array deployed without incident, and the second array deployed about 80% before astronauts noticed a 76-centimetre (2.5 ft) tear. The arrays had been deployed in earlier phases of the space station's construction, and the retraction necessary to move the truss to its final position had gone less smoothly than planned.

A second, smaller tear was noticed upon further inspection, and the mission's spacewalks were replanned in order to devise a repair. Normally, such spacewalks take several months to plan and are settled upon well in advance. On November 3, spacewalker Scott Parazynski, assisted by Douglas Wheelock, fixed the torn panels using makeshift cufflinks and riding on the end of the Space Shuttle's OBSS inspection arm. Parazynski was the first ever spacewalker to use the robotic arm in this way. The spacewalk was regarded as significantly more dangerous than most because of the possibility of shock from the electricity generating solar arrays, the unprecedented usage of the OBSS, and the lack of spacewalk planning and training for the impromptu procedure. Parazynski was, however, able to repair the damage as planned, and the repaired array was fully deployed.

During STS-120, a problem was detected in the starboard Solar Alpha Rotary Joint (SARJ). This joint, together with a similar device on the port side of the station's truss structure, rotates the large solar arrays to keep them facing the Sun. Excessive vibration and high-current spikes in the array drive motor were noted, resulting in a decision to substantially curtail motion of the starboard SARJ until the cause was understood. Inspections during EVAs on STS-120 and STS-123 showed extensive contamination from metallic shavings and debris in the large drive gear and confirmed damage to the large metallic race ring at the heart of the joint. The station had sufficient operating power to carry out its near-term programme with only modest impacts on operations, so to prevent further damage, the joint was locked in place.

On 25 September 2008, NASA announced significant progress in diagnosing the source of the starboard SARJ problem and a programme to repair it on orbit. The repair programme began with the flight of the Space Shuttle Endeavour on STS-126. The crew carried out servicing of both the starboard and port SARJs, lubricating both joints and replacing 11 of 12 Trundle Bearings on the starboard SARJ. It was hoped that this servicing would provide a temporary solution to the problem. A long-term solution is a 10-EVA plan called 'SARJ-XL', which calls for the installation of structural supports between the two segments of the SARJ and a new race ring to be inserted between them to completely replace the failed joint. However, following the cleaning and lubrication of the joint, the results that have been noted so far have been extremely encouraging, to the point that it is now believed that the joint could be maintained by occasional servicing EVAs by resident station crews. Nevertheless, the data from the SARJ will require some time to fully analyse before a decision as to the future of the joint is made.

On 14 January 2009, an incorrect command sequence caused the Zvezda service module orbital altitude maintenance rocket propulsion control system to misfire during an altitude re-boost manoeuvre. This resulted in resonant vibrations into the station structure which persisted for over two minutes. While no damage to the station was immediately reported, some components may have been stressed beyond their design limits. Further analysis confirmed that the station was unlikely to have suffered any structural damage, and it appears that "structures will still meet their normal lifetime capability". Further evaluations are under way.

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Amateur Radio on the International Space Station

Amateur Radio on the International Space Station (ARISS) is a project sponsored by various entities and carried out by astronauts and cosmonauts on the International Space Station who also have an amateur radio license. The program was previously called SAREX, the Space Amateur Radio Experiment, and before that the Shuttle Amateur Radio Experiment. Kenwood Electronics recently launched an advertising campaign capitalising on the fact that their TM-D700A transceiver is currently in use on the ISS.

Amateur radio operators all over the world are able to speak directly to astronauts/cosmonauts via their handheld, mobile, or home radio stations. Low power radios and small antennae can be used to establish communications. It is also possible to send digital data to the space station via laptop computers hooked up to the same radio and antenna, similar to an email communication, except that it uses radio frequencies instead of telephone or cable connections.

The space station occupants work around the clock in shifts, and also take their breaks in shifts, and while on break, many of them will spend some time communicating with "earthlings" via amateur radio.

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List of International Space Station visitors

This is a list of visitors to the International Space Station in alphabetical order. Station crew names are in bold. The suffix (twice) refers to the individual's number of ISS visits, not his or her total number of space flights. Entries without a flag symbol indicate that the person was an American citizen at launch.

As of March 28, 2009, 171 individuals have visited ISS, including the six people currently at the station (ISS Expedition 18 and 19). Note that this list does not double count for individuals with dual citizenship (for example, the Iranian-American space tourist Anousheh Ansari is only listed under the United States).

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Electrical system of the International Space Station

The electrical system of the International Space Station is a critical resource for the International Space Station (ISS) because it allows the crew to live comfortably, to safely operate the station, and to perform scientific experiments. The ISS electrical system uses solar cells to directly convert sunlight to electricity. Large numbers of cells are assembled in arrays to produce high power levels. This method of harnessing solar power is called photovoltaics.

The process of collecting sunlight, converting it to electricity, and managing and distributing this electricity builds up excess heat that can damage spacecraft equipment. This heat must be eliminated for reliable operation of the space station in orbit. The ISS power system uses radiators to dissipate the heat away from the spacecraft. The radiators are shaded from sunlight and aligned toward the cold void of deep space.

Each ISS solar array wing (often abbreviated "SAW") consists of two retractable "blankets" of solar cells with a mast between them. Each wing uses nearly 33,000 solar cells and when fully extended is 35 m (115 ft) long and 12 m (38 ft) wide. When retracted, each wing folds into a solar array blanket box just 51 cm (20 inches) high and 4.57 m (15 ft) long. The ISS now has the full complement of eight solar array wings.

The solar arrays normally track the Sun, with the alpha gimbal used as the primary rotation to follow the Sun as the space station moves around the Earth, and the beta gimbal used to adjust for the angle of the space station's orbit to the ecliptic. Several different tracking modes are used in operations, ranging from full Sun-tracking, to the drag-reduction mode ("Night glider" and "Sun slicer" modes), to a drag-maximization mode used to lower the altitude. See more details in the article at Night Glider mode.

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 batteries, and the battery charge/discharge units (BCDUs), are manufactured by Space Systems/Loral (SS/L), under contract to Boeing.

The power management and distribution subsystem disburses power, as of December 30, 2005, at 160 volts of direct current (abbreviated as "DC") around the station through a series of switches. This voltage may change as the solar arrays degrade over time and the solar arrays' voltage-max-power (Vmp) point changes. This Vmp is the operating voltage at which the arrays provide the most power. The switches that route power throughout the station have built-in microprocessors that are controlled by software and are connected to a computer network running throughout the station.

Eighty-two separate strings, or power lines, lead from each solar array to a sequential shunt unit (SSU) that provides coarse electrical power regulation. The job of the SSU is to shunt, or short, the excess current from the solar array to maintain the desired 160 volt bus voltage. The SSUs are provided by SS/L.

To meet operational requirements, DC-to-DC converter units step down and condition the voltage from 160 to 124.5 volts DC to form a secondary power system to service the loads. By transmitting power at higher voltages and stepping it down to lower voltages where the power is to be used, much like municipal power systems, the station can use smaller wires to transmit this electrical power and thus reduce launch loads. The converters also isolate the secondary system from the primary system and maintain uniform power quality throughout the station.

The station-to-shuttle power transfer system (SSPTS) allows visiting shuttles to derive power from the station to extend their missions. It is on the Pressurized Mating Adapter-2 (PMA-2), at the forward end of the Harmony module where shuttles dock.

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