Station	International Space Station
Station image	STS-134_International_Space_Station_after_undocking.jpg
Station image alt	A rearward view of the ISS backdropped by the limb of the Earth. In view are the station's four large, gold-coloured solar array wings, two on either side of the station, mounted to a central truss structure. Further along the truss are six large, white radiators, three next to each pair of arrays. In between the solar arrays and radiators is a cluster of pressurised modules arranged in an elongated T shape, also attached to the truss. A set of blue solar arrays are mounted to the module at the aft end of the cluster.
Station image size	300px
Extra image size	300px
Extra image caption	The International Space Station on 30 May 2011 as seen from the departing during STS-134.
Insignia	ISS insignia.svg
Insignia size	150px
Insignia caption	ISS Insignia
Insignia alt	A silhouette of the ISS shown orbiting above the Earth. This image is suspended within an orange and purple shield, with the words 'International Space Station' above the image, and laurel leaves beneath.
Sign	Alpha
Crew	6Expedition 28
Launch	1998–2012
Launch pad	Baikonur LC-81/23, LC-1/5KSC LC-39,
Mass	(as of 03/09/2011)
Length	51 m (167.3 ft)from PMA-2 to Zvezda
Width	109 m (357.5 ft)along truss, arrays extended
Height	c. 20 m (c. 66 ft)nadir–zenith, arrays forward–aft(27 November 2009)
Volume	(21 March 2011)
Pressure	101.3 kPa (29.91 inHg, 1 atm)
Perigee	352 km (190 nmi) AMSL(21 March 2011)
Apogee	355 km (192 nmi) AMSL(21 March 2011)
Inclination	51.6 degrees
Speed	7,706.6 m/s(27,743.8 km/h, 17,239.2 mph)
Period	91 minutes
In orbit	()
Occupied	()
Orbits	* 15.69661858595413)}}

() | decay = 2 km/month | NSSDC_ID = 1998-067A | as_of = 9 March 2011(unless noted otherwise) | stats_ref = | configuration_image = ISS configuration 2011-05 en.svg | configuration_alt = The components of the ISS in an exploded diagram, with modules on-orbit highlighted in orange, and those still awaiting launch in blue or pink. | configuration_caption = Station elements (exploded view) | configuration_size = 300px }} The International Space Station (ISS) is an internationally-developed research facility assembled in low Earth orbit and is the largest space station ever constructed. On-orbit construction of the station began in 1998 and is expected to be finished in 2012. The station is expected to remain in operation until at least 2020, and potentially to 2028. Like many artificial satellites, the ISS can be seen from Earth with the naked eye. The ISS serves as a research laboratory that has a microgravity environment in which crews conduct experiments in biology, human biology, physics, astronomy and meteorology. The station has a unique environment for the testing of the spacecraft systems that will be required for missions to the Moon and Mars. The ISS is operated by Expedition crews, and has been continuously staffed since 2 November 2000—an uninterrupted human presence in space for the past . , the crew of Expedition 28 is aboard.

The ISS is a synthesis of several space station projects that includes the American Freedom, the Soviet/Russian Mir-2, the European Columbus and the Japanese Kibō. Budget constraints led to the merger of these projects into a single multi-national programme. The ISS project began in 1994 with the Shuttle-Mir program, and the first module of the station, Zarya, was launched in 1998 by Russia. Since then, pressurised modules, external trusses and other components have been launched by American space shuttles, Russian Proton rockets and Russian Soyuz rockets. , the station consisted of 15 pressurised modules and an extensive integrated truss structure (ITS). The planned final module, the Russian laboratory module, is expected to launch in 2012. Power is provided by 16 solar arrays mounted on the external truss, in addition to four smaller arrays on the Russian modules. The station is maintained at an orbit between and altitude, and travels at an average ground speed of 27,724 km (17,227 mi) per hour, completing 15.7 orbits per day.

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

Purpose

According to the original Memorandum of Understanding between NASA and RSA, the International Space Station was intended to be a laboratory, observatory and factory in space. It was also planned to provide transportation, servicing and act as a staging base for possible future missions to the Moon, Mars and asteroids. In the 2010 United States National Space Policy, the ISS was given additional roles of serving commercial, diplomatic, and educational purposes.

The ISS is a long-term platform in the space environment where extended studies are conducted. The presence of a permanent crew affords the ability to monitor, replenish, repair, and replace experiments and components of the spacecraft itself. The ISS provides a platform to conduct experiments that require one or more of the unusual conditions present on the station. The primary fields of research include human research, space medicine, life sciences, physical sciences, astronomy and meteorology. Scientists on Earth have access to the crew's data and can modify experiments or launch new ones; benefits generally unavailable on unmanned spacecraft. Crews fly expeditions of several months duration, providing approximately 160 man-hours a week of labor with a crew of 6.

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

The ISS provides a location in the relative safety of Low Earth Orbit to test spacecraft systems that will be required for long-duration missions to the Moon and Mars. This provides experience in the maintenance, repair, and replacement of systems on-orbit, which will be essential in operating spacecraft farther from Earth. Mission risks are reduced, and the capabilities of interplanetary spacecraft are advanced. The ESA states that "Whereas the ISS is essential for answering questions concerning the possible impact of weightlessness, radiation and other space-specific factors, other aspects such as the effect of long-term isolation and confinement can be more appropriately addressed via ground-based simulations".

A Mars exploration mission may be a multinational effort involving space agencies and countries outside the current ISS partnership. In 2010 ESA Director-General Jean-Jacques Dordain stated his agency was ready to propose to the other 4 partners that China, India and South Korea be invited to join the ISS partnership. NASA chief Charlie Bolden stated in Feb 2011 "Any mission to Mars is likely to be a global effort". As of 2011, the space agencies of Europe, Russia and China are carrying out the ground-based preparations in the Mars500 project, which complement the ISS-based preparations for a manned mission to Mars. China is planning to launch its own space station in 2011, and has officially initiated its programme for a modular station. However, China has indicated a willingness to cooperate further with other countries on manned exploration.

Another part of the crew's mission is educational outreach and international cooperation. The ISS crews provide opportunities for students on Earth by running student-developed experiments, making educational demonstrations, allowing for student participation in classroom versions of ISS experiments, and directly engaging students using radio, videolink and email. The ISS programme itself allows more than 20 nations to live and work together in space, providing lessons in international cooperation for future multi-national missions. Amateur Radio on the ISS (ARISS) is a volunteer programme which inspires students worldwide to pursue careers in science, technology, engineering and mathematics through amateur radio communications opportunities with the ISS crew. ARISS is an international working group, consisting of delegations from 9 countries including several countries in Europe as well as Japan, Russia, Canada, and the United States. The organization is run by volunteers from the national amateur radio organizations and the international AMSAT (Radio Amateur Satellite Corporation) organizations from each country.

Origins

The International Space Station represents a union of several national space station projects that originated during the Cold War. In the early 1980s, NASA planned to launch a modular space station called Freedom as a counterpart to the Soviet Salyut and Mir space stations, while the Soviets were planning to construct Mir-2 in the 1990s as a replacement for Mir. Because of budget and design constraints, Freedom never progressed past mock-ups and minor component tests and, with the fall of the Soviet Union and the end of the Space Race, Freedom was nearly cancelled by the United States House of Representatives. The post-Soviet economic chaos in Russia in turn led to the cancellation of Mir-2, though only after its base block, DOS-8, had been constructed. Similar budgetary difficulties were faced by other nations with space station projects, which prompted the American government to negotiate with European states, Russia, Japan, and Canada in the early 1990s to begin a collaborative project.

In June 1992, American president George H. W. Bush and Russian president Boris Yeltsin agreed to cooperate on space exploration. The resulting Agreement between the United States of America and the Russian Federation Concerning Cooperation in the Exploration and Use of Outer Space for Peaceful Purposes called for a short, joint space program, with one American astronaut deployed to the Russian space station Mir and two Russian cosmonauts deployed to 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 program as part of an agreement that later included Space Shuttle orbiters docking with Mir. According to the plan, the International Space Station programme would combine the proposed space stations of all participant agencies: NASA's Freedom, the RSA's Mir-2 (with DOS-8 later becoming Zvezda), ESA's Columbus, and the Japanese Kibō laboratory. When the first module, Zarya, was launched in 1998, the station was expected to be completed by 2003. Delays have led to a revised estimated completion date of 2011.

The Russian Orbital Segment is the eleventh Soviet-Russian space station. Mir and the ISS are successors to the Salyut and Almaz stations. Salyut 6 included Soviet crews and cosmonauts from Czechoslovakia, Hungary, Bulgaria, Poland, Romania, Cuba, Mongolia, Vietnam, and East Germany. Salyut 7 included crew from India and France during its almost 9-year lifespan. Mir was visited by crews from a dozen nations during the station's 15-year lifespan, and ISS expands on this international co-operation with crew from more than 14 nations.

Station structure

Assembly

The assembly of the International Space Station, a major endeavour in space architecture, began in November 1998. Russian modules launch and dock robotically, with the exception of Rassvet. All other modules were delivered by space shuttle, which required installation by ISS and shuttle crewmembers using the SSRMS and EVAs; , they had added 159 components during more than 1,000 hours of EVA activity. 127 of these spacewalks originated from the station, while the remaining 32 were launched from the airlocks of docked space shuttles. The beta angle of the station had to be considered at all times during construction, as the station's beta angle is directly related to the percentage of its orbit that the station (as well as any docked or docking spacecraft) is exposed to the sun; the space shuttle would not perform optimally above a limit called the "beta cutoff". Rassvet was delivered by NASA's Atlantis Space Shuttle in 2010 in exchange for the Russian Proton delivery of the United States-funded Russian-built Zarya Module in 1998. Robot arms rather than EVAs were utilized in its installation (docking).

The first segment of the ISS, Zarya, was launched on 20 November 1998 on an autonomous Russian Proton rocket. It provided propulsion, orientation control, communications, electrical power, but lacked long-term life support functions. Two weeks later a passive NASA module Unity was launched aboard Space Shuttle flight STS-88 and attached to Zarya by astronauts during EVAs. This module has two Pressurized Mating Adapters (PMAs), one connects permanently to Zarya, the other allows the space shuttle to dock to the space station. At this time, the Russian station MIR was still inhabited. The ISS remained unmanned for two years, during which time MIR was de-orbited. On July 12, 2000 Zvezda was launched into orbit. Preprogrammed commands onboard deployed its solar arrays and communications antenna. It then became the passive vehicle for a rendezvous with the Zarya and Unity. As a passive "target" vehicle, the Zvezda maintained a stationkeeping orbit as the Zarya-Unity vehicle performed the rendezvous and docking via ground control and the Russian automated rendezvous and docking system. Zarya's computer transferred control of the station to Zvezda's computer soon after docking. Zvezda added sleeping quarters, a toilet, kitchen, CO2 scrubbers, dehumidifier, oxygen generators, exercise equipment, plus data, voice and television communications with mission control. This enabled permanent habitation of the station.

The first resident crew, Expedition 1, arrived in November 2000 on Soyuz TM-31, midway between the flights of STS-92 and STS-97. These two Space Shuttle flights each added segments of the station's Integrated Truss Structure, which provided the station with Ku-band communication for U.S. television, additional attitude support needed for the additional weight of the USOS, and substantial solar arrays supplementing the station's existing 4 solar arrays.

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

The expansion schedule was interrupted by the destruction of the on STS-107 in 2003, with the resulting hiatus in the Space Shuttle program 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, which delivered the station's second set of solar arrays. Several more truss segments and a third set of arrays were delivered on STS-116, STS-117, and STS-118. As a result of the major expansion of the station's power-generating capabilities, more pressurised modules could be accommodated, and the Harmony node and Columbus European laboratory were added. These were followed shortly after by the first two components of Kibō. In March 2009, STS-119 completed the Integrated Truss Structure with the installation of the fourth and final set of solar arrays. The final section of Kibō was delivered in July 2009 on STS-127, followed by the Russian Poisk module. The third node, Tranquility, was delivered in February 2010 during STS-130 by the Space Shuttle Endeavour, alongside the Cupola, closely followed in May 2010 by the penultimate Russian module, Rassvet, delivered by Space Shuttle Atlantis on STS-132. The last pressurised module of the USOS, Leonardo, was brought to the station by Discovery on her final flight, STS-133, followed by the Alpha Magnetic Spectrometer on STS-134, delivered by Endeavour.

, the station consisted of fifteen pressurised modules and the Integrated Truss Structure. Still to be launched are the Russian Multipurpose Laboratory Module Nauka and a number of external components, including the European Robotic Arm. Assembly is expected to be completed by 2012, by which point the station will have a mass in excess of 400 metric tons (440 short tons).

Pressurised modules

When completed in late 2011, the ISS will consist of sixteen pressurised modules with a combined volume of around . These modules include laboratories, docking compartments, airlocks, nodes and living quarters. Fifteen of these components are already in orbit, with the remaining one awaiting launch. Each module was or will be launched either by the Space Shuttle, Proton rocket or Soyuz rocket.

		! Assembly mission	! Launch date	! Launch system	Nation	Isolated view	Notes
	1A/R	20 November 1998	Proton-K	Russia (builder)USA (financier)	>rowspan="2"
rowspan="2"	2A	4 December 1998	Space Shuttle <	, STS-88	USA	>rowspan="2"
colspan="4"
rowspan="2"	1R	12 July 2000	Proton-K	Russia	<		rowspan="2" |
colspan="4">The station's service module, which provides the main living quarters for resident crews, environmental systems and attitude & orbit control. The module also provides additional docking locations for Soyuz spacecraft, Progress spacecraft and the Automated Transfer Vehicle, and its addition rendered the ISS permanently habitable for the first time.
rowspan="2"	5A	7 February 2001	Space Shuttle <	, STS-98	USA	>rowspan="2"
rowspan="2"	7A	12 July 2001	Space Shuttle Atlantis, STS-104	USA	<		rowspan="2" |
colspan="4">The USOS airlock, Quest hosts spacewalks with both United States 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. This module has a separately controlled atmosphere. Crew sleep in this module, breathing a low nitrogen mixture the night before scheduled EVAs, to avoid decompression sickness (known as "the bends") in the low pressure suits.
rowspan="2"	4R	14 September 2001	Soyuz-U, Progress M-SO1	Russia	<		rowspan="2" |
rowspan="2"	10A	23 October 2007	Space Shuttle , STS-120	Europe (builder)USA (operator)	>rowspan="2"
rowspan="2"	1E	7 February 2008	Space Shuttle Atlantis, STS-122	Europe	<		rowspan="2" |
colspan="4">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.
rowspan="2"	1J/A	11 March 2008	Space Shuttle Endeavour, STS-123	Japan	<		rowspan="2" |
rowspan="2"	1J	31 May 2008	Space Shuttle Discovery, STS-124	Japan	>rowspan="2"
rowspan="2"	5R	10 November 2009	Soyuz-U, Progress M-MIM2	Russia	<		rowspan="2" |
rowspan="2"	20A	8 February 2010	Space Shuttle Endeavour, STS-130	Europe (builder)USA (operator)	>rowspan="2"
rowspan="2"	20A	8 February 2010	Space Shuttle Endeavour, STS-130	Europe (builder)<	USA (operator)	>rowspan="2"
rowspan="2"	ULF4	14 May 2010	Space Shuttle Atlantis, STS-132	Russia	<		rowspan="2" |
rowspan="2"	ULF5	24 February 2011	Space Shuttle Discovery, STS-133	Italy (Builder)USA (Operator)	>rowspan="2"
colspan="4"

Launch schedule

		! Assembly mission	! Launch date	! Launch system	Nation	Isolated view	Notes
rowspan="2"	3R	May 2012	Proton-M	Russia	>rowspan="2"

Cancelled modules

Several modules planned for the station have been cancelled over the course of the ISS programme, whether for budgetary reasons, because the modules became unnecessary, or following a redesign of the station after the 2003 Columbia disaster. The cancelled modules include: The United States Centrifuge Accommodations Module for Science experiments, not people, in varying levels of artificial gravity. The United States Habitation Module, which would have served as the station's living quarters. The sleep stations are now spread throughout the station. The United States Interim Control Module and ISS Propulsion Module were intended to replace functions of Zvezda in case of a launch failure. The Russian Universal Docking Module, to which the cancelled Russian Research modules and spacecraft would have docked. The Russian Science Power Platform would have provided the Russian Orbital Segment with a power supply independent of the ITS solar arrays. Two Russian Research Modules that were planned to be used for scientific research.

Unpressurised elements

The ISS features a large number of external components that do not require pressurization. The largest such component is the Integrated Truss Structure (ITS), to which the station's main solar arrays and thermal radiators are mounted. The ITS consists of ten separate segments forming a structure 108.5 m (356 ft) long.

The ITS serves as a base for the main remote manipulator system called the Mobile Servicing System (MSS). This consists of the Mobile Base System (MBS), the Canadarm2, and the Special Purpose Dexterous Manipulator. The MBS rolls along rails built into some of the ITS segments to allow the arm to reach all parts of the United States segment of the station. The MSS had its reach increased by an Enhanced Orbiter Boom Sensor System, installed by Astronauts in EVA during the STS-134 mission in May, 2011. To gain access to the extreme extents of the Russian Segment the crew also placed a PDGF to the forward docking section of Zarya, so that the SSRMS may inchworm itself onto that point

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

The station in its complete form has several smaller external components, such as the three External Stowage Platforms (ESPs), launched on STS-102, STS-114 and STS-118 as well as four ExPrESS Logistics Carriers (ELCs). ELCs 1 and 2 were delivered on STS-129 in November 2009. ELC 4 was installed on February 2011 by STS-133 and ELC 3 by STS-134 in May 2011. Whilst these platforms allow experiments (including MISSE, the STP-H3 and the Robotic Refuelling Mission) to be deployed and conducted in the vacuum of space such as by providing the necessary electricity and computing to process experimental data locally, the platforms' primary function is to support Orbital Replacement Units (ORUs). ORUs are key elements of the ISS that can be readily replaced when the unit either passes its design life or fails. Examples of ORUs include pumps, storage tanks, antennas and battery units. Such units are replaced either by astronauts during EVA or by the SPDM. While spare parts/ORUs were routinely transported to and from the station via space shuttle resupply missions, there was a heavy emphasis on ORU transport once the station approached completion. Several shuttle missions were dedicated to the delivery of ORUs, including STS-129, STS-133 and STS-134. To date only one other mode of transportation of ORUs has been utilised - the Japanese cargo vessel HTV-2 - which delivered an FHRC and CTC-2 via its Exposed Pallet (EP).

There are also smaller exposure facilities mounted directly to laboratory modules; the JEM Exposed Facility serves as an external 'porch' for the Japanese Experiment Module complex, and a facility on the European Columbus laboratory provides power and data connections for experiments such as the European Technology Exposure Facility and the Atomic Clock Ensemble in Space. A remote sensing instrument, SAGE III-ISS, is due to be delivered to the station in 2014 aboard a Dragon capsule. The largest such scientific payload externally mounted to the ISS is the Alpha Magnetic Spectrometer (AMS), a particle physics experiment, was launched on STS-134 in May 2011, and mounted externally on the ITS. The AMS will measure cosmic rays and look for evidence of dark matter and antimatter.

Power supply

Photovoltaic (PV) arrays power the ISS. The Russian segment of the station, like the space shuttle and most aircraft, uses 28 volt DC partly provided by four solar arrays mounted directly to Zarya and Zvezda. The rest of the station uses 130–180 V DC from the United States PV array.

The United States solar arrays are arranged as four wing pairs, with each wing producing nearly 32.8 kW. These arrays normally track the sun to maximise power generation. Each array is about 375 m² (450 yd²) in area and long. In the complete configuration, the solar arrays track the sun by rotating the alpha gimbal once per orbit while the beta gimbal follows slower changes in the angle of the sun to the orbital plane. The Night Glider mode aligns the solar arrays parallel to the ground at night to reduce the significant aerodynamic drag at the station's relatively low orbital altitude.

The station uses rechargeable nickel-hydrogen batteries (NiH₂) for continuous power during the 35 minutes of every 90 minute orbit that it is eclipsed by the Earth. The batteries are recharged on the day side of the earth. They have a 6.5 year lifetime (over 37,000 charge/discharge cycles) and will be regularly replaced over the anticipated 20-year life of the station.

Power is stabilised and distributed at 160 V DC and converted to the user-required 124 V DC. The higher distribution voltage allows smaller, lighter conductors. The two station segments share power with converters, essential since the Columbia disaster forced the cancellation of the Russian Science Power Platform and Jaxa centrifuge modules.

Thermal Control System

The International Space Station (ISS) External Active Thermal Control System (EATCS) maintains an equilibrium when the ISS environment or heat loads exceed the capabilities of the Passive Thermal Control System (PTCS). Note Elements of the PTCS are external surface materials, insulation such as MLI, or Heat Pipes. The EATCS provides heat rejection capabilities for all the US pressurised modules, including the JEM and COF as well as the main power distribution electronics of the S0, S1 and P1 Trusses. The EATCS consists of two independent Loops (Loop A & Loop B), they both use mechanically pumped Ammonia in fluid state, in closed-loop circuits. The EATCS is capable of rejecting up to 70 kW, and provides a substantial upgrade in heat rejection capacity from the 14 kW capability of the Early External Active Thermal Control System (EEATCS) via the Early Ammonia Servicer (EAS), which was launched on STS-105 and installed onto the P6 Truss.

Orbit control

The ISS is maintained in a nearly circular orbit with a minimum mean altitude of 278 km (173 mi) and a maximum of 460 km (286 mi). It travels at an average speed of 27,724 kilometres (17,227 mi) per hour, and completes 15.7 orbits per day. The normal maximum altitude is 425 km (264 mi) to allow NASA Shuttle rendezvous missions. It is likely that, with the retirement of the shuttle, the nominal orbit of the space station will be raised in altitude. As the ISS constantly loses altitude because of a slight atmospheric drag, it needs to be boosted to a higher altitude several times each year. This boost 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) for the boost to a higher altitude to be completed.

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 allow station-keeping to be done more economically than at present. The station's navigational position and velocity, or state vector, is independently established using the United States Global Positioning System (GPS) and a combination of state vector updates from Russian Ground Sites and the Russian GLONASS system.

The Russian orbital segment handles Guidance, Navigation & Control for the entire Station. Initially, Zarya, the first module of the station, controlled the station until a short time after the Russian service module Zvezda docked and was transferred control. Zvezda contains the ESA built DMS-R Data Management System. The attitude (orientation) of the station is independently determined by a set of sun, star and horizon sensors on Zvezda and the United States GPS with antennas on the S0 truss and a receiver processor in the United States lab. The attitude knowledge is propagated between updates by rate sensors. Attitude control is maintained by either of two mechanisms; normally, a system of four control moment gyroscopes (CMGs) keeps the station oriented, with Destiny forward of Unity, the P truss on the port side, and Rassvet on the Earth-facing (nadir) side. Once, during Expedition 10, an incorrect command was sent to the station's computer, and the CMG system became 'saturated' (when the set of CMGs exceed their operational range or cannot track a series of rapid movements) Attitude control was automatically taken over by the Russian Attitude Control System thrusters for about one orbit, using about 14 kilograms of propellant before the fault was noticed and fixed. Thrusters are deactivated during EVA's for crew safety. When a space shuttle or Soyuz is docked to the station, it can also be used to maintain station attitude such as for troubleshooting. Shuttle control was used exclusively during installation of the S3/S4 truss, which provides electrical power and data interfaces for station's electronics.

Communications & computers

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

The Russian Orbital Segment communicates directly with the ground via the Lira antenna mounted to Zvezda. The Lira antenna also has the capability to use the Luch data relay satellite system. This system, used for communications with Mir, fell into disrepair during the 1990s, and as a result is no longer in use, although two new Luch satellites—Luch-5A and Luch-5B—are planned for launch in 2011 to restore the operational capability of the system. Another Russian communications system is the Voskhod-M, which enables internal telephone communications between Zvezda, Zarya, Pirs, Poisk and the USOS, and also provides a VHF radio link to ground control centres via antennas on Zvezda's exterior.

The US Orbital Segment (USOS) makes use of two separate radio links mounted in the Z1 truss structure: the S band (used for audio) and K_u band (used for audio, video and data) systems. These transmissions are routed via the United States Tracking and Data Relay Satellite System (TDRSS) in geostationary orbit, which allows for almost continuous real-time communications with NASA's Mission Control Center (MCC-H) in Houston. Data channels for the Canadarm2, European Columbus laboratory and Japanese Kibō modules are routed via the S band and K_u 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. UHF is employed by other spacecraft that dock to or undock from the station, such as Soyuz, Progress, HTV, ATV and the Space Shuttle (except the shuttle also makes use of the S band and K_u band systems via TDRSS), to receive commands from Mission Control and ISS crewmembers. Automated spacecraft are fitted with their own communications equipment; the ATV uses a laser attached to the spacecraft and equipment attached to Zvezda, known as the Proximity Communications Equipment, to accurately dock to the station.

The ISS is equipped with approximately 100 IBM and Lenovo ThinkPad model A31 and T61P laptop computers. Each computer is a commercial off-the-shelf purchase which is then modified for safety and operation including updates to connectors, cooling and power to accommodate the station's 28V DC power system and weightless environment. As of September 2000, the ThinkPad is the only laptop certified for long duration flight aboard the ISS though Mac and other laptop computers have been used aboard the ISS for specific experiments. All laptops aboard the ISS are connected to the station's wireless LAN via Wi-Fi and are connected to the ground at 3 Mbit/s up and 10 Mbit/s down, comparable to home DSL connection speeds.

Life support

The ISS Environmental Control and Life Support System (ECLSS) provides or controls atmospheric pressure, fire detection and suppression, oxygen levels, waste management and water supply. The highest priority for the ECLSS is the ISS atmosphere, but the system also collects, processes, and stores waste and water produced and used by the crew—a process that recycles fluid from the sink, toilet, and condensation from the air. The Elektron system aboard Zvezda and a similar system in Destiny generate oxygen aboard the station. 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 similar to the Earth's. 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 a fire such as that responsible for the deaths of the Apollo 1 crew.

Microgravity

Gravity is the only significant force acting upon the ISS, which is in constant freefall. This state of freefall, or perceived weightlessness, is not perfect however, being disturbed by four separate effects:

The drag resulting from the residual atmosphere. When the ISS enters the earth's shadow, the main solar panels are rotated to minimize this aerodynamic drag, helping reduce orbital decay.

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 center of mass will tend to follow its own orbit. That is, parts on the underside, closer to the earth are pulled harder, towards the earth. Conversely, parts on the top of the station, further from earth, try to fling off into space. However, as each point is physically part of the station, this is impossible, and so each component is subject to small forces which keep them attached to the station as it orbits. This is also called the tidal force.

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

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

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

Sightings

Before sunrise or after sunset, the ISS can appear to observers on the ground, with the naked eye as a slow moving, bright, white dot, slowly crossing the sky in 2 to 5 minutes. This happens when after sunset or before sunrise the ISS is still sunlit, which is typically the case up to a few hours after sunset or before sunrise. Because of its size, the ISS is the brightest man made object in the sky, with an approximate brightness of magnitude -4 when overhead, similar to Venus. The ISS can also produce flares as sunlight glints off reflective surfaces as it orbits of up to 8 or 16 times the brightness of Venus. The ISS is also visible during broad daylight conditions, albeit with much much more effort.

Tools are provided by a number of websites such as Heavens-Above as well as smartphone applications that use the known orbital data and the observer's longitude and latitude to predict when the ISS will be visible (weather permitting), where the station will appear to rise to the observer, the altitude above the horizon it will reach and the duration of the pass before the station disappears to the observer either by setting below the horizon or entering into Earth's shadow.

The ISS orbits at an inclination of 51.6 degrees to Earth's equator, necessary to ensure that Russian Soyuz and Progress spacecraft launched from the Baikonur Cosmodrome may be safely launched to reach the station. While this orbit makes the station visible from 95% of the inhabited land on Earth, it is not visible from extreme northern or southern latitudes.

Life on board

Crew schedule

The time zone used on board the ISS is Coordinated Universal Time (UTC). 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 before starting work at around 08:10. The first scheduled exercise of the day follows, after which the crew continues work until 13:05. Following a one-hour lunch break, the afternoon consists of more exercise and work before the crew carries out its pre-sleep activities beginning at 19:30, including dinner and a crew conference. The scheduled sleep period begins at 21:30. In general, the crew works ten hours per day on a weekday, and five hours on Saturdays, with the rest of the time their own for relaxation or work catch-up.

Food and drink

Most of the food eaten by station crews is stored frozen, refrigerated or canned. Menus are prepared by the astronauts, with the help of a dietitian, before the astronauts' flight to 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 individual food packages and cooks them using the onboard galley, which features two food warmers, a refrigerator, and a water dispenser that provides both heated and unheated water. Drinks are provided in dehydrated powder form and are mixed with water before consumption. Drinks and soups are sipped from plastic bags with straws, while solid food is eaten with a knife and fork, which are attached to a tray with magnets to prevent them from floating away. Any food that does float away, including crumbs, must be collected to prevent it from 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 these adverse physiological effects, the station is equipped with two treadmills (including the COLBERT), the aRED (advanced Resistive Exercise Device) which enables various weightlifting exercises, and a stationary bicycle; each astronaut spends at least two hours per day exercising on the equipment. Astronauts use bungee cords to strap themselves to the treadmill. 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.

Sleeping in space

The station provides crew quarters for each member of the expedition's crew, with two 'sleep stations' in the Zvezda and four more installed in Harmony. The American quarters are private, approximately person-sized soundproof booths. The Russian crew quarters include a small window, but don't provide the same amount of ventilation or block the same amount of noise as their American counterparts. A crewmember can sleep in a crew quarter in a tethered sleeping bag, listen to music, use a laptop, and store personal items 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 have no allocated sleep module, and attach a sleeping bag to an available space on a wall—it is possible to sleep floating freely through the station, but this is generally avoided because of the possibility of bumping into sensitive equipment. It is important that crew accommodations be well ventilated; otherwise, astronauts can wake up oxygen-deprived and gasping for air, because a bubble of their own exhaled carbon dioxide has formed around their heads.

Hygiene

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

There are two space toilets on the ISS, both of Russian design, located in Zvezda and Tranquility. 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 away. Solid waste is collected in individual 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.

Station operations

Expeditions

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

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

The International Space Station is the most-visited spacecraft in the history of space flight. , it had received 297 visitors (196 different people). Mir had 137 visitors (104 different people).

Visiting spacecraft

Spacecraft (or 'visiting vehicles') from four different space agencies visit the ISS, serving a variety of purposes. The Automated Transfer Vehicle from the European Space Agency, the Russian Roskosmos Progress spacecraft and the HTV 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 United States serviced the ISS through its Space Shuttle programme, providing resupply missions, assembly and logistics flights, and crew rotation. , there have been 25 Soyuz, 41 Progress, 2 ATV, 2 HTV and 35 space shuttle flights to the station. Expeditions require, on average, of supplies, and , crews had consumed a total of around . 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.

Following the retirement of the Space Shuttle, a number of other spacecraft are expected to fly to the station. Two, the Orbital Sciences Cygnus and SpaceX Dragon, will fly under NASA's Commercial Orbital Transportation Services and Commercial Resupply Services contracts, delivering cargo to the station until at least 2015.

Docking

Spacecraft from Russia and Europe are able to launch, fly and dock themselves without human intervention. This includes both Russian manned and unmanned spacecraft. American craft are manually docked, while Japanese craft are berthed with the use of manually controlled robot arms. Russian and European Supply craft can remain at the ISS for 6 months, allowing great flexibility in crew time for loading and unloading of supplies and trash. Japanese spacecraft berth for 1–2 months. American space shuttles can remain in space for up to 17 days, the longest docking being 11 days.

The American Manual approach to docking allows greater initial flexibility and less complexity. The downside to this mode of operation is that each mission becomes unique and requires specialized training and planning, making the process more labor-intensive and expensive. The Russians pursued an automated methodology that used the crew in override or monitoring roles. Although the initial development costs were high, the system has become very reliable with standardizations that provide significant cost benefits in repetitive routine operations. The Russian approach allows assembly of space stations orbiting other worlds in preparation for manned missions. The Nauka module of the ISS will be used in the 12th Russian(/Soviet) space station, OPSEK, whose main goal is supporting manned deep space exploration.

Currently docked

, there are 3 spacecrafts docked with the ISS and the following spacecrafts have the ability to dock with the ISS: Soyuz, Progress, ATV and HTV. The docking of STS-135 was last time that a Space Shuttle docked with the ISS.

!	Spacecraft	Mission	Docking port	Docked (UTC)	Undocking (UTC)	Notes
	Soyuz TMA-21			6 April 2011 23:09	September 2011
	Progress M-10M	Progress 42 Cargo		29 April 2011 14:29	25 October 2011
	Soyuz TMA-02M			9 June 2011 21:18	November 2011

From 26 February 2011 to 7 March 2011, during the docked phase of STS-133, four of the governmental partners (United States, ESA, Japan and Russia) had their current visiting vehicles (Space Shuttle, ATV, HTV, Progress and Soyuz) docked at the ISS at one time, the only time this has happened to date.

Docking schedule

All dates are UTC. Dates are the earliest possible dates and may change. Forward ports are at the front of the station according to its normal direction of travel and orientation (attitude). Aft is at the rear of the station, used by spacecraft boosting the station's orbit. Nadir is closest the earth, Zenith is on top.

!	Spacecraft	Launch	Mission	Planned Docking (UTC)	Docking port	Notes
	Soyuz TMA-22	30 September 2011.		2 October 2011
		26 October 2011	Progress 45 Cargo	28 October 2011
	Soyuz TMA-03M	30 November 2011.		1 December 2011
	Dragon C2	30 November 2011	Dragon Demo	7 December 2011
		27 December 2011	Progress 46 Cargo	29 December 2011
		18 February 2012	HTV-3 Cargo	23 February 2012
		23 February 2012	Cygnus 1 Cargo	TBD	Harmony nadir
		7 March 2012	ATV-3 Cargo	15 March 2012
	Soyuz TMA-04M	30 March 2012.		TBD
		12 April 2012	Dragon 1 Cargo	TBA	TBD

Mission control centres

The components of the ISS are operated and monitored by their respective space agencies at control centres across the globe, including: Roskosmos's Mission Control Center at Korolyov, Moscow Oblast, controls the Russian Orbital Segment which handles Guidance, Navigation & Control for the entire Station., in addition to individual Soyuz and Progress missions and main relif mission contorl to the United States segments in case the Lyndon B. Johnson Space Center is evacuated like in Hurricane Rita in 2005 and Hurricane Ike in 2008. ESA's ATV Control Centre, at the Toulouse Space Centre (CST) in Toulouse, France, controls flights of the unmanned European Automated Transfer Vehicle. JAXA's JEM Control Centre and HTV Control Centre at Tsukuba Space Centre (TKSC) in Tsukuba, Japan, are responsible for operating the Japanese Experiment Module complex and all flights of the 'White Stork' HTV Cargo spacecraft, respectively. NASA's Mission Control Center at Lyndon B. Johnson Space Center in Houston, Texas, serves as the primary control facility for the United States segment of the ISS and also controls the Space Shuttle missions that visit the station. NASA's Payload Operations and Integration Center at Marshall Space Flight Center in Huntsville, Alabama, serves as the centre that coordinates all payload operations in the United States Segment. ESA's Columbus Control Centre at the German Aerospace Centre (DLR) in Oberpfaffenhofen, Germany, controls the European Columbus research laboratory. CSA's MSS Control at Saint-Hubert, Quebec, Canada, controls and monitors the Mobile Servicing System, or Canadarm2.

Utilisation rights

The Russian part of the station is operated and controlled by the Russian Federation's space agency and provides Russia with the right to nearly one-half of the crew time for the ISS. The allocation of remaining crew time (three to four crew members of the total permanent crew of six) and hardware within the other sections of the station has been assigned as follows: Columbus: 51% for the ESA, 46.7% for NASA, and 2.3% for CSA. 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.

Safety aspects

Orbital debris

At the low altitudes at which the ISS orbits there is a variety of space debris, consisting of many different objects including entire spent rocket stages, dead satellites, explosion fragments—including materials from anti-satellite weapon tests, paint flakes, slag from solid rocket motors, coolant released by RORSAT nuclear powered satellites and some of the 480,000,000 small needles from the American military Project West Ford. These objects, in addition to natural micrometeoroids, are a significant threat to life for the crew, and the station's structure, despite their small size, because of their kinetic energy and direction in relation to the station. Debris poses a risk to spacewalking crew, as such objects could puncture their spacesuits, causing them to depressurise.

Space debris objects are tracked remotely from the ground, and the station crew can be notified of many objects with sufficient size to cause damage on impact. This allows for a Debris Avoidance Manoeuvre (DAM) to be conducted, which uses thrusters on the Russian Orbital Segment to alter the station's orbital altitude, avoiding the debris. DAMs are not uncommon, taking place if computational models show the debris will approach within a certain threat distance. Eight DAMs had been performed prior to March 2009, the first seven between October 1999 and May 2003. Usually the orbit is raised by one or two kilometres by means of an increase in orbital velocity of the order of 1 m/s. Unusually there was a lowering of 1.7 km on 27 August 2008, the first such lowering for 8 years. There were two DAMs in 2009, on 22 March and 17 July. If a threat from orbital debris is identified too late for a DAM to be safely conducted, the station crew close all the hatches aboard the station and retreat into their Soyuz spacecraft, so that they would be able to evacuate in the event it was damaged by the debris. This partial station evacuation has occurred twice, on 13 March 2009 and 28 June 2011.

Space environment

The space environment is hostile to life. Unprotected presence in space is characterised by an intense radiation field (consisting primarily of protons and other subatomic charged particles from the solar wind, in addition to cosmic rays), high vacuum, extreme temperatures, and microgravity. Some simple forms of life including Tardigrades can survive in this environment in a desiccated state.

The ISS is partially protected from this environment by the Earth's magnetic field. From an average distance of about 70,000 km, depending on Solar activity, the magnetosphere begins to deflect solar wind around the Earth and ISS. However, solar flares are still a hazard to the crew, who may receive only a few minutes warning. The crew of Expedition 10 took shelter as a precaution in 2005 in a more heavily shielded part of the ROS designed for this purpose during the initial 'proton storm' of an X-3 class solar flare.

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

Despite efforts to improve radiation shielding on the ISS compared to previous stations such as Mir, radiation levels within the station have not been vastly reduced, and it is thought that further technological advancement will be required to make long-duration human spaceflight further into the Solar System a possibility. Large, acute doses of radiation from Coronal Mass Ejection can cause radiation sickness and can be fatal. Without the protection of the Earth's magnetosphere, interplanetary manned missions are especially vulnerable.

The radiation levels experienced on ISS are about 5 times greater than those experienced by airline passengers and crew. The Earth's electromagnetic field provides almost the same level of protection against solar and other radiation in low Earth orbit as in the stratosphere. Airline passengers, however, experience this level of radiation for no more than 15 hours for the longest transcontinental flights. For example, on a 12 hour flight an airline passenger would experience 0.1 millisievert of radiation, or a rate of 0.2 millisieverts per day; only 1/5 the rate experienced by an astronaut in LEO.

Anomalies

Unexpected problems and failures have impacted the station's assembly time-line and work schedules leading to periods of reduced capabilities and, in some cases, could have forced abandonment of the station for safety reasons, had these problems not been resolved.

During STS-120 on 2007, following the relocation of the P6 truss and solar arrays, it was noted during the redeployment of the array that it had become torn and was not deploying properly. An EVA was carried out by Scott Parazynski, assisted by Douglas Wheelock, the men took extra precautions to reduce the risk of electric shock, as the repairs were carried out with the solar array exposed to sunlight. The issues with the array were followed in the same year by problems with the starboard Solar Alpha Rotary Joint (SARJ), which rotates the arrays on the starboard side of the station. 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, and so the joint was locked to prevent further damage. Repairs to the joint were carried out during STS-126 with lubrication of both joints and the replacement 11 of 12 trundle bearings on the joint.

More recently, problems have been noted with the station's engines and cooling. In 2009, the engines on Zvezda were issued an incorrect command which caused excessive vibrations to propagate throughout 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". 2009 also saw damage to the S1 radiator, one of the components of the station's cooling system. The problem was first noticed in Soyuz imagery in September 2008, but was not thought to be serious. The imagery showed that the surface of one sub-panel has peeled back from the underlying central structure, possibly due to micro-meteoroid or debris impact. It is also known that a Service Module thruster cover, jettisoned during an EVA in 2008, had struck the S1 radiator, but its effect, if any, has not been determined. On 15 May 2009 the damaged radiator panel's ammonia tubing was mechanically shut off from the rest of the cooling system by the computer-controlled closure of a valve. The same valve was used immediately afterwards to vent the ammonia from the damaged panel, eliminating the possibility of an ammonia leak from the cooling system via the damaged panel.

Early on 1 August 2010, a failure in cooling Loop A (starboard side), one of two external cooling loops, left the station with only half of its normal cooling capacity and zero redundancy in some systems. The problem appeared to be in the ammonia pump module that circulates the ammonia cooling fluid. Several subsystems, including two of the four CMGs, were shut down.

Planned operations on the ISS were interrupted through a series of EVAs to address the cooling system issue. A first EVA on 7 August 2010, to replace the failed pump module, was not fully completed due to an ammonia leak in one of four quick-disconnects. A second EVA on 11 August successfully removed the failed pump module. A third EVA was required to restore Loop A to normal functionality.

The USOS's cooling system is largely built by the American company Boeing, which is also the manufacturer of the failed pump.

An air leak from the USOS in 2004, the venting of smoke from an Elektron oxygen generator in 2006, and the failure of the computers in the ROS in 2007 during STS-117 which left the station without thruster, Elektron, Vozdukh and other environmental control system operations, the root cause of which was found to be condensation inside the electrical connectors leading to a short-circuit.

Politics

Legal aspects and costs

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

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

In addition to these main intergovernmental agreements, Brazil originally joined the programme as a bilateral partner of the United States by a contract with NASA to supply hardware. In return, NASA would provide Brazil with access to its ISS facilities on-orbit, as well as a flight opportunity for one Brazilian astronaut during the course of the ISS programme. However, due to cost issues, the subcontractor Embraer was unable to provide the promised ExPrESS pallet, and Brazil left the 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. The Chinese, who have their own space station, Project 921-2, scheduled for launch late in 2011, have reportedly expressed interest in the project, especially if it would be able to work with the RKA. Chinese manned spacecraft and space stations have Russian compatible docking systems. However, China remains uninvolved. The heads of both the South Korean and Indian space agency ISRO announced at the first plenary session of the 2009 International Astronautical Congress that their nations intend to join the ISS programme, with talks due to begin in 2010. The heads of agency also expressed support for extending ISS lifetime. European countries not part of the programme will be allowed access to the station in a three-year trial period, ESA officials say.

The cost estimates for the entire ISS programme range from 35 billion to 160 billion US$. ESA the only partner agency which publishes global costs for the ISS, estimates €100 billion for the entire station over 30 years. This overall cost comprises contributions from all partner agencies:

The NASA budget for 2007 estimates costs for the ISS (excluding space shuttle costs) at US$25.6 billion for the years 1994 to 2005, with the annual United States contribution increasing from 2010 to US$2.3 billion. This level is likely to remain relatively constant until around 2017. Based on costs incurred plus a projected $2.5 Billion per year from 2011–2017, NASA spending since 1993 comes to approximately US$53 billion. An additional 33 Shuttle assembly and supply flights equates to $35 Billion. With addition costs from development of Space Station Freedom, NASA's contribution comes to approximately US$100 billion. ESA spending on a 30-year projected station lifespan is €8 billion, consisting of Columbus development (€1 Billion) plus ATV costs including ATV's development up until ATV-1 (€1.35 billion), subsequent ATV launches (€875 Million for each of four spacecraft) and Ariane 5 launch costs (€125 million each), giving ATV total costs of €2.85 billion. JAXA's costs include the Kibō laboratory (¥7,100 billion), consisting of development (c.¥250 billion), equipment development(¥450 billion), totalling approximately ¥2360 billion in costs and expenses of shuttle launches, plus operating costs of US$350–400 million annually. Other costs include HTV development (¥68 billion) and launch costs (c.¥250 billion), plus astronaut training, ground facilities and experiment-related expenses totalling approximately ¥110 billion. This gives total annual costs to JAXA of about ¥400 billion yen. RSA costs are difficult to determine as substantial development costs of the Progress spacecraft, Soyuz spacecraft and Proton rockets used for module launches, are spread across previous Soviet rocket programmes. Cost of development for module design such as DOS base blocks, life support and docking systems are spread across the budgets of the Salyut, Almaz, and Mir 1 and 2 programmes. Russian Prime Minister Vladimir Putin stated in Januray 2011 that the government will spend 115 billion rubles (US$3.8 billion) on national space programmes in 2011, however this includes the entire space programme which will launch a spacecraft on average once per week during 2011. CSA spending over the last 20 years is estimated at CA$1.4 Billion, including development of the Canadarm2 and SPDM.

Criticism

Critics of the ISS contend that the time and money spent on the ISS could be better spent on other projects—whether they be robotic spacecraft missions, space exploration, investigations of problems on Earth, colonisation of Mars, or tax savings.

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

End of mission and deorbit

As recently as 2009, NASA had stated plans to end the ISS programme and deorbit the ISS in early 2016. This was in accordance with the then-President Bush's policy. President Obama announced new policy in 2010, extending the programme through 2020.

All five ISS-participating space agencies had indicated in 2010 their desire to see the platform continue flying beyond 2015, but Europe struggled to agree on funding arrangements within its member states, until agreement was reached in March 2011. Russia and ISS partners in a 2011 statement said that work is being done to make sure other modules can be used beyond 2015. So far, the partners have only manifested missions through about 2015. The first Russian module was launched in 1998, and the 30th anniversary of that module's launch has been chosen as a target date for certification of all components of the ISS.

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

According to the Outer Space Treaty the United States is legally responsible for all modules it has launched. In ISS planning, NASA examined options including returning the station to Earth via shuttle missions (deemed too expensive, as the station is not designed for disassembly and this would require at least 27 shuttle missions), natural orbital decay with random reentry similar to Skylab, boosting the station to a higher altitude (which would simply delay reentry) and a controlled targeted de-orbit to a remote ocean area.

The technical feasibility of a controlled targeted deorbit into a remote ocean was found to be within the capability of the ISS, only with the United States combining its resources with Russia. At the time ISS was launched, the Russian Space Agency had experience from de-orbiting the Salyut 4, 5, 6, and 7 space stations, while NASA's first intentional controlled de-orbit of a satellite (the Compton Gamma Ray Observatory) would not occur for another two years. NASA currently has no spacecraft capable of de-orbiting the ISS at the time of decommissioning.

While the entire USOS cannot be reused and will be discarded, final decisions are still to be made on which ROS modules will be used in OPSEK, and which modules will be discarded. Pirs is to be de-orbited before the decommissioning of the ISS and Nauka will be re-used.

References

External links

;Official International Space Station webpages of the participating space agencies

NASA

RSC Energia

Russian Federal Space Agency

Canadian Space Agency

European Space Agency

Japanese Space Agency

Italian Space Agency

Daily ISS reports

;Interactive and multimedia

NASA's ISS interactive reference guide

NASA's ISS image gallery search page

Current position of the ISS

ISS and satellite tracking website

ISS Webcam

Animation showing ISS assembly sequence

;Experiments and science

ESA – Columbus

JAXA – Space Environment Utilization and Space Experiment

NASA – Station Science

RSC Energia – Science Research on ISS Russian Segment

Category:Artificial satellites orbiting Earth Category:Human spaceflight Category:Joint ventures Category:Manned spacecraft Category:Populated places Category:Spaceflights Category:Space stations

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