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The International Space Station (ISS) is a space station, or a habitable artificial satellite, in low Earth orbit. It is a modular structure whose first component was launched in 1998.[7] Now the largest artificial body in orbit, it can often be seen with the naked eye from Earth.[8] The ISS consists of pressurised modules, external trusses, solar arrays and other components. ISS components have been launched by American Space Shuttles as well as Russian Proton and Soyuz rockets.[9] In 1984, the ESA was invited to participate in Space Station Freedom.[10] After the USSR dissolved, the United States and Russia merged Mir-2 and Freedom together in 1993.[9]

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 station has been continuously occupied for 14 years and 66 days, since the arrival of Expedition 1 on 2 November 2000. This is the longest continuous human presence in space, having surpassed the previous record of 9 years and 357 days held by Mir. The station is serviced by a variety of visiting spacecraft: Soyuz, Progress, the Automated Transfer Vehicle, the H-II Transfer Vehicle,[15] Dragon, and Cygnus. It has been visited by astronauts and cosmonauts from 15 different nations.[16]

After the U.S. Space Shuttle program ended in 2011, Soyuz rockets became the only provider of transport for astronauts at the International Space Station, while Dragon became the only provider of bulk cargo-return-to-Earth services (downmass capability of Soyuz capsules is very limited).

The ISS programme is a joint project among five participating space agencies: NASA, Roscosmos, JAXA, ESA, and CSA.[15][17] The ownership and use of the space station is established by intergovernmental treaties and agreements.[18] 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. The ISS maintains an orbit with an altitude of between 330 km (205 mi) and 435 km (270 mi) by means of reboost manoeuvres using the engines of the Zvezda module or visiting spacecraft. It completes 15.53 orbits per day.[19] As of January 2014, the US-portion of the ISS was funded until 2024, and may operate until 2028.[20][21][22] The Russian Federal Space Agency, Roskosmos (RKA) has proposed using the ISS to commission modules for a new space station, called OPSEK, before the remainder of the ISS is deorbited. The Russian ISS program head, Alexey B. Krasnov, said in July 2014 that "the Ukraine crisis is why Roscosmos has received no government approval to continue the station partnership beyond 2020."[23]

ISS is the ninth space station to be inhabited by crews, following the Soviet and later Russian Salyut, Almaz, and Mir stations, and Skylab from the US.

Purpose[edit]

Sunrise at Zvezda
Fisheye view of several labs

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.[24] In the 2010 United States National Space Policy, the ISS was given additional roles of serving commercial, diplomatic[25] and educational purposes.[26]

Scientific research[edit]

The ISS provides a platform to conduct scientific research. While small unmanned spacecraft can provide platforms for zero gravity and exposure to space, 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.[16][27]

The Station 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][28][29] 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.[27] Crews fly expeditions of several months duration, providing approximately 160-man-hours a week of labour with a crew of 6.[11][30]

Kibō is intended to accelerate Japan's progress in science and technology, gain new knowledge and apply it to such fields as industry and medicine.[31]

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 telescope, and says could not be accommodated on a free flying satellite platform due in part to its power requirements and data bandwidth needs.[32][33] On 3 April 2013, NASA scientists reported that hints of dark matter may have been detected by the Alpha Magnetic Spectrometer.[34][35][36][37][38][39] 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."

Comet Lovejoy photographed by Expedition 30 commander Dan Burbank
Expedition 8 Commander and Science Officer Michael Foale conducts an inspection of the Microgravity Science Glovebox

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.[40] Some simple forms of life called extremophiles,[41] including small invertebrates called tardigrades[42] 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.[43][44] 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.[45][46][47]

Microgravity[edit]

A comparison between the combustion of a candle on Earth (left) and in a microgravity environment, such as that found on the ISS (right)

The Earth's gravity is only slightly weaker at the altitude of the ISS than at the surface. However, 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:[48]

  • 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, however, these items experience small forces that keep the station moving as a rigid body.
ISS crew member storing samples

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]

The investigation of the physics of fluids in microgravity will allow researchers to model the behaviour of fluids better. Because fluids can be almost completely combined in microgravity, physicists investigate fluids that do not mix well on Earth. In addition, an examination of reactions that are slowed by low gravity and temperatures will give scientists a deeper understanding of superconductivity.[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.[49] 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[edit]

A 3D plan of the Russia-based MARS-500 complex, used for ground-based experiments which complement ISS-based preparations for a manned mission to Mars

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".[50] 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. [51]

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."[52] A manned mission to Mars, however, may be a multinational effort involving space agencies and countries outside the current ISS partnership. In 2010, ESA Director-General Jean-Jacques Dordain stated his agency was ready to propose to the other 4 partners that China, India and South Korea be invited to join the ISS partnership.[53] NASA chief Charlie Bolden stated in Feb 2011 "Any mission to Mars is likely to be a global effort".[54] Currently, American legislation prevents NASA co-operation with China on space projects.[55]

Education and cultural outreach[edit]

Japan's Kounotori 4 docking
Expedition 31's Soyuz being blessed

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.[15][56] ESA offers a wide range of free teaching materials that can be downloaded for use in classrooms.[57] 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.[58]

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."[59] 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.[60][61] 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.[62]

Original Jules Verne manuscripts displayed by crew inside Jules Verne ATV

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."[31]

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 9 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. [63]

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.[64] The film was streamed through the website firstorbit.org in a global YouTube premiere in 2011, under a free license.[65]

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.[66] It was the first music video ever to be filmed in space.[67]

Origins[edit]

Assembly[edit]

S0, S1 and P1 truss structures installed
Partially constructed ISS in December 2002
S3-S4 Truss Installed in 2007
Soyuz TMA-19 departs in 2010
ISS in 2007, with fewer solar arrays

The assembly of the International Space Station, a major endeavour in space architecture, began in November 1998.[3] 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 SSRMS and EVAs; as of 5 June 2011, they had added 159 components during more than 1,000 hours of EVA. 127 of these spacewalks originated from the station, while the remaining 32 were launched from the airlocks of docked Space Shuttles.[2] The beta angle of the station had to be considered at all times during construction, as the station's beta angle is directly related to the percentage of its orbit that the station (as well as any docked or docking spacecraft) is exposed to the sun; the Space Shuttle would not perform optimally above a limit called the "beta cutoff".[68]

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 this time, the Russian station Mir was still inhabited. The ISS remained unmanned for two years, during which time 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 vehicle for a rendezvous with the Zarya and Unity. As a passive "target" vehicle, the Zvezda maintained a stationkeeping orbit as the Zarya-Unity vehicle performed the rendezvous and docking via ground control and the Russian automated rendezvous and docking system. Zarya's computer transferred control of the station to Zvezda's computer soon after docking. Zvezda added sleeping quarters, a toilet, kitchen, CO2 scrubbers, dehumidifier, oxygen generators, exercise equipment, plus data, voice and television communications with mission control. This enabled permanent habitation of the station.[69][70]

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".[71] The name "Alpha" had previously been used for the station in the early 1990s,[72] and following the request, its use was authorised for the whole of Expedition 1.[73] 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."[74] 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.[73][75][76]

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.[77]

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.

Aft view showing a Progress spacecraft docked to Zvezda

The expansion schedule was interrupted by the Space Shuttle Columbia disaster in 2003, with the resulting two year hiatus in the Space Shuttle programme halting station assembly. The space shuttle was grounded until 2005 with STS-114 flown by Discovery.[78]

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 followed shortly after by the first two components of Kibō. In March 2009, STS-119 completed the Integrated Truss Structure with the installation of the fourth and final set of solar arrays. The final section of Kibō was delivered in July 2009 on STS-127, followed by the Russian Poisk module. The third node, Tranquility, was delivered in February 2010 during STS-130 by the Space Shuttle Endeavour, alongside the Cupola, closely followed in May 2010 by the penultimate Russian module, Rassvet. 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.[79] The last pressurised module of the USOS, Leonardo, was brought to the station by Discovery on her final flight, STS-133,[80] followed by the Alpha Magnetic Spectrometer on STS-134, delivered by Endeavour.[81]

The Cupola arrived in 2010

As of June 2011, the station consisted of fifteen pressurised modules and the Integrated Truss Structure. Still to be launched are the Russian Multipurpose Laboratory Module Nauka and a number of external components, including the European Robotic Arm. Assembly is expected to be completed by April 2014,[needs update] by which point the station will have a mass in excess of 400 tonnes (440 short tons).[3][82]

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 03/09/2011).[83] 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[edit]

Russian Orbital Segment Windows
USOS International Space Station window locations

The ISS is a third generation[84] modular space station.[85] 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
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Leonardo
cargo bay
 
 
 
 
 
 
 
 
Rassvet
(MRM-1)
Russian
docking port
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
PMA 1
 
 
 
 
 
 
 
 
 
Quest
Airlock
 
 
Unity
Node 1
 
Tranquility
Node 3
PMA 3
docking port
 
 
 
 
 
 
 
 
 
 
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
 
 
Canadarm2
 
 
 
 
 
 
 
 
 
 
 
Solar array
 
 
Solar array
 
 
 
 
 
 
 
 
 
 
Solar array
 
 
Solar array
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
External
stowage
Destiny
Laboratory
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Kibō logistics
Cargo Bay
 
 
 
 
 
 
 
 
 
 
 
HTV/Dragon/Cygnus
berth (docking port)
 
 
HTV/Dragon/Cygnus
berth (docking port)
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Kibō
Robotic Arm
 
 
 
 
External
Payloads
Columbus
Laboratory
 
Harmony
(Node 2)
 
Kibō
Laboratory
Kibō
External Platform
 
 
 
 
 
 
 
 
 
 
 
 
 
PMA 2
docking port
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Pressurised modules[edit]

Zarya[edit]

Zarya as seen by Space Shuttle Endeavour during STS-88

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 specialized functionality, Zarya is now primarily used for storage, both inside the pressurized section and in the externally mounted fuel tanks. The Zarya is a descendant of the TKS spacecraft designed for the Soviet Salyut program. The name Zarya was given to the FGB because it signified the dawn of a new era of international cooperation 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 November 20, 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 December 4, 1998, to attach the Unity Module.

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

Unity[edit]

Unity as pictured by Space Shuttle Endeavour

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 aluminum. 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 December 6, 1998, the STS-88 crew mated the aft berthing port of Unity with the forward hatch of the already orbiting Zarya module.

Zvezda[edit]

Zvezda (Russian: Звезда́, meaning "star"), also known as DOS-8, Service Module or SM (Russian: СМ). It provides all of the station's critical systems,[clarification needed] its addition rendered the station permanently habitable for the first time, adding life support for up to six crew and living quarters for two. Zvezda's DMS-R computer handles guidance, navigation and control for the entire space station.[86] 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. 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, its engines are 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.

Destiny[edit]

Destiny interior in 2001

Destiny is the primary research facility for United States payloads aboard the ISS. In 2011, NASA solicited proposals for a not-for-profit group to manage 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.[87]

Quest[edit]

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.[88]

Pirs and Poisk[edit]

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 of these modules have 2 identical hatches. An outward opening hatch on the MIR space station failed after it swung open too fast after unlatching, due to a small amount of air pressure remaining in the airlock.[89] A different entry was used, and the hatch 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.[90]

Harmony node in 2011
Tranquility node in 2011

Harmony[edit]

Harmony 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 two of the radial ports, the other two can used for the HTV. American Shuttle Orbiters docked with the ISS via PMA-2, attached to the forward port.

Tranquility[edit]

Tranquility 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. Three of the four berthing locations are not used. One location has the cupola installed, and one has the docking port adapter installed.

Columbus module in 2008

Columbus[edit]

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.[91][92] 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[edit]

Not large enough for crew using spacesuits, the airlock on Kibō has a sliding drawer for external experiments.

Kibō (Japanese: きぼう, "hope") is the largest single ISS module. This laboratory is used to carry out research in space medicine, biology, Earth observations, materials production, biotechnology, communications research, and has facilities for growing plants and fish. During August 2011, an observatory mounted on Kibō, which utilises the ISS's orbital motion to image the whole sky in the X-ray spectrum, detected for the first time the moment a star was swallowed by a black hole.[93][94] The laboratory contains a total of 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.

A smaller pressurised module is attached to the top of Kibō, serving as a cargo bay. The dedicated Interorbital communications system 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. Ground staff use telepresence robotics to conduct on-orbit research without crew intervention.

Cupola[edit]

The Cupola's design has been compared to the Millennium Falcon from Star Wars.
Dmitri Kondratyev and Paolo Nespoli in the Cupola. Background left to right, Progress M-09M, Soyuz TMA-20, the Leonardo module and HTV-2.

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 the ESA resulted in the Cupola's development being resumed in 1998 by the ESA. It was built by Thales Alenia Space in Torino, 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 a 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;[95][96] the original prop lightsaber used by actor Mark Hamill as Luke Skywalker in the 1977 film was flown to the station in 2007.[97]

Rassvet[edit]

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 as a docking port for visiting spacecraft. It was flown to the ISS aboard NASA's Space Shuttle Atlantis on the STS-132 mission and connected in May 2010,[98][99] 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.[100] Rassvet was launched with the Russian Nauka Laboratory's Experiments airlock temporarily attached to it, and spare parts for the European Robotic Arm.

Leonardo[edit]

Leonardo installed

Leonardo Permanent Multipurpose Module (PMM) is a storage module attached to the Unity node.[101] 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.[102] 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.[103] The Permanent Multipurpose Module was created by converting Leonardo into a module that could be permanently attached to the station.[104][105][106]

Scheduled additional modules[edit]

Nauka[edit]

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.[107] However, the date has been postponed to February 2017.[108] Prior to the arrival of the Nauka module, a Progress spacecraft was used to remove Pirs from the station, deorbiting 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 is a module in the 20 ton[which?] class and has its own engines. Smaller Russian modules such as Pirs and Poisk were delivered by modified Progress spacecraft, however the larger modules; Zvezda, Zarya, and Nauka, are launched by Proton rockets. They are the only modules on the ISS that contain engines or navigation computers with star, sun and horizon sensors, to enable flight and station-keeping.[citation needed] Russia plans to separate Nauka, along with the rest of the Russian Orbital Segment, before the ISS is deorbited, to form the OPSEK space station.

Uzlovoy Module[edit]

The Uzlovoy Module (UM), or Node Module is a 4 metric ton[109] ball shaped module will support the 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 TMA and Progress M spacecraft. UM is to be incorporated into the ISS in 2016. It will be integrated with a special version of the Progress cargo ship and launched by a standard Soyuz rocket. The 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. However more importantly, 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.[110][111] 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)

Bigelow Expandable Activity Module[edit]

On 16 January 2013, Bigelow Aerospace was contracted by NASA to provide a Bigelow Expandable Activity Module (BEAM), scheduled to arrive at the space station in 2015 for a two-year technology demonstration.[112] BEAM is an inflatable module that will be attached to the aft hatch of the port-side Tranquility module of the International Space Station. During its two-year test run, instruments will 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. Following the test run, the module will be detached and jettisoned from the station.[113]

The cancelled Habitation module under construction in 1997

Cancelled components[edit]

Several modules planned for the station have been cancelled over the course of the ISS programme, whether for budgetary reasons, because the modules became unnecessary, or following a redesign of the station after the 2003 Columbia disaster. The US Centrifuge Accommodations Module was intended to host science experiments in varying levels of artificial gravity.[114] The US Habitation Module would have served as the station's living quarters. Instead, the sleep stations are now spread throughout the station.[115] The US Interim Control Module and ISS Propulsion Module were intended to replace functions of Zvezda in case of a launch failure.[116] The Russian Universal Docking Module, to which the cancelled Russian Research modules and spacecraft would have docked.[109] The Russian Science Power Platform would have provided the Russian Orbital Segment with a power supply independent of the ITS solar arrays,[109] and two Russian Research Modules that were planned to be used for scientific research.[117]

Unpressurised elements[edit]

ISS Truss Components breakdown showing Trusses and all ORUs in situ

The ISS features a large number of external components that do not require pressurisation. The largest such component is the Integrated Truss Structure (ITS), to which the station's main solar arrays and thermal radiators are mounted.[118] The ITS consists of ten separate segments forming a structure 108.5 m (356 ft) long.[3]

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).[82][119] Whilst 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, the platforms' primary function is to store Orbital Replacement Units (ORUs). ORUs are spare parts that can be replaced when the item either passes its design life or fails. Examples of ORUs include pumps, storage tanks, antennas and battery units. Such units are replaced either by astronauts during EVA or by robotic arms. While spare parts were routinely transported to and from the station via Space Shuttle resupply missions, there was a heavy emphasis on ORU transport once the NASA Shuttle approached retirement.[120] Several shuttle missions were dedicated to the delivery of ORUs, including STS-129,[121] STS-133[80] and STS-134.[81] 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).[122][dated info]

Construction of the Integrated Truss Structure over New Zealand.

There are also smaller exposure facilities mounted directly to laboratory modules; the JEM Exposed Facility serves as an external 'porch' for the Japanese Experiment Module complex,[123] and a facility on the European Columbus laboratory provides power and data connections for experiments such as the European Technology Exposure Facility[124][125] and the Atomic Clock Ensemble in Space.[126] A remote sensing instrument, SAGE III-ISS, is due to be delivered to the station in 2014 aboard a Dragon capsule, and the NICER experiment in 2016.[127][128] 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.[129]

Cranes and robotic arms[edit]

Canadarm2, the largest robotic arm on the ISS, has a mass of 1,800 kilograms and is used to dock and manipulate spacecraft and modules on the USOS, and hold crew members and equipment during EVAs.[130] 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 2 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.

Commander Volkov stands on Pirs with his back to the Soyuz whilst operating the manual Strela crane holding photographer Kononenko. Zarya is seen to the left and Zvezda across the bottom of the image.
Dextre, like many of the station's experiments and robotic arms, can be operated from Earth and perform tasks while the crew sleeps.

The Integrated Truss Structure serves as a base for the main remote manipulator system called the Mobile Servicing System (MSS). This consists of the Mobile Base System (MBS), the Canadarm2, and Dextre. Dextre is a 1,500 kg agile robotic manipulator with two 'arms' which have 7 degrees of movement each, a 'torso' which bends at the waist and rotates at the base, a tool holster, lights and video. Staff on earth can operate Dextre via remote control, performing work without crew intervention. The MBS rolls along rails built into some of the ITS segments to allow the arm to reach all parts of the United States segment of the station.[131] The MSS had its reach increased with an Orbiter Boom Sensor System in May 2011, used to inspect tiles on the NASA shuttle, and converted for permanent station use. To gain access to the extreme extents of the Russian Segment the crew also placed a "Power Data Grapple Fixture" to the forward docking section of Zarya, so that the Canadarm2 may inchworm itself onto that point.[132]

The European Robotic Arm, which will service the Russian Orbital Segment, will be launched alongside the Multipurpose Laboratory Module in 2017.[133] The Japanese Experiment Module's Remote Manipulator System (JFM RMS), which services the JEM Exposed Facility,[134] was launched on STS-124 and is attached to the JEM Pressurised Module.[135]

Comparison[edit]

The ISS follows Salyut and Almaz series, Cosmos 557, 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, Russian OPSEK, and the as-yet unfinished Chinese space station. The first space station, Salyut 1, and other one-piece or 'monolithic' first generation space stations, such as Salyut 2,3,4,5, DOS 2, Kosmos 557, Almaz and NASA's Skylab stations were not designed for re-supply.[136] 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.[137]

Station systems[edit]

Life support[edit]

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 Service Module Zvezda. 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[edit]

A flowchart diagram showing the components of the ISS life support system.
The interactions between the components of the ISS Environmental Control and Life Support System (ECLSS)

The atmosphere on board the ISS is similar to the Earth's.[138] Normal air pressure on the ISS is 101.3 kPa (14.7 psi);[139] 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.[140] Earth-like atmospheric conditions have been maintained on all Russian and Soviet spacecraft.[141]

The Elektron system aboard Zvezda and a similar system in Destiny generate oxygen aboard the station.[142] The crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters, a chemical oxygen generator system.[143] 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.[143]

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 Elektron unit is the primary oxygen supply, O
2
and H
2
are produced by electrolysis, with the H
2
being vented overboard. The 1 kW system uses approximately 1 litre of water per crew member per day from stored water from Earth, or water 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.[144]

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.[145] Hydrogen produced is combined with carbon dioxide from the cabin atmosphere and converted to water and methane.

Power and thermal control[edit]

Russian solar arrays, backlit by sunset.
One of the eight truss mounted pairs of USOS solar arrays

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.[146]

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.[118]

The USOS solar arrays are arranged as four wing pairs, with each wing producing nearly 32.8 kW.[118] These arrays normally track the sun to maximise power generation. Each array is about 375 m2 (450 yd2) in area and 58 metres (63 yd) long. In the complete configuration, the solar arrays track the sun by rotating the alpha gimbal once per orbit while the beta gimbal follows slower changes in the angle of the sun to the orbital plane. The Night Glider mode aligns the solar arrays parallel to the ground at night to reduce the significant aerodynamic drag at the station's relatively low orbital altitude.[147]

The station uses rechargeable nickel-hydrogen batteries (NiH2) for continuous power during the 35 minutes of every 90-minute orbit that it is eclipsed by the Earth. The batteries are recharged on the day side of the Earth. They have a 6.5-year lifetime (over 37,000 charge/discharge cycles) and will be regularly replaced over the anticipated 20-year life of the station.[148]

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.[149]

ISS External Active Thermal Control System (EATCS) diagram

The large amount of electrical power consumed by the station's systems and experiments is turned almost entirely into heat. The heat which can be dissipated through the walls of the stations modules is insufficient to keep the internal ambient temperature within comfortable, workable limits. Ammonia is continuously pumped through pipework throughout the station to collect heat, then into external radiators exposed to the cold of space, and back into the station.

The International Space Station (ISS) External Active Thermal Control System (EATCS) maintains an equilibrium when the ISS environment or heat loads exceed the capabilities of the Passive Thermal Control System (PTCS). Note Elements of the PTCS are external surface materials, insulation such as MLI, or Heat Pipes. The EATCS provides heat rejection capabilities for all the US pressurised modules, including the JEM and COF as well as the main power distribution electronics of the S0, S1 and P1 Trusses. The EATCS consists of two independent loops (Loop A & Loop B), both using mechanically pumped liquid ammonia in closed-loop circuits. The EATCS is capable of rejecting up to 70 kW, and provides a substantial upgrade in heat rejection capacity from the 14 kW capability of the Early External Active Thermal Control System (EEATCS) via the Early Ammonia Servicer (EAS), which was launched on STS-105 and installed onto the P6 Truss.[150]

Communications and computers[edit]

Diagram showing communications links between the ISS and other elements. See adjacent text for details.
The communications systems used by the ISS
* Luch satellite not currently in use

Radio communications provide telemetry and scientific data links between the station and Mission Control Centres. Radio links are also used during rendezvous and docking procedures and for audio and video communication between crewmembers, flight controllers and family members. As a result, the ISS is equipped with internal and external communication systems used for different purposes.[151]

The Russian Orbital Segment communicates directly with the ground via the Lira antenna mounted to Zvezda.[15][152] The Lira antenna also has the capability to use the Luch data relay satellite system.[15] This system, used for communications with Mir, fell into disrepair during the 1990s, and as a result is no longer in use,[15][153][154] 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.[155] 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.[156]

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 Ku 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.[9][15][151] Data channels for the Canadarm2, European Columbus laboratory and Japanese Kibō modules are routed via the S band and Ku band systems, although the European Data Relay System and a similar Japanese system will eventually complement the TDRSS in this role.[9][157] Communications between modules are carried on an internal digital wireless network.[158]

Laptop computers surround the Canadarm2 console.

UHF radio is used by astronauts and cosmonauts conducting EVAs. UHF is employed by other spacecraft that dock to or undock from the station, such as Soyuz, Progress, HTV, ATV and the Space Shuttle (except the shuttle also makes use of the S band and Ku band systems via TDRSS), to receive commands from Mission Control and ISS crewmembers.[15] 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.[159][160]

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 surrounding 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 Ku band. This provides speeds of 10 Mbit/s to and 3 Mbit/s from the station, comparable to home DSL connection speeds.[161][162]

The operating system used for key station functions is the Debian version of Linux.[163] The migration from Microsoft Windows was made in May 2013 for reasons of reliability, stability and flexibility.[164]

Station operations[edit]

Expeditions and private flights[edit]

Zarya and Unity were entered for the first time on December 10, 1998.
Soyuz TM-31 being prepared to bring the first resident crew to the station in October 2000
ISS was slowly assembled over a decade of spaceflights and crews
Expeditions have included male and female crew-members from many nations

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.[165][166] 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.[167][168]

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 on board MIR.[169] 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. Commander Michael Fincke has spent a total of 382 days in space – more than any other American astronaut.

Travellers who pay for their own passage into space are termed spaceflight participants by Roskosmos and NASA, and are sometimes informally 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.[175] The remaining seats are sold for around US$40 million to members of the public who can pass a medical. 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] Toyohiro Akiyama was flown to Mir for a week, he was classed as a business traveller, as his employer, Tokyo Broadcasting System, paid for his ticket, and he gave a daily TV broadcast from orbit.

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."[176] 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."[177] In the film, some Kazakhs are shown waiting in the middle of the steppes for four rocket stages to literally fall from the sky. Film-maker Christian Frei states "Filming the work of the Kazakh scrap metal collectors was anything but easy. The Russian authorities finally gave us a film permit in principle, but they imposed crippling preconditions on our activities. The real daily routine of the scrap metal collectors could definitely not be shown. Secret service agents and military personnel dressed in overalls and helmets were willing to re-enact their work for the cameras – in an idealised way that officials in Moscow deemed to be presentable, but not at all how it takes place in reality."

Spaceflight participant Richard Garriott placed a geocache aboard the ISS during his flight.[178] This is currently the only non-terrestrial geocache in existence.[179]

Orbit[edit]

Graph showing the changing altitude of the ISS from November 1998 until January 2009
Animation of ISS orbit from a North American geostationary point of view (sped up 1800 times)

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.[180][181] The orbital inclination chosen was also low enough to allow American space shuttles launched from Florida to reach the ISS.

It travels at an average speed of 27,724 kilometres (17,227 mi) per hour, and completes 15.53 orbits per day (93 minutes per orbit).[1][19] 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.[182][183] Other, more frequent supply ships do not require this adjustment as they are substantially lighter vehicles.[27][184]

Orbits of the ISS, shown in April 2013.

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.[184] 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.[185] This technology could allow station-keeping to be done more economically than at present.[186][187]

The Russian Orbital Segment contains the station's engines and control bridge, which handles Guidance, Navigation and Control (ROS GNC) for the entire station.[86] 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.[188] 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. 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', reaching their maximum speed, 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.[189][190][191] 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.[192]

Mission controls[edit]

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



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