International Space Station

International Space Station

The International Space Station on 23 May 2010 as seen from the departing Space Shuttle Atlantis during STS-132.
Station statistics
COSPAR ID	1998-067A
SATCAT №	25544
Call sign	Alpha, Station
Crew	Fully crewed: 6 Currently aboard: 6 (Expedition 52)
Launch	20 November 1998 (1998-11-20)
Launch pad	Baikonur 1/5 and 81/23 Kennedy LC-39
Mass	≈ 419,455 kg (924,740 lb)[1]
Length	72.8 m (239 ft)
Width	108.5 m (356 ft)
Height	≈ 20 m (66 ft) nadir–zenith, arrays forward–aft (27 November 2009)
Pressurised volume	931.57 m3 (32,898 cu ft)[2] (28 May 2016)
Atmospheric pressure	101.3 kPa (29.9 inHg; 1.0 atm)
Perigee	401.1 km (249.2 mi) AMSL[3]
Apogee	408.0 km (253.5 mi) AMSL[3]
Orbital inclination	51.64 degrees[3]
Orbital speed	7.67 km/s[3] (27,600 km/h; 17,200 mph)
Orbital period	92.65 minutes[3]
Orbits per day	15.54[3]
Orbit epoch	7 July 2017, 13:10:09 UTC[3]
Days in orbit	18 years, 8 months, 30 days (19 August 2017)
Days occupied	16 years, 9 months, 17 days (19 August 2017)
No. of orbits	102,491 as of July 2017[3]
Orbital decay	2 km/month
Statistics as of 9 March 2011 (unless noted otherwise) References: [1][3][4][5][6][7]
Configuration
Station elements as of June 2017 (exploded view)

The International Space Station (ISS) is a space station, or a habitable artificial satellite, in low Earth orbit. Its first component launched into orbit in 1998, and the ISS is now the largest human-made body in low Earth orbit and can often be seen with the naked eye from Earth.^[8][9] The ISS consists of pressurised modules, external trusses, solar arrays, and other components. ISS components have been launched by Russian Proton and Soyuz rockets, and American Space Shuttles.^[10]

The ISS serves as a microgravity and space environment research laboratory in which crew members conduct experiments in biology, human biology, physics, astronomy, meteorology, and other fields.^[11][12][13] The station is suited for the testing of spacecraft systems and equipment required for missions to the Moon and Mars.^[14] The ISS maintains an orbit with an altitude of between 330 and 435 km (205 and 270 mi) by means of reboost manoeuvres using the engines of the Zvezda module or visiting spacecraft. It completes 15.54 orbits per day.^[15]

The ISS is the ninth space station to be inhabited by crews, following the Soviet and later Russian Salyut, Almaz, and Mir stations as well as Skylab from the US. The station has been continuously occupied for 7003613400000000000♠16 years and 290 days since the arrival of Expedition 1 on 2 November 2000. This is the longest continuous human presence in low Earth orbit, having surpassed the previous record of 7003364400000000000♠9 years and 357 days held by Mir. The station is serviced by a variety of visiting spacecraft: the Russian Soyuz and Progress, the American Dragon and Cygnus, the Japanese H-II Transfer Vehicle,^[16] and formerly the Space Shuttle and the European Automated Transfer Vehicle. It has been visited by astronauts, cosmonauts and space tourists from 17 different nations.^[17]

After the US Space Shuttle programme ended in 2011, Soyuz rockets became the only provider of transport for astronauts at the International Space Station, and Dragon became the only provider of bulk cargo return to Earth (called downmass). Soyuz has very limited downmass capability.

The ISS programme is a joint project among five participating space agencies: NASA, Roscosmos, JAXA, ESA, and CSA.^[16][18] The ownership and use of the space station is established by intergovernmental treaties and agreements.^[19] The station is divided into two sections, the Russian Orbital Segment (ROS) and the United States Orbital Segment (USOS), which is shared by many nations. As of January 2014, the American portion of ISS is being funded until 2024.^[20][21][22] Roscosmos has endorsed the continued operation of ISS through 2024^[23] but has proposed using elements of the Russian Orbital Segment to construct a new Russian space station called OPSEK.^[24]

On 28 March 2015, Russian sources announced that Roscosmos and NASA had agreed to collaborate on the development of a replacement for the current ISS.^[25][26] NASA later issued a guarded statement expressing thanks for Russia's interest in future co-operation in space exploration but fell short of confirming the Russian announcement.^[27][28]

Purpose

According to the original Memorandum of Understanding between NASA and Rosaviakosmos, the International Space Station was intended to be a laboratory, observatory and factory in low Earth orbit. It was also planned to provide transportation, maintenance, and act as a staging base for possible future missions to the Moon, Mars and asteroids.^[29] In the 2010 United States National Space Policy, the ISS was given additional roles of serving commercial, diplomatic^[30] and educational purposes.^[31]

Scientific research

The ISS provides a platform to conduct scientific research. Small unmanned spacecraft can provide platforms for zero gravity and exposure to space, but space stations offer a long-term environment where studies can be performed potentially for decades, combined with ready access by human researchers over periods that exceed the capabilities of manned spacecraft.^[17][32]

The ISS simplifies individual experiments by eliminating the need for separate rocket launches and research staff. The wide variety of research fields include astrobiology, astronomy, human research including space medicine and life sciences, physical sciences, materials science, space weather, and weather on Earth (meteorology).^{[11][12][13][33][34]} Scientists on Earth have access to the crew's data and can modify experiments or launch new ones, which are benefits generally unavailable on unmanned spacecraft.^[32] Crews fly expeditions of several months duration, providing approximately 160-man-hours per week of labour with a crew of 6.^[11][35]

To detect dark matter and answer other fundamental questions about our universe, engineers and scientists from all over the world built the Alpha Magnetic Spectrometer (AMS), which NASA compares to the Hubble space telescope, and says could not be accommodated on a free flying satellite platform partly because of its power requirements and data bandwidth needs.^[36][37] On 3 April 2013, NASA scientists reported that hints of dark matter may have been detected by the Alpha Magnetic Spectrometer.^{[38][39][40][41][42][43]} According to the scientists, "The first results from the space-borne Alpha Magnetic Spectrometer confirm an unexplained excess of high-energy positrons in Earth-bound cosmic rays."

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.^[44] Some simple forms of life called extremophiles,^[45] including small invertebrates called tardigrades^[46] can survive in this environment in an extremely dry state called desiccation.

Medical research 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 colonisation 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.^[47][48] Medical studies are conducted aboard the ISS on behalf of the National Space 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 on board 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.^[49][50][51]

Microgravity

The Earth's gravity is only slightly weaker at the altitude of the ISS than at the surface, but objects in orbit are in a continuous state of freefall, resulting in an apparent state of weightlessness. This perceived weightlessness is disturbed by five separate effects:^[52]

• Drag from the residual atmosphere; when the ISS enters the Earth's shadow, the main solar panels are rotated to minimise this aerodynamic drag, helping reduce orbital decay.
• Vibration from movements of mechanical systems and the crew.
• Actuation of the on-board attitude control moment gyroscopes.
• Thruster firings for attitude or orbital changes.
• Gravity-gradient effects, also known as tidal effects. Items at different locations within the ISS would, if not attached to the station, follow slightly different orbits. Being mechanically interconnected these items experience small forces that keep the station moving as a rigid body.

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

Investigating the physics of fluids in microgravity will provide better models of the behaviour of fluids. Because fluids can be almost completely combined in microgravity, physicists investigate fluids that do not mix well on Earth. In addition, examining reactions that are slowed by low gravity and low temperatures will improve our understanding of superconductivity.^[12]

The study of materials science is an important ISS research activity, with the objective of reaping economic benefits through the improvement of techniques used on the ground.^[53] 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.^[12]

Exploration

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 operations, maintenance as well as repair and replacement activities on-orbit, which will be essential skills in operating spacecraft farther from Earth, mission risks can be reduced and the capabilities of interplanetary spacecraft advanced.^[14] Referring to the MARS-500 experiment, ESA states that "Whereas the ISS is essential for answering questions concerning the possible impact of weightlessness, radiation and other space-specific factors, aspects such as the effect of long-term isolation and confinement can be more appropriately addressed via ground-based simulations".^[54] Sergey Krasnov, the head of human space flight programmes for Russia's space agency, Roscosmos, in 2011 suggested a "shorter version" of MARS-500 may be carried out on the ISS.^[55]

In 2009, noting the value of the partnership framework itself, Sergey Krasnov wrote, "When compared with partners acting separately, partners developing complementary abilities and resources could give us much more assurance of the success and safety of space exploration. The ISS is helping further advance near-Earth space exploration and realisation of prospective programmes of research and exploration of the Solar system, including the Moon and Mars."^[56] A manned mission to Mars 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 four partners that China, India and South Korea be invited to join the ISS partnership.^[57] NASA chief Charlie Bolden stated in February 2011, "Any mission to Mars is likely to be a global effort".^[58] Currently, American legislation prevents NASA co-operation with China on space projects.^[59]

Education and cultural outreach

The ISS crew provides 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.^[16][60] ESA offers a wide range of free teaching materials that can be downloaded for use in classrooms.^[61] In one lesson, students can navigate a 3-D model of the interior and exterior of the ISS, and face spontaneous challenges to solve in real time.^[62]

JAXA aims both to "Stimulate the curiosity of children, cultivating their spirits, and encouraging their passion to pursue craftsmanship", and to "Heighten the child's awareness of the importance of life and their responsibilities in society."^[63] Through a series of education guides, a deeper understanding of the past and near-term future of manned space flight, as well as that of Earth and life, will be learned.^[64][65] In the JAXA Seeds in Space experiments, the mutation effects of spaceflight on plant seeds aboard the ISS is explored. Students grow sunflower seeds which flew on the ISS for about nine months as a start to 'touch the Universe'. In the first phase of Kibō utilisation from 2008 to mid-2010, researchers from more than a dozen Japanese universities conducted experiments in diverse fields.^[66]

Cultural activities are another major objective. Tetsuo Tanaka, director of JAXA's Space Environment and Utilization Center, says "There is something about space that touches even people who are not interested in science."^[67]

Amateur Radio on the ISS (ARISS) is a volunteer programme which encourages 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 nine countries including several countries in Europe as well as Japan, Russia, Canada, and the United States. In areas where radio equipment cannot be used, speakerphones connect students to ground stations which then connect the calls to the station.^[68]

First Orbit is a feature-length documentary film about Vostok 1, the first manned space flight around the Earth. By matching the orbit of the International Space Station to that of Vostok 1 as closely as possible, in terms of ground path and time of day, documentary filmmaker Christopher Riley and ESA astronaut Paolo Nespoli were able to film the view that Yuri Gagarin saw on his pioneering orbital space flight. This new footage was cut together with the original Vostok 1 mission audio recordings sourced from the Russian State Archive. Nespoli, during Expedition 26/27, filmed the majority of the footage for this documentary film, and as a result is credited as its director of photography.^[69] The film was streamed through the website firstorbit.org in a global YouTube premiere in 2011, under a free license.^[70]

In May 2013, commander Chris Hadfield shot a music video of David Bowie's "Space Oddity" on board the station; the film was released freely on YouTube.^[71] It was the first music video ever to be filmed in space.^[72]

Assembly

The assembly of the International Space Station, a major endeavour in space architecture, began in November 1998.^[5] Russian modules launched and docked robotically, with the exception of Rassvet. All other modules were delivered by the Space Shuttle, which required installation by ISS and shuttle crewmembers using the Canadarm2 (SSRMS) and extra-vehicular activities (EVAs); as of 5 June 2011, they had added 159 components during more than 1,000 hours of EVA (see List of ISS spacewalks). 127 of these spacewalks originated from the station, and the remaining 32 were launched from the airlocks of docked Space Shuttles.^[4] The beta angle of the station had to be considered at all times during construction, as it directly affects how long during its orbit the station (and any docked or docking spacecraft) is exposed to the sun; the Space Shuttle would not perform optimally above a limit called the "beta cutoff".^[73] Many of the modules that launched on the Space Shuttle were integrated and tested on the ground at the Space Station Processing Facility to find and correct issues prior to launch.

The first module of the ISS, Zarya, was launched on 20 November 1998 on an autonomous Russian Proton rocket. It provided propulsion, attitude 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 that time, the Russian station Mir was still inhabited. The ISS remained unmanned for two years, while Mir was de-orbited. On 12 July 2000, Zvezda was launched into orbit. Preprogrammed commands on board deployed its solar arrays and communications antenna. It then became the passive target for a rendezvous with Zarya and Unity: it maintained a station-keeping orbit while 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.^[74][75]

The first resident crew, Expedition 1, arrived in November 2000 on Soyuz TM-31. At the end of the first day on the station, astronaut Bill Shepherd requested the use of the radio call sign "Alpha", which he and cosmonaut Krikalev preferred to the more cumbersome "International Space Station".^[76] The name "Alpha" had previously been used for the station in the early 1990s,^[77] and following the request, its use was authorised for the whole of Expedition 1.^[78] Shepherd had been advocating the use of a new name to project managers for some time. Referencing a naval tradition in a pre-launch news conference he had said: "For thousands of years, humans have been going to sea in ships. People have designed and built these vessels, launched them with a good feeling that a name will bring good fortune to the crew and success to their voyage."^[79] Yuri Semenov, the President of Russian Space Corporation Energia at the time, disapproved of the name "Alpha"; he felt that Mir was the first space station, and so he would have preferred the names "Beta" or "Mir 2" for the ISS.^[78][80][81]

Expedition 1 arrived 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 US television, additional attitude support needed for the additional mass of the USOS, and substantial solar arrays supplementing the station's existing 4 solar arrays.^[82]

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 Space Shuttle Columbia disaster in 2003 and a resulting two-year hiatus in the Space Shuttle programme. The space shuttle was grounded until 2005 with STS-114 flown by Discovery.^[83]

Assembly resumed in 2006 with the arrival of STS-115 with Atlantis, 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 soon followed 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, followed in May 2010 by the penultimate Russian module, Rassvet. Rassvet was delivered by Space Shuttle Atlantis on STS-132 in exchange for the Russian Proton delivery of the Zarya module in 1998 which had been funded by the United States.^[84] The last pressurised module of the USOS, Leonardo, was brought to the station by Discovery on her final flight, STS-133,^[85] followed by the Alpha Magnetic Spectrometer on STS-134, delivered by Endeavour.^[86]

As of June 2011, the station consisted of 15 pressurised modules and the Integrated Truss Structure. Five modules are still to be launched, including the Nauka with the European Robotic Arm, the Uzlovoy Module, and two power modules called NEM-1 and NEM-2.^[87] As of November 2015, Russia's future primary research module Nauka is set to launch in February 2017, along with the European Robotic Arm which will be able to relocate itself to different parts of the Russian modules of the station. After the Nauka module is attached, the Uzlovoy Module will be attached to one of its docking ports. When completed, the station will have a mass of more than 400 tonnes (440 short tons).^[5]

The gross mass of the station changes over time. The total launch mass of the modules on orbit is about 417,289 kg (919,965 lb) (as of 3 September 2011).^[88] The mass of experiments, spare parts, personal effects, crew, foodstuff, clothing, propellants, water supplies, gas supplies, docked spacecraft, and other items add to the total mass of the station. Hydrogen gas is constantly vented overboard by the oxygen generators.

Station structure

The ISS is a third generation^[89] modular space station.^[90] Modular stations can allow the mission to be changed over time and new modules can be added or removed from the existing structure, allowing greater flexibility.

Below is a diagram of major station components. The blue areas are pressurised sections accessible by the crew without using spacesuits. The station's unpressurised superstructure is indicated in red. Other unpressurised components are yellow. Note that the Unity node joins directly to the Destiny laboratory. For clarity, they are shown apart.

Russian docking port

Solar array

Zvezda DOS-8 (service module)

Solar array

Russian docking port

Poisk (MRM-2) airlock

Pirs airlock

Russian docking port

Nauka lab to replace Pirs

European robotic arm

Solar array

Zarya FGB (first module)

Solar array

Rassvet (MRM-1)

Russian docking port

PMA 1

Cargo spacecraft berthing port

Leonardo cargo bay

BEAM habitat

Quest airlock

Unity Node 1

Tranquility Node 3

ESP-2

Cupola

Solar array

Solar array

Heat radiator

Heat radiator

Solar array

Solar array

ELC 2, AMS

Z1 truss

ELC 3

S5/6 Truss

S3/S4 Truss

S1 Truss

S0 Truss

P1 Truss

P3/P4 Truss

P5/6 Truss

ELC 4, ESP 3

ELC 1

Dextre robotic arm

Canadarm2 robotic arm

Solar array

Solar array

Solar array

Solar array

ESP-1

Destiny laboratory

Kibō logistics cargo bay

Cargo spacecraft berthing port

PMA 3 docking port

Kibō robotic arm

External payloads

Columbus laboratory

Harmony Node 2

Kibō laboratory

Kibō external platform

PMA 2 docking port

IDA 2 docking adapter

Comparison

The ISS follows Salyut and Almaz series, Skylab, and Mir as the 11th space station launched, as the Genesis prototypes were never intended to be manned. Other examples of modular station projects include the Soviet/Russian Mir and the planned Russian OPSEK and Chinese space station. First generation space stations, such as early Salyuts and NASA's Skylab were not designed for re-supply.^[91] Generally, each crew had to depart the station to free the only docking port for the next crew to arrive, Skylab had more than one docking port but was not designed for resupply. Salyut 6 and 7 had more than one docking port and were designed to be resupplied routinely during crewed operation.^[92]

Pressurised modules

Zarya

Zarya (Russian: Заря́; lit. dawn), also known as the Functional Cargo Block or FGB (from the Russian "Функционально-грузовой блок", Funktsionalno-gruzovoy blok or ФГБ), was the first module of the International Space Station to be launched. The FGB provided electrical power, storage, propulsion, and guidance to the ISS during the initial stage of assembly. With the launch and assembly in orbit of other modules with more specialised functionality, Zarya is now primarily used for storage, both inside the pressurised section and in the externally mounted fuel tanks. Zarya is a descendant of the TKS spacecraft designed for the Soviet Salyut programme. The name Zarya was given to the FGB because it signified the dawn of a new era of international co-operation in space. Although it was built by a Russian company, it is owned by the United States. Zarya weighs 19,300 kg (42,500 lb), is 12.55 m (41.2 ft) long and 4.1 m (13 ft) wide, discounting solar arrays.

Built from December 1994 to January 1998 in Russia at the Khrunichev State Research and Production Space Center (KhSC) in Moscow, Zarya's control system was developed by the Khartron Corp. (Kharkiv, Ukraine).

Zarya was launched on 20 November 1998, on a Russian Proton rocket from Baikonur Cosmodrome Site 81 in Kazakhstan to a 400 km (250 mi) high orbit with a designed lifetime of at least 15 years. After Zarya reached orbit, STS-88 launched on 4 December 1998, to attach the Unity module.

Although only designed to fly autonomously for six to eight months, Zarya did so for almost two years because of delays with the Russian Service Module, Zvezda, which finally launched on 12 July 2000, and docked with Zarya on 26 July using the Russian Kurs docking system.

Unity

Unity, or Node 1, is one of three nodes, or passive connecting modules, in the US Orbital Segment of the station. It was the first US-built component of the Station to be launched. Cylindrical in shape, with six berthing locations facilitating connections to other modules, Unity was carried into orbit by Space Shuttle Endeavour as the primary cargo of STS-88 in 1998. Essential space station resources such as fluids, environmental control and life support systems, electrical and data systems are routed through Unity to supply work and living areas of the station. More than 50,000 mechanical items, 216 lines to carry fluids and gases, and 121 internal and external electrical cables using six miles of wire were installed in the Unity node. Unity is made of aluminium. Prior to its launch aboard Endeavour, conical Pressurized Mating Adapters (PMAs) were attached to the aft and forward berthing mechanisms of Unity. Unity and the two mating adapters together weighed about 11,600 kg (25,600 lb). The adapters allow the docking systems used by the Space Shuttle and by Russian modules to attach to the node's hatches and berthing mechanisms.

Unity was carried into orbit as the primary cargo of the Space Shuttle Endeavour on STS-88, the first Space Shuttle mission dedicated to assembly of the station. On 6 December 1998, the STS-88 crew mated the aft berthing port of Unity with the forward hatch of the already orbiting Zarya module.

Zvezda

Zvezda (Russian: Звезда́, meaning "star"), also known as DOS-8, Service Module or SM (Russian: СМ). Early in the station's life, Zvezda provided all of its critical systems.^[93][94] It made the station permanently habitable for the first time, adding life support for up to six crew and living quarters for two.^[10] Zvezda's DMS-R computer handles guidance, navigation and control for the entire space station.^[95] A second computer which performs the same functions will be installed in the Nauka module, FGB-2.

The hull of Zvezda was completed in February 1985, with major internal equipment installed by October 1986.^[94] The module was launched by a Proton-K rocket from Site 81/23 at Baikonur, on 12 July 2000. Zvezda is at the rear of the station according to its normal direction of travel and orientation, and its engines may be used to boost the station's orbit. Alternatively Russian and European spacecraft can dock to Zvezda's aft port and use their engines to boost the station.^[96][97]

Destiny

Destiny, also known as the U.S. Lab, is the primary research facility for United States payloads aboard the ISS. In 2011, NASA chose the not-for-profit group Center for the Advancement of Science in Space (CASIS) to be the sole manager of all American science on the station which does not relate to manned exploration. The module houses 24 International Standard Payload Racks, some of which are used for environmental systems and crew daily living equipment. Destiny also serves as the mounting point for the station's Truss Structure.^[98]

Quest

Quest is the only USOS airlock, and hosts spacewalks with both United States EMU and Russian Orlan spacesuits. It consists of two segments: the equipment lock, which 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.^[99]

Pirs and Poisk

Pirs (Russian: Пирс, meaning "pier"), (Russian: Стыковочный отсек), "docking module", SO-1 or DC-1 (docking compartment), and Poisk (Russian: По́иск; lit. Search), also known as the Mini-Research Module 2 (MRM 2), Малый исследовательский модуль 2, or МИМ 2. Pirs and Poisk are Russian airlock modules, each having 2 identical hatches. An outward-opening hatch on the Mir space station failed after it swung open too fast after unlatching, because of a small amount of air pressure remaining in the airlock.^[100] A different entry was used, and the hatch was repaired. All EVA hatches on the ISS open inwards and are pressure-sealing. Pirs was used to store, service, and refurbish Russian Orlan suits and provided contingency entry for crew using the slightly bulkier American suits. The outermost docking ports on both airlocks allow docking of Soyuz and Progress spacecraft, and the automatic transfer of propellants to and from storage on the ROS.^[101]

Harmony

Harmony, also known as Node 2, is the second of the station's node modules and the utility hub of the USOS. The module contains four racks that provide electrical power, bus electronic data, and acts as a central connecting point for several other components via its six Common Berthing Mechanisms (CBMs). The European Columbus and Japanese Kibō laboratories are permanently berthed to the starboard and port radial ports respectively. The nadir and zenith ports can be used for docking visiting spacecraft including HTV, Dragon, and Cygnus, with the nadir port serving as the primary docking port. American Shuttle Orbiters docked with the ISS via PMA-2, attached to the forward port.

Tranquility

Tranquility, also known as Node 3, is the third and last of the station's US nodes, it contains an additional life support system to recycle waste water for crew use and supplements oxygen generation. Like the other US nodes, it has six berthing mechanisms, five of which are currently in use. The first one connects to the station's core via the Unity module, others host the Cupola, the PMA docking port #3, the Leonardo PMM and the Bigelow Expandable Activity Module. The final zenith port remains free.

Columbus

Columbus, the primary research facility for European payloads aboard the ISS, 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.^[102][103] ESA's development of technologies on all the main areas of life support has been ongoing for more than 20 years and are/have been used in modules such as Columbus and the ATV. The German Aerospace Center DLR manages ground control operations for Columbus and the ATV is controlled from the French CNES Toulouse Space Center.

Kibō

Kibō (Japanese: きぼう, "hope") is a laboratory and the largest ISS module. It is used to do research in space medicine, biology, Earth observations, materials production, biotechnology and communications, and has facilities for growing plants and fish. During August 2011, the MAXI observatory mounted on Kibō, which uses the ISS's orbital motion to image the whole sky in the X-ray spectrum, detected for the first time the moment when a star was swallowed by a black hole.^[104][105] The laboratory contains 23 racks, including 10 experiment racks, and has a dedicated airlock for experiments. In a 'shirt sleeves' environment, crew attach an experiment to the sliding drawer within the airlock, close the inner, and then open the outer hatch. By extending the drawer and removing the experiment using the dedicated robotic arm, payloads are placed on the external platform. The process can be reversed and repeated quickly, allowing access to maintain external experiments without the delays caused by EVAs.

• 
  Pressurized Module
• 
  Experiment Logistics Module
• 
  Exposed Facility
• 
  Experiment Logistics Module
• 
  Remote Manipulator System

A smaller pressurised module is attached to the top of Kibō, serving as a cargo bay. The dedicated Interorbital Communications System (ICS) allows large amounts of data to be beamed from Kibō's ICS, first to the Japanese KODAMA satellite in geostationary orbit, then to Japanese ground stations. When a direct communication link is used, contact time between the ISS and a ground station is limited to approximately 10 minutes per visible pass. When KODAMA relays data between a LEO spacecraft and a ground station, real-time communications are possible in 60% of the flight path of the spacecraft. Japanese ground controllers use telepresence robotics to remotely conduct onboard research and experiments, thus reducing the workload of station astronauts. Ground controllers also use a free-floating autonomous ball camera to photodocument astronaut and space station activities, further freeing up astronaut time.

Cupola

Cupola is a seven-window observatory, used to view Earth and docking spacecraft. Its name derives from the Italian word cupola, which means "dome". The Cupola project was started by NASA and Boeing, but cancelled due to budget cuts. A barter agreement between NASA and ESA led to ESA resuming development of Cupola in 1998. It was built by Thales Alenia Space in Turin, Italy. The module comes equipped with robotic workstations for operating the station's main robotic arm and shutters to protect its windows from damage caused by micrometeorites. It features 7 windows, with an 80-centimetre (31 in) round window, the largest window on the station (and the largest flown in space to date). The distinctive design has been compared to the 'turret' of the fictitious Millennium Falcon from the motion picture Star Wars;^[106][107] the original prop lightsaber used by actor Mark Hamill as Luke Skywalker in the 1977 film was flown to the station in 2007.^[108]

Rassvet

Rassvet (Russian: Рассве́т; lit. "dawn"), also known as the Mini-Research Module 1 (MRM-1) (Russian: Ма́лый иссле́довательский модуль, МИМ 1) and formerly known as the Docking Cargo Module (DCM), is similar in design to the Mir Docking Module launched on STS-74 in 1995. Rassvet is primarily used for cargo storage and for docking by visiting spacecraft. It was flown to the ISS aboard NASA's Space Shuttle Atlantis on the STS-132 mission and connected in May 2010,^[109][110] Rassvet is the only Russian-owned module launched by NASA, to repay for the launch of Zarya, which is Russian designed and built, but partially paid for by NASA.^[111] Rassvet was launched with the Russian Nauka laboratory's experiments airlock temporarily attached to it, and spare parts for the European Robotic Arm.

Leonardo

Leonardo Permanent Multipurpose Module (PMM) is a storage module attached to the Tranquility node.^[112][113] The three NASA Space Shuttle MPLM cargo containers—Leonardo, Raffaello and Donatello—were built for NASA in Turin, Italy by Alcatel Alenia Space, now Thales Alenia Space.^[114] The MPLMs were provided to NASA's ISS programme by Italy (independent of their role as a member state of ESA) and are considered to be US elements. In a bartered exchange for providing these containers, the US gave Italy research time aboard the ISS out of the US allotment in addition to that which Italy receives as a member of ESA.^[115] The Permanent Multipurpose Module was created by converting Leonardo into a module that could be permanently attached to the station.^{[116][117][118]}

Bigelow Expandable Activity Module

Bigelow Expandable Activity Module (BEAM) is a prototype inflatable space habitat serving as a two-year technology demonstration.^[119] It was built by Bigelow Aerospace under a contract established by NASA on 16 January 2013. BEAM was delivered to the ISS aboard SpaceX CRS-8 on 10 April 2016, was berthed to the aft port of the Tranquility node on 16 April,^[120][121] and was fully expanded on 28 May.^[122]

During its two-year test run, instruments are expected to measure its structural integrity and leak rate, along with temperature and radiation levels. The hatch leading into the module will remain mostly closed except for periodic visits by space station crew members for inspections and data collection. The module was originally planned to be jetissoned from the station following the test,^[123] but following positive data after a year in orbit, NASA has suggested that it could remain on the station as a storage area.^[124]

Scheduled additional modules

Nauka

Nauka (Russian: Нау́ка; lit. "science"), also known as the Multipurpose Laboratory Module (MLM) or FGB-2 (Russian: Многофункциональный лабораторный модуль, МЛМ), is the major Russian laboratory module. It was scheduled to arrive at the station in 2014, docking to the port that was occupied by the Pirs module.^[125] Due to deterioration during many years spent in storage, it proved necessary to build a new propulsion module,^[126] and the launch date was postponed to 2018.^[127] Before the Nauka module arrives, a Progress spacecraft will remove Pirs from the station and deorbit it to reenter over the Pacific Ocean. Nauka contains an additional set of life support systems and attitude control. Originally it would have routed power from the single Science-and-Power Platform, but that single module design changed over the first ten years of the ISS mission, and the two science modules, which attach to Nauka via the Uzlovoy Module, or Russian node, each incorporate their own large solar arrays to power Russian science experiments in the ROS.

Nauka's mission has changed over time. During the mid-1990s, it was intended as a backup for the FGB, and later as a universal docking module (UDM); its docking ports will be able to support automatic docking of both spacecraft, additional modules and fuel transfer. Nauka has its own engines. Like Zvezda and Zarya, Nauka will be launched by a Proton rocket, while smaller Russian modules such as Pirs and Poisk were delivered by modified Progress spacecraft. Russia plans to separate Nauka, along with the rest of the Russian Orbital Segment, to form the OPSEK space station before the ISS is deorbited.

Uzlovoy Module

The Uzlovoy Module (UM), or Node Module is a 4-metric-ton^[128] ball-shaped module that will allow docking of two scientific and power modules during the final stage of the station assembly, and provide the Russian segment additional docking ports to receive Soyuz MS and Progress MS spacecraft. UM is due to be launched in late 2018. It will be integrated with a special version of the Progress cargo ship and launched by a standard Soyuz rocket. Progress would use its own propulsion and flight control system to deliver and dock the Node Module to the nadir (Earth-facing) docking port of the Nauka MLM/FGB-2 module. One port is equipped with an active hybrid docking port, which enables docking with the MLM module. The remaining five ports are passive hybrids, enabling docking of Soyuz and Progress vehicles, as well as heavier modules and future spacecraft with modified docking systems. The node module was conceived to serve as the only permanent element of the future Russian successor to the ISS, OPSEK. Equipped with six docking ports, the Node Module would serve as a single permanent core of the future station with all other modules coming and going as their life span and mission required.^[129][130] This would be a progression beyond the ISS and Russia's modular Mir space station, which are in turn more advanced than early monolithic first generation stations such as Skylab, and early Salyut and Almaz stations.

Science Power Modules 1 & 2 (NEM-1, NEM-2) (Russian: Нау́чно-Энергетический Модуль-1 и -2)

NanoRacks Airlock Module

The NanoRacks Airlock Module is a commercially-funded airlock module intended to be launched in 2019. The module is being built by NanoRacks and Boeing,^[131] and will be used to deploy CubeSats, small satellites, and other external payloads for NASA, CASIS, and other commercial and governmental customers. It is intended to be manifested with a Commercial Resupply Services mission.^[132]

Cancelled components

Several modules planned for the station were cancelled over the course of the ISS programme. Reasons include budgetary constraints, the modules becoming unnecessary, and station redesigns after the 2003 Columbia disaster. The US Centrifuge Accommodations Module would have hosted science experiments in varying levels of artificial gravity.^[133] The US Habitation Module would have served as the station's living quarters. Instead, the sleep stations are now spread throughout the station.^[134] The US Interim Control Module and ISS Propulsion Module would have replaced the functions of Zvezda in case of a launch failure.^[135] Two Russian Research Modules were planned for scientific research.^[136] They would have docked to a Russian Universal Docking Module.^[137] The Russian Science Power Platform would have supplied power to the Russian Orbital Segment independent of the ITS solar arrays.

Unpressurised elements

The ISS has a large number of external components that do not require pressurisation. The largest of these is the Integrated Truss Structure (ITS), to which the station's main solar arrays and thermal radiators are mounted.^[138] The ITS consists of ten separate segments forming a structure 108.5 m (356 ft) long.^[5]

The station in its complete form has several smaller external components, such as the six robotic arms, the three External Stowage Platforms (ESPs) and four ExPRESS Logistics Carriers (ELCs).^[139][140] While these platforms allow experiments (including MISSE, the STP-H3 and the Robotic Refueling Mission) to be deployed and conducted in the vacuum of space by providing electricity and processing experimental data locally, their primary function is to store spare Orbital Replacement Units (ORUs). ORUs are parts that can be replaced when they fail or pass their design life. Examples of ORUs include pumps, storage tanks, antennas and battery units. Such units are replaced either by astronauts during EVA or by robotic arms. Spare parts were routinely transported to and from the station via Space Shuttle resupply missions, with a heavy emphasis on ORU transport once the NASA Shuttle approached retirement.^[141] Several shuttle missions were dedicated to the delivery of ORUs, including STS-129,^[142] STS-133^[143] and STS-134.^[144] As of January 2011, only one other mode of transportation of ORUs had been utilised – the Japanese cargo vessel HTV-2 – which delivered an FHRC and CTC-2 via its Exposed Pallet (EP).^[145]

There are also smaller exposure facilities mounted directly to laboratory modules; the Kibō Exposed Facility serves as an external 'porch' for the Kibō complex,^[146] and a facility on the European Columbus laboratory provides power and data connections for experiments such as the European Technology Exposure Facility^[147][148] and the Atomic Clock Ensemble in Space.^[149] A remote sensing instrument, SAGE III-ISS, was delivered to the station in 2014 aboard a Dragon capsule, and the NICER experiment is scheduled to be delivered in 2016.^[150][151] The largest such scientific payload externally mounted to the ISS is the Alpha Magnetic Spectrometer (AMS), a particle physics experiment launched on STS-134 in May 2011, and mounted externally on the ITS. The AMS measures cosmic rays to look for evidence of dark matter and antimatter.^[152]

Robotic arms and cargo cranes

The Integrated Truss Structure serves as a base for the station's primary remote manipulator system, called the Mobile Servicing System (MSS), which is composed of three main components. Canadarm2, the largest robotic arm on the ISS, has a mass of 1,800 kilograms (4,000 lb) and is used to dock and manipulate spacecraft and modules on the USOS, hold crew members and equipment in place during EVAs and move Dextre around to perform tasks.^[153] Dextre is a 1,560 kg (3,440 lb) robotic manipulator with two arms, a rotating torso and has power tools, lights and video for replacing orbital replacement units (ORUs) and performing other tasks requiring fine control.^[154] The Mobile Base System (MBS) is a platform which rides on rails along the length of the station's main truss. It serves as a mobile base for Canadarm2 and Dextre, allowing the robotic arms to reach all parts of the USOS.^[155] To gain access to the Russian Segment a grapple fixture was added to Zarya on STS-134, so that Canadarm2 can inchworm itself onto the ROS.^[156] Also installed during STS-134 was the 15 m (50 ft) Orbiter Boom Sensor System (OBSS), which had been used to inspect heat shield tiles on Space Shuttle missions and can be used on station to increase the reach of the MSS.^[156] Staff on Earth or the station can operate the MSS components via remote control, performing work outside the station without space walks.

Japan's Remote Manipulator System, which services the Kibō Exposed Facility,^[157] was launched on STS-124 and is attached to the Kibō Pressurised Module.^[158] The arm is similar to the Space Shuttle arm as it is permanently attached at one end and has a latching end effector for standard grapple fixtures at the other.

The European Robotic Arm, which will service the Russian Orbital Segment, will be launched alongside the Multipurpose Laboratory Module in 2017.^[159] The ROS does not require spacecraft or modules to be manipulated, as all spacecraft and modules dock automatically and may be discarded the same way. Crew use the two Strela (Russian: Стрела́; lit. Arrow) cargo cranes during EVAs for moving crew and equipment around the ROS. Each Strela crane has a mass of 45 kg (99 lb).

Station systems

Life support

The critical systems are the atmosphere control system, the water supply system, the food supply facilities, the sanitation and hygiene equipment, and fire detection and suppression equipment. The Russian Orbital Segment's life support systems are contained in the Zvezda service module. Some of these systems are supplemented by equipment in the USOS. The MLM Nauka laboratory has a complete set of life support systems.

Atmospheric control systems

The atmosphere on board the ISS is similar to the Earth's.^[160] Normal air pressure on the ISS is 101.3 kPa (14.7 psi);^[161] 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.^[162] Earth-like atmospheric conditions have been maintained on all Russian and Soviet spacecraft.^[163]

The Elektron system aboard Zvezda and a similar system in Destiny generate oxygen aboard the station.^[164] The crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters, a chemical oxygen generator system.^[165] 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.^[165]

Part of the ROS atmosphere control system is the oxygen supply. Triple-redundancy is provided by the Elektron unit, solid fuel generators, and stored oxygen. The primary supply of oxygen is the Elektron unit which produces O
2 and H
2 by electrolysis of water and vents H2 overboard. The 1 kW system uses approximately one litre of water per crew member per day. This water is either brought from Earth or recycled from other systems. Mir was the first spacecraft to use recycled water for oxygen production. The secondary oxygen supply is provided by burning O
2-producing Vika cartridges (see also ISS ECLSS). Each 'candle' takes 5–20 minutes to decompose at 450–500 °C, producing 600 litres of O
2. This unit is manually operated.^[166]

The US Orbital Segment has redundant supplies of oxygen, from a pressurised storage tank on the Quest airlock module delivered in 2001, supplemented ten years later by ESA-built Advanced Closed-Loop System (ACLS) in the Tranquility module (Node 3), which produces O
2 by electrolysis.^[167] Hydrogen produced is combined with carbon dioxide from the cabin atmosphere and converted to water and methane.

Power and thermal control

Double-sided solar, or Photovoltaic, arrays provide electrical power for the ISS. These bifacial cells are more efficient and operate at a lower temperature than single-sided cells commonly used on Earth, by collecting sunlight on one side and light reflected off the Earth on the other.^[168]

The Russian segment of the station, like the Space Shuttle and most spacecraft, uses 28 volt DC from four rotating solar arrays mounted on Zarya and Zvezda. The USOS uses 130–180 V DC from the USOS PV array, 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, at the expense of crew safety. The ROS uses low voltage; the two station segments share power with converters.

The USOS solar arrays are arranged as four wing pairs, for a total production of 75 to 90 kilowatts.^[169] These arrays normally track the sun to maximise power generation. Each array is about 375 m² (4,036 sq ft) in area and 58 m (190 ft) long. In the complete configuration, the solar arrays track the sun by rotating the alpha gimbal once per orbit; 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.^[170]

The station uses rechargeable nickel–hydrogen batteries (NiH
2) 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.^[171] As of 2017, the nickel–hydrogen batteries are being replaced by lithium-ion batteries, which are expected to last until the end of the ISS program.^[172]

The station's large solar panels generate a high potential voltage difference between the station and the ionosphere. This could cause arcing through insulating surfaces and sputtering of conductive surfaces as ions are accelerated by the spacecraft plasma sheath. To mitigate this, plasma contactor units (PCU)s create current paths between the station and the ambient plasma field.^[173]

The station's systems and experiments consume a large amount of electrical power, almost all of which converts to heat. Little of this heat dissipates through the walls of the station. To keep the internal ambient temperature within comfortable, workable limits, ammonia is continuously pumped through pipes throughout the station to collect heat, then into external radiators to emit infrared radiation, then back into the station.^[174] Thus this passive thermal control system (PTCS) is made of external surface materials, insulation such as MLI, and heat pipes.

If the PTCS cannot keep up with the heat load, an External Active Thermal Control System (EATCS) maintains the temperature. The EATCS consists of an internal, non-toxic, water coolant loop used to cool and dehumidify the atmosphere, which transfers collected heat into an external liquid ammonia loop that can withstand the much lower temperature of space, and is circulated through radiators to remove the heat. The EATCS provides cooling for all the US pressurised modules, including Kibō and Columbus, as well as the main power distribution electronics of the S0, S1 and P1 trusses. It can reject up to 70 kW. This is much more than the 14 kW 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.^[175]

Communications and 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 crew members, flight controllers and family members. As a result, the ISS is equipped with internal and external communication systems used for different purposes.^[176]

The Russian Orbital Segment communicates directly with the ground via the Lira antenna mounted to Zvezda.^[16][177] The Lira antenna also has the capability to use the Luch data relay satellite system.^[16] This system, used for communications with Mir, fell into disrepair during the 1990s, and so is no longer in use,^{[16][178][179]} although two new Luch satellites—Luch-5A and Luch-5B—were launched in 2011 and 2012 respectively to restore the operational capability of the system.^[180] 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.^[181]

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.^{[10][16][176]} 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 System and a similar Japanese system will eventually complement the TDRSS in this role.^[10][182] Communications between modules are carried on an internal digital wireless network.^[183]

UHF radio is used by astronauts and cosmonauts conducting EVAs. UHF is used 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.^[16] 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.^[184][185]

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. Heat generated by the laptops does not rise but stagnates around the laptop, so additional forced ventilation is required. Laptops aboard the ISS are connected to the station's wireless LAN via Wi-Fi and to the ground via K_u band. This provides speeds of 10 Mbit/s to and 3 Mbit/s from the station, comparable to home DSL connection speeds.^[186][187]

The operating system used for key station functions is the Debian Linux distribution.^[188] The migration from Microsoft Windows was made in May 2013 for reasons of reliability, stability and flexibility.^[189]

Station operations

Expeditions and private flights

See also the list of International Space Station expeditions (professional crew), space tourism (private travellers), and the list of human spaceflights to the ISS (both).

Each permanent crew is given an expedition number. Expeditions run up to six months, from launch until undocking, an 'increment' covers the same time period, but includes cargo ships and all activities. Expeditions 1 to 6 consisted of 3 person crews, Expeditions 7 to 12 were reduced to the safe minimum of two following the destruction of the NASA Shuttle Columbia. From Expedition 13 the crew gradually increased to 6 around 2010.^[190][191] With the arrival of the American Commercial Crew vehicles in the middle of the 2010s, expedition size may be increased to seven crew members, the number ISS is designed for.^[192][193]

Sergei Krikalev, member of Expedition 1 and Commander of Expedition 11, has spent more time in space than anyone else, a total of 803 days and 9 hours and 39 minutes. His awards include the Order of Lenin, Hero of the Soviet Union, Hero of the Russian Federation, and 4 NASA medals. On 16 August 2005 at 1:44 am EDT, he passed the record of 748 days held by Sergei Avdeyev, who had 'time travelled' 1/50th of a second into the future aboard Mir.^[194] He participated in psychosocial experiment SFINCSS-99 (Simulation of Flight of International Crew on Space Station), which examined inter-cultural and other stress factors affecting integration of crew in preparation for the ISS spaceflights. Scott Kelly has spent the most time in space of any American. Kelly returned from the ISS on 1 March 2016 having spent 340 consecutive days in orbit.

Travellers who pay for their own passage into space are termed spaceflight participants by Roscosmos and NASA, and are sometimes referred to as space tourists, a term they generally dislike.^{[note 1]} All seven were transported to the ISS on Russian Soyuz spacecraft. When professional crews change over in numbers not divisible by the three seats in a Soyuz, and a short-stay crewmember is not sent, the spare seat is sold by MirCorp through Space Adventures. When the space shuttle retired in 2011, and the station's crew size was reduced to 6, space tourism was halted, as the partners relied on Russian transport seats for access to the station. Soyuz flight schedules increase after 2013, allowing 5 Soyuz flights (15 seats) with only two expeditions (12 seats) required.^[200] The remaining seats are sold for around US$40 million to members of the public who can pass a medical exam. ESA and NASA criticised private spaceflight at the beginning of the ISS, and NASA initially resisted training Dennis Tito, the first man to pay for his own passage to the ISS.^{[note 2]}

Anousheh Ansari (Persian: انوشه انصاری‎‎) became the first Iranian in space and the first self-funded woman to fly to the station. Officials reported that her education and experience make her much more than a tourist, and her performance in training had been "excellent."^[201] Ansari herself dismisses the idea that she is a tourist. She did Russian and European studies involving medicine and microbiology during her 10-day stay. The documentary Space Tourists follows her journey to the station, where she fulfilled "an age-old dream of man: to leave our planet as a "normal person" and travel into outer space."^[202]

In 2008, spaceflight participant Richard Garriott placed a geocache aboard the ISS during his flight.^[203] This is currently the only non-terrestrial geocache in existence.^[204] At the same time, the Immortality Drive, an electronic record of eight digitised human DNA sequences, was placed aboard the ISS.^[205]

Orbit

The ISS is maintained in a nearly circular orbit with a minimum mean altitude of 330 km (205 mi) and a maximum of 410 km (255 mi), in the centre of the thermosphere, 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. Spent rocket stages must be dropped into uninhabited areas and this limits the directions rockets can be launched from the spaceport.^[206][207] It travels at an average speed of 27,724 kilometres per hour (17,227 mph), and completes 15.54 orbits per day (93 minutes per orbit).^[3][15] The station's altitude was allowed to fall around the time of each NASA shuttle mission. Orbital boost burns would generally be delayed until after the shuttle's departure. This allowed shuttle payloads to be lifted with the station's engines during the routine firings, rather than have the shuttle lift itself and the payload together to a higher orbit. This trade-off allowed heavier loads to be transferred to the station. After the retirement of the NASA shuttle, the nominal orbit of the space station was raised in altitude.^[208][209] Other, more frequent supply ships do not require this adjustment as they are substantially lighter vehicles.^[32][210]

Orbital boosting can be performed by the station's two main engines on the Zvezda service module, or Russian or European spacecraft docked to Zvezda's aft port. The ATV has been designed with the possibility of adding a second docking port to its other end, allowing it to remain at the ISS and still allow other craft to dock and boost the station. It takes approximately two orbits (three hours) for the boost to a higher altitude to be completed.^[210] Maintaining ISS altitude uses about 7.5 tonnes of chemical fuel per annum^[211] at an annual cost of about $210 million.^[212]

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.^[213] This technology could allow station-keeping to be done more economically than at present.^[214][215]

The Russian Orbital Segment contains the Data Management System, which handles Guidance, Navigation and Control (ROS GNC) for the entire station.^[95] 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.^[216] Using two fault-tolerant computers (FTC), Zvezda computes the station's position and orbital trajectory using redundant Earth horizon sensors, Solar horizon sensors as well as Sun and star trackers. The FTCs each contain three identical processing units working in parallel and provide advanced fault-masking by majority voting.

Orientation

Zvezda uses gyroscopes and thrusters to turn itself around. Gyroscopes do not require propellant, rather they use electricity to 'store' momentum in flywheels by turning in the opposite direction to the station's movement. The USOS has its own computer controlled gyroscopes to handle the extra mass of that section. When gyroscopes 'saturate', thrusters are used to cancel out the stored momentum. During Expedition 10, an incorrect command was sent to the station's computer, using about 14 kilograms of propellant before the fault was noticed and fixed. When attitude control computers in the ROS and USOS fail to communicate properly, it can result in a rare 'force fight' where the ROS GNC computer must ignore the USOS counterpart, which has no thrusters.^{[217][218][219]} When an ATV, NASA 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 the station's electronics.^[220]

Mission controls

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

• Roscosmos's Mission Control Center at Korolyov, Moscow Oblast, controls the Russian Orbital Segment which handles Guidance, Navigation & Control for the entire Station.,^[95][216] in addition to individual Soyuz and Progress missions.^[16]
• ESA's ATV Control Centre, at the Toulouse Space Centre (CST) in Toulouse, France, controls flights of the unmanned European Automated Transfer Vehicle.^[16]
• JAXA's JEM Control Center and HTV Control Center at Tsukuba Space Center (TKSC) in Tsukuba, Japan, are responsible for operating the Kibō complex and all flights of the 'White Stork' HTV Cargo spacecraft, respectively.^[16]
• 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 controlled the Space Shuttle missions that visited the station.^[16]
• NASA's Payload Operations and Integration Center at Marshall Space Flight Center in Huntsville, Alabama, coordinates payload operations in the USOS.^[16]
• ESA's Columbus Control Centre at the German Aerospace Centre (DLR) in Oberpfaffenhofen, Germany, manages the European Columbus research laboratory.^[16]
• CSA's MSS Control at Saint-Hubert, Quebec, Canada, controls and monitors the Mobile Servicing System, or Canadarm2.^[16]

Repairs

Orbital Replacement Units (ORUs) are spare parts that can be readily replaced when a unit either passes its design life or fails. Examples of ORUs are pumps, storage tanks, controller boxes, antennas, and battery units. Some units can be replaced using robotic arms. Many are stored outside the station, either on small pallets called ExPRESS Logistics Carriers (ELCs) or share larger platforms called External Stowage Platforms which also hold science experiments. Both kinds of pallets have electricity as many parts which could be damaged by the cold of space require heating. The larger logistics carriers also have computer local area network connections (LAN) and telemetry to connect experiments. A heavy emphasis on stocking the USOS with ORU's occurred around 2011, before the end of the NASA shuttle programme, as its commercial replacements, Cygnus and Dragon, carry one tenth to one quarter the payload.

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 in 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.^[221] 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.^[222] 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.^[223] Repairs to the joint were carried out during STS-126 with lubrication of both joints and the replacement of 11 out of 12 trundle bearings on the joint.^[224][225]

2009 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.^[226] The imagery showed that the surface of one sub-panel has peeled back from the underlying central structure, possibly because of 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.^[226]

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.^{[227][228][229]} 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 because of an ammonia leak in one of four quick-disconnects. A second EVA on 11 August successfully removed the failed pump module.^[230][231] A third EVA was required to restore Loop A to normal functionality.^[232][233]

The USOS's cooling system is largely built by the American company Boeing,^[234] which is also the manufacturer of the failed pump.^[235]

An air leak from the USOS in 2004,^[236] the venting of fumes from an Elektron oxygen generator in 2006,^[237] and the failure of the computers in the ROS in 2007 during STS-117 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.

The four Main Bus Switching Units (MBSUs, located in the S0 truss), control the routing of power from the four solar array wings to the rest of the ISS. In late 2011 MBSU-1, while still routing power correctly, ceased responding to commands or sending data confirming its health, and was scheduled to be swapped out at the next available EVA. In each MBSU, two power channels feed 160V DC from the arrays to two DC-to-DC power converters (DDCUs) that supply the 124V power used in the station. A spare MBSU was already on board, but 30 August 2012 EVA failed to be completed when a bolt being tightened to finish installation of the spare unit jammed before electrical connection was secured.^[238] The loss of MBSU-1 limits the station to 75% of its normal power capacity, requiring minor limitations in normal operations until the problem can be addressed.

On 5 September 2012, in a second, 6 hr, EVA to replace MBSU-1, astronauts Sunita Williams and Akihiko Hoshide successfully restored the ISS to 100% power.^[239]

On 24 December 2013, astronauts made a rare Christmas Eve space walk, installing a new ammonia pump for the station's cooling system. The faulty cooling system had failed earlier in the month, halting many of the station's science experiments. Astronauts had to brave a "mini blizzard" of ammonia while installing the new pump. It was only the second Christmas Eve spacewalk in NASA history.^[240]

Fleet operations

A wide variety of manned and unmanned spacecraft have supported the station's activities. More than 60 Progress spacecraft, including M-MIM2 and M-SO1 which installed modules, and more than 40 Soyuz spacecraft have flown to the ISS. 35 flights of the retired NASA Space Shuttle were made to the station.^[4] There have been five European ATV, five Japanese HTV 'Kounotori', eight SpaceX Dragon and four Orbital ATK Cygnus flights.

Currently docked/berthed

See also the list of professional crew, private travellers, both or just unmanned spaceflights.

Key

Spacecraft and mission[241]			Location	Arrival (UTC)	Departure (planned)
	Soyuz MS-04	Expedition 51/52	Poisk zenith	20 April 2017	3 September 2017[242]
	Progress MS-06	Progress 67 cargo	Zvezda aft	16 June 2017	December 2017
	Soyuz MS-05	Expedition 52/53	Rassvet nadir	28 July 2017[242]	14 December 2017[242]

Scheduled missions

• 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 operated by government agencies are indicated with 'Gov' and under commercial arrangements are indicated with 'Com'.

Key

Docking

All Russian spacecraft and self-propelled modules are able to rendezvous and dock to the space station without human intervention using the Kurs docking system. Radar allows these vehicles to detect and intercept ISS from over 200 kilometres away. The European ATV uses star sensors and GPS to determine its intercept course. When it catches up it uses laser equipment to optically recognise Zvezda, along with the Kurs system for redundancy. Crew supervise these craft, but do not intervene except to send abort commands in emergencies. The Japanese H-II Transfer Vehicle parks itself in progressively closer orbits to the station, and then awaits 'approach' commands from the crew, until it is close enough for a robotic arm to grapple and berth the vehicle to the USOS. The American Space Shuttle was manually docked, and on missions with a cargo container, the container would be berthed to the Station with the use of manual robotic arms. Berthed craft can transfer International Standard Payload Racks. Japanese spacecraft berth for one to two months. Russian and European Supply craft can remain at the ISS for six months,^[248][249] allowing great flexibility in crew time for loading and unloading of supplies and trash. NASA Shuttles could remain docked for 11–12 days.^[250]

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 specialised training and planning, making the process more labour-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 standardisations that provide significant cost benefits in repetitive routine operations.^[251] An automated approach could allow assembly of modules orbiting other worlds prior to crew arrival.

Soyuz spacecraft used for crew rotation also serve as lifeboats for emergency evacuation; they are replaced every six months and have been used once to remove excess crew after the Columbia disaster.^[252] Expeditions require, on average, 2,722 kg of supplies, and as of 9 March 2011, crews had consumed a total of around 22,000 meals.^[4] Soyuz crew rotation flights and Progress resupply flights visit the station on average two and three times respectively each year,^[253] with the ATV and HTV planned to visit annually from 2010 onwards. Cygnus and Dragon were contracted to fly cargo to the station after retirement of the NASA Shuttle.^[254][255]

From 26 February 2011 to 7 March 2011 four of the governmental partners (United States, ESA, Japan and Russia) had their spacecraft (NASA Shuttle, ATV, HTV, Progress and Soyuz) docked at the ISS, the only time this has happened to date.^[256] On 25 May 2012, SpaceX became the world's first privately held company to send cargo, via the Dragon spacecraft, to the International Space Station.^[257]

Launch and docking windows

Prior to a ship's docking to the ISS, navigation and attitude control (GNC) is handed over to the ground control of the ships' country of origin. GNC is set to allow the station to drift in space, rather than fire its thrusters or turn using gyroscopes. The solar panels of the station are turned edge-on to the incoming ships, so residue from its thrusters does not damage the cells. When a NASA Space Shuttle docked to the station, other ships were grounded, as the Shuttle's reinforced carbon-carbon wing leading edges, cameras, windows, and instruments were too much at risk from damage or contamination by thruster residue from other ships' movements.

Approximately 30% of NASA shuttle launch delays were caused by poor weather. Occasional priority was given to the Soyuz arrivals at the station where the Soyuz carried crew with time-critical cargoes such as biological experiment materials, also causing shuttle delays. Departure of the NASA shuttle was often delayed or prioritised according to weather over its two landing sites.^[258] Whilst the Soyuz is capable of landing anywhere, anytime, its planned landing time and place is chosen to give consideration to helicopter pilots and ground recovery crew, to give acceptable flying weather and lighting conditions. Soyuz launches occur in adverse weather conditions, but the cosmodrome has been shut down on occasions when buried by snow drifts up to 6 metres in depth, hampering ground operations.

Life aboard

Crew activities

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.^[259]

The time zone used aboard 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 per day. During visiting space shuttle missions, the ISS crew mostly follows the shuttle's Mission Elapsed Time (MET), which is a flexible time zone based on the launch time of the shuttle mission.^{[260][261][262]}

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.^[263][264] The American quarters are private, approximately person-sized soundproof booths. The Russian crew quarters include a small window, but provide less ventilation and sound proofing. A crew member 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.^{[265][266][267]} 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.^[268] 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.^[265]

Food

Most of the food aboard is vacuum sealed in plastic bags. Cans are rare because they are heavy and expensive to transport. Preserved food is not highly regarded by the crew, and taste is reduced in microgravity.^[265] Therefore, effort is made to make the food more palatable, such as using more spices than in regular cooking. The crew looks forward to the arrival of any ships from Earth, as they bring fresh fruit and vegetables. Care is taken that foods do not create crumbs. Sauces are often used to avoid contaminating station equipment. Each crew member has individual food packages and cooks them using the on-board galley. The galley features two food warmers, a refrigerator added in November 2008, and a water dispenser that provides both heated and unheated water.^[266] Drinks are provided as dehydrated powder that is mixed with water before consumption.^[266][267] Drinks and soups are sipped from plastic bags with straws. Solid food is eaten with a knife and fork attached to a tray with magnets to prevent them from floating away. Any food that floats away, including crumbs, must be collected to prevent it from clogging the station's air filters and other equipment.^[267]

Hygiene

Showers on space stations were introduced in the early 1970s on Skylab and Salyut 3.^[269]:139 By Salyut 6, in the early 1980s, the crew complained of the complexity of showering in space, which was a monthly activity.^[270] The ISS does not feature a shower; instead, crewmembers wash using a water jet and wet wipes, with soap dispensed from a toothpaste tube-like container. Crews are also provided with rinseless shampoo and edible toothpaste to save water.^[268][271]

There are two space toilets on the ISS, both of Russian design, located in Zvezda and Tranquility.^[266] 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.^[265] 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.^[266][272] 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 that men and women can use the same toilet. The diverted urine is collected and transferred to the Water Recovery System, where it is recycled into drinking water.^[267]

Crew health and safety

Radiation

The ISS is partially protected from the space environment by Earth's magnetic field. From an average distance of about 70,000 km (43,000 mi), depending on Solar activity, the magnetosphere begins to deflect solar wind around Earth and ISS. Solar flares are still a hazard to the crew, who may receive only a few minutes warning. In 2005, during the initial 'proton storm' of an X-3 class solar flare, the crew of Expedition 10 took shelter in a more heavily shielded part of the ROS designed for this purpose.^[273][274]

Subatomic charged particles, primarily protons from cosmic rays and solar wind, are normally absorbed by Earth's atmosphere. When they interact in sufficient quantity, their effect is visible to the naked eye in a phenomenon called an aurora. Outside Earth's atmosphere, crews are exposed to about 1 millisievert each day, which is about a year of natural exposure on Earth. This results in a higher risk of cancer for astronauts. Radiation can penetrate living tissue and damage the DNA and chromosomes of lymphocytes. These cells are central to the immune system, and so any damage to them could contribute to the lower immunity experienced by astronauts. Radiation has also been linked to a higher incidence of cataracts in astronauts. Protective shielding and drugs may lower risks to an acceptable level.^[47]

Radiation levels on the ISS are about five times greater than those experienced by airline passengers and crew. Earth's electromagnetic field provides almost the same level of protection against solar and other radiation in low Earth orbit as in the stratosphere. For example, on a 12-hour flight an airline passenger would experience 0.1 millisieverts of radiation, or a rate of 0.2 millisieverts per day; only 1/5 the rate experienced by an astronaut in LEO. Additionally, airline passengers experience this level of radiation for a few hours of flight, while ISS crew are exposed for their whole stay.^[275]

Stress

There is considerable evidence that psychosocial stressors are among the most important impediments to optimal crew morale and performance.^[276] Cosmonaut Valery Ryumin wrote in his journal during a particularly difficult period on board the Salyut 6 space station: "All the conditions necessary for murder are met if you shut two men in a cabin measuring 18 feet by 20 and leave them together for two months."

NASA's interest in psychological stress caused by space travel, initially studied when their manned missions began, was rekindled when astronauts joined cosmonauts on the Russian space station Mir. Common sources of stress in early American missions included maintaining high performance under public scrutiny and isolation from peers and family. The latter is still often a cause of stress on the ISS, such as when the mother of NASA Astronaut Daniel Tani died in a car accident, and when Michael Fincke was forced to miss the birth of his second child.

A study of the longest spaceflight concluded that the first three weeks are a critical period where attention is adversely affected because of the demand to adjust to the extreme change of environment.^[277] Skylab's three crews remained one, two, and three months respectively, long term crews on Salyut 6, Salyut 7, and the ISS last about five to six months and Mir's expeditions often lasted longer. The ISS working environment includes further stress caused by living and working in cramped conditions with people from very different cultures who speak a different language. First-generation space stations had crews who spoke a single language; second and third-generation stations have crew from many cultures who speak many languages. The ISS is unique because visitors are not classed automatically into 'host' or 'guest' categories as with previous stations and spacecraft, and may not suffer from feelings of isolation in the same way. Crew members with a military pilot background and those with an academic science background or teachers and politicians may have problems understanding each other's jargon and worldview.

Medical

Medical effects of long-term weightlessness include muscle atrophy, deterioration of the skeleton (osteopenia), 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, and puffiness of the face.^[47]

Sleep is disturbed on the ISS regularly because of mission demands, such as incoming or departing ships. Sound levels in the station are unavoidably high; because the atmosphere is unable to thermosiphon, fans are required at all times to allow processing of the atmosphere which would stagnate in the freefall (zero-g) environment.

To prevent some of these adverse physiological effects, the station is equipped with two treadmills (including the COLBERT), and the aRED (advanced Resistive Exercise Device) which enables various weightlifting exercises which add muscle but do not compensate for or raise astronauts' reduced bone density,^[278] and a stationary bicycle; each astronaut spends at least two hours per day exercising on the equipment.^[265][266] Astronauts use bungee cords to strap themselves to the treadmill.^[279][280]

Microbiological environmental hazards

Hazardous moulds which can foul air and water filters may develop aboard space stations. They can produce acids which degrade metal, glass, and rubber. They can also be harmful for the crew's health. Microbiological hazards have led to a development of the LOCAD-PTS that can identify common bacteria and moulds faster than standard methods of culturing, which may require a sample to be sent back to Earth.^[281] As of 2012, 76 types of unregulated micro-organisms have been detected on the ISS.^[282]

Reduced humidity, paint with mould-killing chemicals, and antiseptic solutions can be used to prevent contamination in space stations. All materials used in the ISS are tested for resistance against fungi.^[283]

Threat of orbital debris

At the low altitudes at which the ISS orbits, there is a variety of space debris,^[284] consisting of different objects including entire spent rocket stages, defunct satellites, explosion fragments—including materials from anti-satellite weapon tests, paint flakes, slag from solid rocket motors, and coolant released by US-A nuclear-powered satellites. These objects, in addition to natural micrometeoroids,^[285] are a significant threat. Large objects could destroy the station, but are less of a threat because their orbits can be predicted.^[286][287] Objects too small to be detected by optical and radar instruments, from approximately 1 cm down to microscopic size, number in the trillions. Despite their small size, some of these objects are a threat because of their kinetic energy and direction in relation to the station. Spacesuits of spacewalking crew could puncture, causing exposure to vacuum.^[288]

Ballistic panels, also called micrometeorite shielding, are incorporated into the station to protect pressurised sections and critical systems. The type and thickness of these panels depends on their predicted exposure to damage. The station's shields and structure have different designs on the ROS and the USOS. On the USOS, a thin aluminium sheet is held apart from the hull and causes objects to shatter into a cloud before hitting the hull, thereby spreading the energy of impact. On the ROS, a carbon plastic honeycomb screen is spaced from the hull, an aluminium honeycomb screen is spaced from that, with a screen-vacuum thermal insulation covering, and glass cloth over the top. It is about 50% less likely to be punctured, and crew move to the ROS when the station is under threat. Punctures on the ROS would be contained within the panels which are 70 cm square.

Space debris is tracked remotely from the ground, and the station crew can be notified.^[289] 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,^[290] the first seven between October 1999 and May 2003.^[291] 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.^[291][292] There were two DAMs in 2009, on 22 March and 17 July.^[293] 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 the station was seriously damaged by the debris. This partial station evacuation has occurred on 13 March 2009, 28 June 2011, 24 March 2012 and 16 June 2015.^[294][295]

End of mission

According to a 2009 report, Space Corporation 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, called the Orbital Piloted Assembly and Experiment Complex (OPSEK). The modules under consideration for removal from the current ISS include the Multipurpose Laboratory Module (Nauka), currently scheduled to be launched in 2017, and other Russian modules which are planned to be attached to Nauka afterwards. Those modules would be within their useful lives in 2016 or 2020. The report presents a statement from an unnamed Russian engineer that, based on the experience from Mir, a 30-year life should be possible, except for micrometeorite damage, because the Russian modules have been built with on-orbit refurbishment in mind.^[296]

According to the Outer Space Treaty the United States and Russia are legally responsible for all modules they have launched.^[297] In ISS planning, NASA examined options including returning the station to Earth via shuttle missions (deemed too expensive, as the USOS is not designed for disassembly and this would require at least 27 shuttle missions^[298]), natural orbital decay with random reentry similar to Skylab, boosting the station to a higher altitude (which would delay reentry) and a controlled targeted de-orbit to a remote ocean area.^[299]

A controlled deorbit into a remote ocean was found to be technically feasible only with Russia's assistance.^[299] The Russian Space Agency has experience from de-orbiting the Salyut 4, 5, 6, 7 and Mir space stations; NASA's first intentional controlled de-orbit of a satellite (the Compton Gamma Ray Observatory) occurred in 2000.^[300] As of late 2010, the preferred plan is to use a slightly modified Progress spacecraft to de-orbit the ISS.^[301] This plan was seen as the simplest, cheapest and with the highest margin.^[301] Skylab, the only space station built and launched entirely by the US, decayed from orbit slowly over 5 years, and no attempt was made to de-orbit it using a deorbital burn. Remains of Skylab hit populated areas of Esperance, Western Australia^[302] without injuries or loss of life.

The Exploration Gateway Platform, a discussion by NASA and Boeing at the end of 2011, suggested using leftover USOS hardware and 'Zvezda 2' [sic] as a refuelling depot and service station located at one of the Earth-Moon Lagrange points, L1 or L2. The entire USOS cannot be reused and will be discarded, but some Russian modules are planned to be reused. Nauka, the Uzlovoy Module, two science power platforms and Rassvet, launched between 2010 and 2015 and joined to the ROS, may be separated to form OPSEK.^[303] Nauka will be used in the station, whose main goal is supporting manned deep space exploration. OPSEK will orbit at a higher inclination of 71 degrees, allowing observation to and from all of the Russian Federation.

In February 2015, Roscosmos announced that it would remain a part of the ISS programme until 2024.^[23] Nine months earlier—in response to US sanctions against Russia over the conflict in the Crimea—Russian Deputy Prime Minister Dmitry Rogozin had stated that Russia would reject a US request to prolong the orbiting station's use beyond 2020, and would only supply rocket engines to the US for non-military satellite launches.^[304]

A proposed modification that would reuse some of the ISS American and European segments is to attach a VASIMR drive module to the vacated Node with its own onboard power source. This would allow long-term reliability testing of the concept for less cost than building a dedicated space station from scratch.^[305]

On 28 March 2015, Russian sources announced that Roscosmos and NASA had agreed to collaborate on the development of a replacement for the current ISS.^[25][26] Igor Komarov, the head of Russia's Roscosmos, made the announcement with NASA administrator Charles Bolden at his side. Komarov said "Roscosmos together with NASA will work on the programme of a future orbital station", "We agreed that the group of countries taking part in the ISS project will work on the future project of a new orbital station", "The first step is that the ISS will operate until 2024", and that Roscosmos and NASA "do not rule out that the station's flight could be extended".^[306] In a statement provided to SpaceNews on 28 March, NASA spokesman David Weaver said the agency appreciated the Russian commitment to extending the ISS, but did not confirm any plans for a future space station.^[28]

On 30 September 2015, Boeing's contract with NASA as prime contractor for the ISS was extended to 30 September 2020. Part of Boeing's services under the contract will relate to extending the station's primary structural hardware past 2020 to the end of 2028.^[307]

Regarding extending the ISS, on 15 November 2016 General Director Vladimir Solntsev of RSC Energia stated "Maybe the ISS will receive continued resources. Today we discussed the possibility of using the station until 2028," and "Much will depend on the political moments in relations with the Americans, with the new administration. It will be discussed."^[308][309]

Cost

The ISS is arguably the most expensive single item ever constructed.^[310] In 2010 the cost was expected to be $150 billion. This includes NASA's budget of $58.7 billion (inflation-unadjusted) for the station from 1985 to 2015 ($72.4 billion in 2010 dollars), Russia's $12 billion, Europe's $5 billion, Japan's $5 billion, Canada's $2 billion, and the cost of 36 shuttle flights to build the station; estimated at $1.4 billion each, or $50.4 billion in total. Assuming 20,000 person-days of use from 2000 to 2015 by two- to six-person crews, each person-day would cost $7.5 million, less than half the inflation-adjusted $19.6 million ($5.5 million before inflation) per person-day of Skylab.^[311]

International co-operation

Participating countries

Former member

•  Brazil

Sightings from Earth

Naked eye

The ISS is visible to the naked eye as a slow-moving, bright white dot because of reflected sunlight, and can be seen in the hours after sunset and before sunrise, when the station remains sunlit but the ground and sky are dark.^[312] The ISS takes about 10 minutes to pass from one horizon to another, and will only be visible part of that time because of moving into or out of the Earth's shadow. Because of the size of its reflective surface area, the ISS is the brightest artificial object in the sky, excluding flares, with an approximate maximum magnitude of −4 when overhead (similar to Venus). The ISS, like many satellites including the Iridium constellation, can also produce flares of up to 8 or 16 times the brightness of Venus as sunlight glints off reflective surfaces.^[313][314] The ISS is also visible during broad daylight conditions, albeit with a great deal more effort.

Tools are provided by a number of websites such as Heavens-Above (see Live viewing below) 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.^{[315][316][317][318]}

In November 2012 NASA launched its "Spot the Station" service, which sends people text and email alerts when the station is due to fly above their town.^[319]

The station is visible from 95% of the inhabited land on Earth, but is not visible from extreme northern or southern latitudes.^[206]

Astrophotography

Using a telescope-mounted camera to photograph the station is a popular hobby for astronomers,^[320] whilst using a mounted camera to photograph the Earth and stars is a popular hobby for crew.^[321] The use of a telescope or binoculars allows viewing of the ISS during daylight hours.^[322]

Parisian engineer and astrophotographer Thierry Legault, known for his photos of spaceships crossing the Sun (called occultation), travelled to Oman in 2011, to photograph the Sun, Moon and space station all lined up.^[323] Legault, who received the Marius Jacquemetton award from the Société astronomique de France in 1999, and other hobbyists, use websites that predict when the ISS will pass in front of the Sun or Moon and from what location those passes will be visible.

See also

• Center for the Advancement of Science in Space - operates the US National Laboratory on the ISS
• List of space stations
• Origins of the International Space Station
• Space architecture

Notes

References

External links

Agency ISS websites

•  Canadian Space Agency
•  European Space Agency
•  Centre national d'études spatiales (National Centre for Space Studies)
•  German Aerospace Centre
•  Italian Space Agency
•  Japan Aerospace Exploration Agency
•  S.P. Korolev Rocket and Space Corporation Energia
•  Russian Federal Space Agency
•  National Aeronautics and Space Administration

Research

• NASA: Daily ISS Reports
• NASA: Station Science
• ESA: Columbus
• RSC Energia: Science Research on ISS Russian Segment

Live viewing

• Live ISS webcam by NASA at uStream.tv
• Live HD ISS webcams by NASA HDEV at uStream.tv
• Real-time position at Heavens-above.com
• Real-time position at N2YO.com

Multimedia

• Interactive reference guide at NASA.gov
• Image gallery search page at NASA.gov
• Assembly sequence animation by USA Today and NASA
• ISS tour with Sunita Williams by NASA at YouTube.com
• ISS tour with André Kuipers by ESA at YouTube.com
• ISS photo tour with Samantha Cristoforetti by ESA
• The Future of Hope, Kibō module documentary by JAXA at YouTube.com
• Journey to the ISS by ESA at YouTube.com
• ISS Photo Library containing the entire collection of photos made by the ISS mission's astronauts