International Space Station

International Space Station
A foreward view of the International Space Station backdropped by the limb of the Earth. In view are the station's four large, maroon-coloured solar array wings, two on either side of the station, mounted to a central truss structure. Further along the truss are six large, white radiators, three next to each pair of arrays. In between the solar arrays and radiators is a cluster of pressurised modules arranged in an elongated T shape, also attached to the truss. A set of blue solar arrays are mounted to the module at the aft end of the cluster.
The ISS on 23 May 2010, as seen from STS-132
ISS insignia.svg
ISS emblem.png
Station statistics
COSPAR ID 1998-067A
SATCAT no. 25544
Call sign Alpha, Station
Crew Fully crewed: 7
Currently aboard: 7
(Expedition 64) (SpaceX Crew-1)
Launch 20 November 1998; 22 years ago (1998-11-20)
Launch pad
Mass 419,725 kg (925,335 lb)[1]
Length 73.0 m (239.4 ft)[1]
Width 109.0 m (357.5 ft)[1]
Pressurised volume 915.6 m3 (32,333 cu ft)[1]
Atmospheric pressure 101.3 kPa (14.7 psi; 1.0 atm)
oxygen 21%, nitrogen 79%
Perigee altitude 418 km (259.7 mi) AMSL[2]
Apogee altitude 420 km (261.0 mi) AMSL[2]
Orbital inclination 51.64° [2]
Orbital speed 7.66 km/s [2]
(27,600 km/h; 17,100 mph)
Orbital period 92.68 minutes [2]
Orbits per day 15.54 [2]
Orbit epoch 05 December 2020 17:39:47 [2]
Days in orbit 22 years, 15 days
(5 December 2020)
Days occupied 20 years, 1 month, 3 days
(5 December 2020)
No. of orbits 116,178 as of May 2019[2]
Orbital decay 2 km/month
Statistics as of 9 March 2011
(unless noted otherwise)
References: [1][2][3][4][5]
Configuration
The components of the ISS in an exploded diagram, with modules on-orbit highlighted in orange, and those still awaiting launch in blue or pink
Station elements as of August 2019
( exploded view)

The International Space Station (ISS) is a modular space station (habitable artificial satellite) in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA (Japan), ESA (Europe), and CSA (Canada).[6][7] The ownership and use of the space station is established by intergovernmental treaties and agreements.[8] The station serves as a microgravity and space environment research laboratory in which scientific research is conducted in astrobiology, astronomy, meteorology, physics, and other fields.[9][10][11] The ISS is suited for testing the spacecraft systems and equipment required for possible future long-duration missions to the Moon and Mars.[12]

The ISS programme evolved from the Space Station Freedom, an American proposal which was conceived in 1984 to construct a permanently manned Earth-orbiting station,[13] and the contemporaneous Soviet/Russian Mir-2 proposal with similar aims. The ISS is the ninth space station to be inhabited by crews, following the Soviet and later Russian Salyut, Almaz, and Mir stations and the U.S. Skylab. It is the largest artificial object in space and the largest satellite in low Earth orbit, regularly visible to the naked eye from Earth's surface.[14][15] It maintains an orbit with an average altitude of 400 kilometres (250 mi) by means of reboost manoeuvres using the engines of the Zvezda Service Module or visiting spacecraft.[16] The ISS circles the Earth in roughly 93 minutes, completing 15.5 orbits per day.[17]

The station is divided into two sections: the Russian Orbital Segment (ROS), operated by Russia; and the United States Orbital Segment (USOS), which is shared by many nations. Roscosmos has endorsed the continued operation of ROS through 2024,[18] having previously proposed using elements of the segment to construct a new Russian space station called OPSEK.[19] The first ISS component was launched in 1998, and the first long-term residents arrived on 2 November 2000.[20] The station has since been continuously occupied for 20 years and 33 days,[21] the longest continuous human presence in low Earth orbit, having surpassed the previous record of 9 years and 357 days held by the Mir space station. The latest major pressurised module, Leonardo, was fitted in 2011 and an experimental inflatable space habitat was added in 2016. Development and assembly of the station continues, with several major new Russian elements scheduled for launch starting in 2020. As of December 2018, the station is expected to operate until 2030.[22]

The ISS consists of pressurised habitation modules, structural trusses, photovoltaic solar arrays, thermal radiators, docking ports, experiment bays and robotic arms. Major ISS modules have been launched by Russian Proton and Soyuz rockets and US Space Shuttles.[23] The station is serviced by a variety of visiting spacecraft: the Russian Soyuz and Progress, the U.S. Dragon and Cygnus, the Japanese H-II Transfer Vehicle,[6] and, formerly, the European Automated Transfer Vehicle. The Dragon spacecraft allows the return of pressurised cargo to Earth, which is used, for example, to repatriate scientific experiments for further analysis. As of September 2019, 239 astronauts, cosmonauts, and space tourists from 19 different nations have visited the space station, many of them multiple times; this includes 151 Americans, 47 Russians, nine Japanese, eight Canadians, and five Italians.[24]

Purpose

The ISS was originally intended to be a laboratory, observatory, and factory while providing transportation, maintenance, and a low Earth orbit staging base for possible future missions to the Moon, Mars, and asteroids. However, not all of the uses envisioned in the initial memorandum of understanding between NASA and Roscosmos have come to fruition.[25] In the 2010 United States National Space Policy, the ISS was given additional roles of serving commercial, diplomatic,[26] and educational purposes.[27]

Scientific research

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
Fisheye view of several labs
CubeSats are deployed by the NanoRacks CubeSat Deployer

The ISS provides a platform to conduct scientific research, with power, data, cooling, and crew available to support experiments. Small uncrewed spacecraft can also provide platforms for experiments, especially those involving 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.[28][29]

The ISS simplifies individual experiments by allowing groups of experiments to share the same launches and crew time. Research is conducted in a wide variety of fields, including astrobiology, astronomy, physical sciences, materials science, space weather, meteorology, and human research including space medicine and the life sciences.[9][10][11][30][31] Scientists on Earth have timely access to the data and can suggest experimental modifications to the crew. If follow-on experiments are necessary, the routinely scheduled launches of resupply craft allows new hardware to be launched with relative ease.[29] Crews fly expeditions of several months' duration, providing approximately 160 person-hours per week of labour with a crew of six. However, a considerable amount of crew time is taken up by station maintenance.[9][32]

Perhaps the most notable ISS experiment is the Alpha Magnetic Spectrometer (AMS), which is intended to detect dark matter and answer other fundamental questions about our universe and is as important as the Hubble Space Telescope according to NASA. Currently docked on station, it could not have been easily accommodated on a free flying satellite platform because of its power and bandwidth needs.[33][34] On 3 April 2013, scientists reported that hints of dark matter may have been detected by the AMS.[35][36][37][38][39][40] 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.[41] Some simple forms of life called extremophiles,[42] as well as small invertebrates called tardigrades[43] can survive in this environment in an extremely dry state through 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 high duration 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.[44][45]

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.[46][47][48]

In August 2020, scientists reported that bacteria from Earth, particularly Deinococcus radiodurans bacteria, which is highly resistant to environmental hazards, were found to survive for three years in outer space, based on studies conducted on the International Space Station. These findings support the notion of panspermia, the hypothesis that life exists throughout the Universe, distributed in various ways, including space dust, meteoroids, asteroids, comets, planetoids or contaminated spacecraft.[49][50]

ISS crew member storing samples
A comparison between the combustion of a candle on Earth (left) and in a free fall environment, such as that found on the ISS (right)

Gravity at the altitude of the ISS is approximately 90% as strong as at Earth's surface, but objects in orbit are in a continuous state of freefall, resulting in an apparent state of weightlessness.[51] This perceived weightlessness is disturbed by five separate effects:[52]

  • Drag from the residual atmosphere.
  • Vibration from the 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.[10]

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.[10]

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.[10]

Exploration

A 3D plan of the Russia-based MARS-500 complex, used for conducting ground-based experiments that complement ISS-based preparations for a human 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.[12] 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 crewed 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 Charles Bolden stated in February 2011, "Any mission to Mars is likely to be a global effort".[58] Currently, US federal legislation prevents NASA co-operation with China on space projects.[59]

Education and cultural outreach

Original Jules Verne manuscripts displayed by crew inside the Jules Verne ATV

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.[6][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 3D model of the interior and exterior of the ISS, and face spontaneous challenges to solve in real time.[62]

JAXA aims to inspire children to "pursue craftsmanship" and to heighten their "awareness of the importance of life and their responsibilities in society".[63] Through a series of education guides, students develop a deeper understanding of the past and near-term future of crewed space flight, as well as that of Earth and life.[64][65] In the JAXA "Seeds in Space" experiments, the mutation effects of spaceflight on plant seeds aboard the ISS are explored by growing sunflower seeds that have flown on the ISS for about nine months. 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 of the ISS programme. Tetsuo Tanaka, the director of JAXA's Space Environment and Utilization Center, has said: "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 that 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 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 space station.[68]

Spoken voice recording by ESA astronaut Paolo Nespoli on the subject of the ISS, produced in November 2017 for Wikipedia

First Orbit is a feature-length documentary film about Vostok 1, the first crewed space flight around the Earth. By matching the orbit of the ISS 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 is credited as the director of photography for this documentary film, as he recorded the majority of the footage himself during Expedition 26/27.[69] The film was streamed in a global YouTube premiere in 2011 under a free licence through the website firstorbit.org.[70]

In May 2013, commander Chris Hadfield shot a music video of David Bowie's "Space Oddity" on board the station, which was released on YouTube.[71][72] It was the first music video ever to be filmed in space.[73]

In November 2017, while participating in Expedition 52/53 on the ISS, Paolo Nespoli made two recordings of his spoken voice (one in English and the other in his native Italian), for use on Wikipedia articles. These were the first content made in space specifically for Wikipedia.[74][75]

Construction

Manufacturing

ISS module Node 2 manufacturing and processing in the Space Station Processing Facility

Since the International Space Station is a multi-national collaborative project, the components for in-orbit assembly were manufactured in various countries around the world. Beginning in the mid 1990s, the U.S. components Destiny, Unity, the Integrated Truss Structure, and the solar arrays were fabricated at the Marshall Space Flight Center and the Michoud Assembly Facility. These modules were delivered to the Operations and Checkout Building and the Space Station Processing Facility (SSPF) for final assembly and processing for launch.[76]

The Russian modules, including Zarya and Zvezda, were manufactured at the Khrunichev State Research and Production Space Center in Moscow. Zvezda was initially manufactured in 1985 as a component for Mir-2, but was never launched and instead became the ISS Service Module.[77]

The European Space Agency Columbus module was manufactured at the EADS Astrium Space Transportation facilities in Bremen, Germany, along with many other contractors throughout Europe.[78] The other ESA-built modules—Harmony, Tranquility, the Leonardo MPLM, and the Cupola—were initially manufactured at the Thales Alenia Space factory in Turin, Italy. The structural steel hulls of the modules were transported by aircraft to the Kennedy Space Center SSPF for launch processing.[79]

The Japanese Experiment Module Kibō, was fabricated in various technology manufacturing facilities in Japan, at the NASDA (now JAXA) Tsukuba Space Center, and the Institute of Space and Astronautical Science. The Kibo module was transported by ship and flown by aircraft to the SSPF.[80]

The Mobile Servicing System, consisting of the Canadarm2 and the Dextre grapple fixture, was manufactured at various factories in Canada (such as the David Florida Laboratory) and the United States, under contract by the Canadian Space Agency. The mobile base system, a connecting framework for Canadarm2 mounted on rails, was built by Northrop Grumman.

Assembly

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 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. 127 of these spacewalks originated from the station, and the remaining 32 were launched from the airlocks of docked Space Shuttles.[81] The beta angle of the station had to be considered at all times during construction.[82]

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 Pressurised Mating Adapters (PMAs), one connects permanently to Zarya, the other allowed the Space Shuttle to dock to the space station. At that time, the Russian station Mir was still inhabited, and the ISS remained uncrewed for two years. 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.[83][84]

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".[85] The name "Alpha" had previously been used for the station in the early 1990s,[86] and its use was authorised for the whole of Expedition 1.[87] 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."[88] Yuri Semenov, the President of Russian Space Corporation Energia at the time, disapproved of the name "Alpha" as he felt that Mir was the first modular space station, so the names "Beta" or "Mir 2" for the ISS would have been more fitting.[87][89][90]

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 four existing solar arrays.[91]

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 hiatus in flights. The Space Shuttle was grounded until 2005 with STS-114 flown by Discovery.[92]

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 US-funded Zarya module in 1998.[93] The last pressurised module of the USOS, Leonardo, was brought to the station in February 2011 on the final flight of Discovery, STS-133.[94] The Alpha Magnetic Spectrometer was delivered by Endeavour on STS-134 the same year.[95]

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 Prichal module, and two power modules called NEM-1 and NEM-2.[96] As of May 2020, Russia's future primary research module Nauka is set to launch in the spring of 2021,[97] along with the European Robotic Arm which will be able to relocate itself to different parts of the Russian modules of the station.[98]

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).[99] 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.

Structure

Technical blueprint of components

The ISS is a third generation[100] modular space station.[101] Modular stations can allow modules to be added to 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. 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
Prichal
Solar array (retracted) Zarya FGB
(first module)
Solar array (retracted)
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
Bishop
airlock
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
IDA 3
docking adapter
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
Axiom modules

Pressurised modules

Zarya as seen by Space Shuttle Endeavour during STS-88
Unity as seen by Space Shuttle Endeavour during STS-88
Zvezda as seen by Space Shuttle Endeavour during STS-97
The Destiny module being installed on the ISS
Quest Joint Airlock Module
The Pirs module attached to the ISS.
Poisk after arriving at the ISS on 12 November 2009.

Pirs (Russian: Пирс, lit. 'Pier') and Poisk (Russian: По́иск, lit. 'Search') are Russian airlock modules, each having two 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.[111] 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.[112]

Pirs was launched on 14 September 2001, as ISS Assembly Mission 4R, on a Russian Soyuz-U rocket, using a modified Progress spacecraft, Progress M-SO1, as an upper stage. Poisk was launched on 10 November 2009[113][114] attached to a modified Progress spacecraft, called Progress M-MIM2, on a Soyuz-U rocket from Launch Pad 1 at the Baikonur Cosmodrome in Kazakhstan.

Harmony shown connected to Columbus, Kibo, and Destiny. PMA-2 faces. The nadir and zenith locations are open.
Tranquility in 2011
The Columbus module on the ISS
Kibō Exposed Facility on the right
The Cupola 's windows with shutters open.
Rassvet as seen from the Cupola module during STS-132 with a Progress in the lower right
Leonardo Permanent Multipurpose Module
Progression of the expansion of BEAM
IDA-1 upright

Unpressurised elements

ISS Truss Components breakdown showing Trusses and all ORUs in situ

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.[130] The ITS consists of ten separate segments forming a structure 108.5 metres (356 ft) long.[3]

The station was intended to have several smaller external components, such as six robotic arms, three External Stowage Platforms (ESPs) and four ExPRESS Logistics Carriers (ELCs).[131][132] 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, including pumps, storage tanks, antennas, and battery units. Such units are replaced either by astronauts during EVA or by robotic arms.[133] Several shuttle missions were dedicated to the delivery of ORUs, including STS-129,[134] STS-133[135] and STS-134.[136] 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).[137][needs update]

Construction of the Integrated Truss Structure over New Zealand.

There are also smaller exposure facilities mounted directly to laboratory modules; the Kibō Exposed Facility serves as an external "porch" for the Kibō complex,[138] and a facility on the European Columbus laboratory provides power and data connections for experiments such as the European Technology Exposure Facility[139][140] and the Atomic Clock Ensemble in Space.[141] A remote sensing instrument, SAGE III-ISS, was delivered to the station in February 2017 aboard CRS-10,[142] and the NICER experiment was delivered aboard CRS-11 in June 2017.[143] The largest 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.[144][145]

The commercial Bartolomeo External Payload Hosting Platform, manufactured by Airbus, was launched on 6 March 2020 aboard CRS-20 and attached to the European Columbus module. It will provide an additional 12 external payload slots, supplementing the eight on the ExPRESS Logistics Carriers, ten on Kibō, and four on Columbus. The system is designed to be robotically serviced and will require no astronaut intervention. It is named after Christopher Columbus's younger brother.[146][147][148]

Commander Volkov stands on Pirs with his back to the Soyuz whilst operating the manual
Strela crane (which is holding photographer Oleg Kononenko).
Dextre, like many of the station's experiments and robotic arms, can be operated from Earth, allowing tasks to be performed while the crew sleeps.

The Integrated Truss Structure serves as a base for the station's primary remote manipulator system, 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.[149]
  • Dextre is a 1,560 kg (3,440 lb) robotic manipulator that has two arms and a rotating torso, with power tools, lights, and video for replacing orbital replacement units (ORUs) and performing other tasks requiring fine control.[150]
  • The Mobile Base System (MBS) is a platform that rides on rails along the length of the station's main truss, which serves as a mobile base for Canadarm2 and Dextre, allowing the robotic arms to reach all parts of the USOS.[151]

A grapple fixture was added to Zarya on STS-134 to enable Canadarm2 to inchworm itself onto the Russian Orbital Segment.[152] 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 which can be used on the station to increase the reach of the MSS.[152] Staff on Earth or the ISS can operate the MSS components using remote control, performing work outside the station without the need for space walks.

Japan's Remote Manipulator System, which services the Kibō Exposed Facility,[153] was launched on STS-124 and is attached to the Kibō Pressurised Module.[154] 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.

Planned components

The European Robotic Arm, which will service the Russian Orbital Segment, will be launched alongside the Multipurpose Laboratory Module in 2020.[155] 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).

Artist's rendering of the Nauka module docked to Zvezda

Nauka (Russian: Нау́ка, lit. 'Science'), also known as the Multipurpose Laboratory Module (MLM), (Russian: Многофункциональный лабораторный модуль, or МЛМ), is a component of the ISS that has yet to be launched into space. The MLM is funded by the Roscosmos State Corporation. In the original ISS plans, Nauka was to use the location of the Docking and Stowage Module (DSM), but the DSM was later replaced by the Rassvet module and moved to Zarya's nadir port. Planners now anticipate that Nauka will dock at Zvezda's nadir port, replacing the Pirs module.[156][157]

The launch of Nauka, initially planned for 2007, has been repeatedly delayed for various reasons.[158] As of May 2020, the launch to the ISS is assigned to no earlier than spring 2021.[97] After this date, the warranties of some of Nauka's systems will expire.

Mockup of the Prichal module at the Yuri Gagarin Cosmonaut Training Center

Prichal, also known as Uzlovoy Module or UM (Russian: Узловой Модуль Причал, lit. 'Nodal Module Berth'),[159] is a 4-tonne (8,800 lb)[160] 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 the third quarter of 2021.[161] It will be integrated with a special version of the Progress cargo ship and launched by a standard Soyuz rocket, docking to the nadir port of the Nauka 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 intended to serve as the only permanent element of the cancelled OPSEK.[161][162][157]

Science Power Module 1 (SPM-1, also known as NEM-1) and Science Power Module 2 (SPM-2, also known as NEM-2) are modules that are planned to arrive at the ISS not earlier than 2024.[163] They will dock to the Prichal module, which is planned to be attached to the Nauka module.[157] If Nauka is cancelled, then Prichal, SPM-1, and SPM-2 would dock at the zenith port of the Zvezda module. SPM-1 and SPM-2 would also be required components for the OPSEK space station.[164]

The NanoRacks Bishop Airlock Module is a commercially-funded airlock module intended to be launched to the ISS on SpaceX CRS-21 in December 2020.[165][166] The module is being built by NanoRacks, Thales Alenia Space, and Boeing.[167] It will be used to deploy CubeSats, small satellites, and other external payloads for NASA, CASIS, and other commercial and governmental customers.[168]

In January 2020, NASA awarded Axiom Space a contract to build a commercial module for the ISS with a launch date of 2024. The contract is under the NextSTEP2 program. NASA negotiated with Axiom on a firm fixed-price contract basis to build and deliver the module, which will attach to the forward port of the space station's Harmony (Node 2) module. Although NASA has only commissioned one module, Axiom plans to build an entire segment consisting of five modules, including a node module, an orbital research and manufacturing facility, a crew habitat, and a "large-windowed Earth observatory". The Axiom segment is expected to greatly increase the capabilities and value of the space station, allowing for larger crews and private spaceflight by other organisations. Axiom plans to convert the segment into a stand-alone space station once the ISS is decommissioned, with the intention that this would act as a successor to the ISS.[169][170][171]

Proposed components

Made by Bigelow Aerospace. In August 2016 Bigelow negotiated an agreement with NASA to develop a full-sized ground prototype Deep Space Habitation based on the B330 under the second phase of Next Space Technologies for Exploration Partnerships. The module is called the Expandable Bigelow Advanced Station Enhancement (XBASE), as Bigelow hopes to test the module by attaching it to the International Space Station.

Nanoracks, after finalizing its contract with NASA, and after winning NextSTEPs Phase II award, is now developing its concept Independence-1 (previously known as Ixion), which would turn spent rocket tanks into a habitable living area to be tested in space. In Spring 2018, Nanoracks announced that Ixion is now known as the Independence-1, the first 'outpost' in Nanoracks' Space Outpost Program.

If produced, this centrifuge will be the first in-space demonstration of sufficient scale centrifuge for artificial partial-g effects. It will be designed to become a sleep module for the ISS crew.

Cancelled components

The cancelled Habitation module under construction at Michoud in 1997

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.[172] The US Habitation Module would have served as the station's living quarters. Instead, the living quarters are now spread throughout the station.[173] The US Interim Control Module and ISS Propulsion Module would have replaced the functions of Zvezda in case of a launch failure.[174] Two Russian Research Modules were planned for scientific research.[175] They would have docked to a Russian Universal Docking Module.[176] The Russian Science Power Platform would have supplied power to the Russian Orbital Segment independent of the ITS solar arrays.

Onboard 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 Nauka laboratory has a complete set of life support systems.

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.[177] Normal air pressure on the ISS is 101.3 kPa (14.69 psi);[178] the same as at sea level on Earth. An Earth-like atmosphere offers benefits for crew comfort, and is much safer than a pure oxygen atmosphere, because of the increased risk of a fire such as that responsible for the deaths of the Apollo 1 crew.[179] Earth-like atmospheric conditions have been maintained on all Russian and Soviet spacecraft.[180]

The Elektron system aboard Zvezda and a similar system in Destiny generate oxygen aboard the station.[181] The crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters, a chemical oxygen generator system.[182] 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.[182]

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 (1.3 hp) 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 (842–932 °F), producing 600 litres (130 imp gal; 160 US gal) of O
2
. This unit is manually operated.[183]

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

Power and thermal control

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

Double-sided solar arrays provide electrical power to the ISS. These bifacial cells collect direct sunlight on one side and light reflected off from the Earth on the other, and are more efficient and operate at a lower temperature than single-sided cells commonly used on Earth.[185]

The Russian segment of the station, like most spacecraft, uses 28 V low voltage 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 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.[186] 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.[187]

The station originally used 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 orbit. They had a 6.5-year lifetime (over 37,000 charge/discharge cycles) and were regularly replaced over the anticipated 20-year life of the station.[188] Starting in 2016, the nickel–hydrogen batteries were replaced by lithium-ion batteries, which are expected to last until the end of the ISS program.[189]

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.[190]

ISS External Active Thermal Control System (EATCS) diagram

The station's systems and experiments consume a large amount of electrical power, almost all of which is converted to heat. To keep the internal temperature within workable limits, a 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. From the heat exchangers, ammonia is pumped into external radiators that emit heat as infrared radiation, then back to the station.[191] 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.[192]

Communications and computers

Diagram showing communications links between the ISS and other elements.
The communications systems used by the ISS
* Luch and the Space Shuttle are not in use as of 2020

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.[193]

The Russian Orbital Segment communicates directly with the ground via the Lira antenna mounted to Zvezda.[6][194] The Lira antenna also has the capability to use the Luch data relay satellite system.[6] This system fell into disrepair during the 1990s, and so was not used during the early years of the ISS,[6][195][196] 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.[197] Another Russian communications system is the Voskhod-M, which enables internal telephone communications between Zvezda, Zarya, Pirs, Poisk, and the USOS and provides a VHF radio link to ground control centres via antennas on Zvezda's exterior.[198]

The US Orbital Segment (USOS) makes use of two separate radio links mounted in the Z1 truss structure: the S band (audio) and Ku band (audio, video and data) systems. These transmissions are routed via the United States Tracking and Data Relay Satellite System (TDRSS) in geostationary orbit, allowing for almost continuous real-time communications with Christopher C. Kraft Jr. Mission Control Center (MCC-H) in Houston.[6][23][193] Data channels for the Canadarm2, European Columbus laboratory and Japanese Kibō modules were originally also routed via the S band and Ku band systems, with the European Data Relay System and a similar Japanese system intended to eventually complement the TDRSS in this role.[23][199] Communications between modules are carried on an internal wireless network.[200]

An array of laptops in the US lab
Laptop computers surround the Canadarm2 console

UHF radio is used by astronauts and cosmonauts conducting EVAs and other spacecraft that dock to or undock from the station.[6] Automated spacecraft are fitted with their own communications equipment; the ATV uses a laser attached to the spacecraft and the Proximity Communications Equipment attached to Zvezda to accurately dock with the station.[201][202]

The ISS is equipped with about 100 IBM/Lenovo ThinkPad and HP ZBook 15 laptop computers. The laptops have run Windows 95, Windows 2000, Windows XP, Windows 7, Windows 10 and Linux operating systems.[203] 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 ethernet, which connects to the ground via Ku band. While originally this provided speeds of 10 Mbit/s download and 3 Mbit/s upload from the station,[204][205] NASA upgraded the system in late August 2019 and increased the speeds to 600 Mbit/s.[206][207] Laptop hard drives occasionally fail and must be replaced.[208] Other computer hardware failures include instances in 2001, 2007 and 2017; some of these failures have required EVAs to replace computer modules in externally mounted devices.[209][210][211][212]

The operating system used for key station functions is the Debian Linux distribution.[213] The migration from Microsoft Windows was made in May 2013 for reasons of reliability, stability and flexibility.[214]

In 2017, an SG100 Cloud Computer was launched to the ISS as part of OA-7 mission.[215] It was manufactured by NCSIST of Taiwan and designed in collaboration with Academia Sinica, and National Central University under contract for NASA.[216]

Operations

Expeditions

Zarya and Unity were entered for the first time on 10 December 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

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 three-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 six around 2010.[217][218] With the planned arrival of crew on US commercial vehicles in the early 2020s,[219] expedition size may be increased to seven crew members, the number ISS is designed for.[220][221]

Gennady Padalka, member of Expeditions 9, 19/20, 31/32, and 43/44, and Commander of Expedition 11, has spent more time in space than anyone else, a total of 878 days, 11 hours, and 29 minutes.[222] Peggy Whitson has spent the most time in space of any American, totalling 665 days, 22 hours, and 22 minutes during her time on Expeditions 5, 16, and 50/51/52.[223]

Private flights

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.[b] 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 six, space tourism was halted, as the partners relied on Russian transport seats for access to the station. Soyuz flight schedules increase after 2013, allowing five Soyuz flights (15 seats) with only two expeditions (12 seats) required.[231] 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 person to pay for his own passage to the ISS.[c]

Anousheh Ansari 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."[232] 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."[233]

In 2008, spaceflight participant Richard Garriott placed a geocache aboard the ISS during his flight.[234] This is currently the only non-terrestrial geocache in existence.[235] At the same time, the Immortality Drive, an electronic record of eight digitised human DNA sequences, was placed aboard the ISS.[236]

Fleet operations

Dragon and Cygnus cargo vessels were docked at the ISS together for the first time in April 2016.
Japan's Kounotori 4 berthing

A wide variety of crewed and uncrewed spacecraft have supported the station's activities. Flights to the ISS include 37 Space Shuttle missions, 75 Progress resupply spacecraft (including the modified M-MIM2 and M-SO1 module transports), 59 crewed Soyuz spacecraft, 5 ATVs, 9 Japanese HTVs, 20 SpaceX Dragon and 13 Cygnus missions.[citation needed]

There are currently 8 available docking ports for visiting spacecrafts. [237]

  1. Harmony forward (with PMA 2 / IDA 2)
  2. Harmony zenith (with PMA 3 / IDA 3)
  3. Harmony nadir
  4. Unity nadir
  5. Pirs nadir
  6. Poisk zenith
  7. Rassvet nadir
  8. Zvezda aft
Rendering of the ISS Visiting Vehicle Launches, Arrivals and Departures. Live link at: nasa.gov/feature/visiting-vehicle-launches-arrivals-and-departures
Key
Spacecraft and mission Location Arrival (UTC) Departure (planned)
Russia Progress MS No. 448 Progress MS-14 Zvezda aft 25 April 2020[242] December 2020[243]
Russia Progress MS No. 444 Progress MS-15 Pirs nadir 23 July 2020[244] 23 April 2021[245]
United States S.S. Kalpana Chawla NG-14 Unity nadir 5 October 2020[246] 6 December 2020[243]
Russia Soyuz MS Favor Soyuz MS-17 Rassvet nadir 14 October 2020 17 April 2021[247]
United States Crew Dragon Resilience Crew-1 PMA 2 / IDA 2 forward 17 November 2020 TBD
  • 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.
Key
The Progress M-14M resupply vehicle as it approaches the ISS in 2012. Over 50 unpiloted Progress spacecraft have been sent with supplies during the lifetime of the station.
Space Shuttle Endeavour, ATV-2, Soyuz TMA-21 and Progress M-10M docked to the ISS, as seen from the departing Soyuz TMA-20.

All Russian spacecraft and self-propelled modules are able to rendezvous and dock to the space station without human intervention using the Kurs radar docking system 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. Progress and ATV supply craft can remain at the ISS for six months,[253][254] allowing great flexibility in crew time for loading and unloading of supplies and trash.

From the initial station programs, the Russians pursued an automated docking 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 operations.[255]

Soyuz spacecraft used for crew rotation also serve as lifeboats for emergency evacuation; they are replaced every six months and were used after the Columbia disaster to return stranded crew from the ISS.[256] 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.[81] Soyuz crew rotation flights and Progress resupply flights visit the station on average two and three times respectively each year.[257]

Other vehicles berth instead of docking. 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. Berthed craft can transfer International Standard Payload Racks. Japanese spacecraft berth for one to two months.[258] The berthing Cygnus and SpaceX Dragon were contracted to fly cargo to the station under the phase 1 of the Commercial Resupply Services program.[259][260]

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.[261] On 25 May 2012, SpaceX delivered the first commercial cargo with a Dragon spacecraft.[262]

Prior to a ship's docking to the ISS, navigation and attitude control (GNC) is handed over to the ground control of the ship's 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. Before its retirement, Shuttle launches were often given priority over Soyuz, with occasional priority given to Soyuz arrivals carrying crew and time-critical cargoes, such as biological experiment materials.[263]

Repairs

Spare parts are called ORUs; some are externally stored on pallets called ELCs and ESPs.
Two black and orange solar arrays, shown uneven and with a large tear visible. A crew member in a spacesuit, attached to the end of a robotic arm, holds a latticework between two solar sails.
While anchored on the end of the OBSS during STS-120, astronaut Scott Parazynski performs makeshift repairs to a US solar array that damaged itself when unfolding.
Mike Hopkins during a spacewalk

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. Most 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 provide electricity for many parts that could be damaged by the cold of space and require heating. The larger logistics carriers also have local area network (LAN) connections for 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. Serious problems include an air leak from the USOS in 2004,[264] the venting of fumes from an Elektron oxygen generator in 2006,[265] and the failure of the computers in the ROS in 2007 during STS-117 that left the station without thruster, Elektron, Vozdukh and other environmental control system operations. In the latter case, the root cause was found to be condensation inside electrical connectors leading to a short circuit.[266]

During STS-120 in 2007 and following the relocation of the P6 truss and solar arrays, it was noted during the solar array had torn and was not deploying properly.[267] An EVA was carried out by Scott Parazynski, assisted by Douglas Wheelock. Extra precautions were taken to reduce the risk of electric shock, as the repairs were carried out with the solar array exposed to sunlight.[268] 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 bearing surfaces, so the joint was locked to prevent further damage.[269][270] Repairs to the joints were carried out during STS-126 with lubrication and the replacement of 11 out of 12 trundle bearings on the joint.[271][272]

In September 2008, damage to the S1 radiator was first noticed in Soyuz imagery. The problem was initially not thought to be serious.[273] 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. 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 then used to vent the ammonia from the damaged panel, eliminating the possibility of an ammonia leak.[273] It is also known that a Service Module thruster cover struck the S1 radiator after being jettisoned during an EVA in 2008, but its effect, if any, has not been determined.

In the early hours of 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.[274][275][276] 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.[277][278] A third EVA was required to restore Loop A to normal functionality.[279][280]

The USOS's cooling system is largely built by the US company Boeing,[281] which is also the manufacturer of the failed pump.[274]

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. Each MBSU has two power channels that feed 160V DC from the arrays to two DC-to-DC power converters (DDCUs) that supply the 124V power used in the station. In late 2011 MBSU-1 ceased responding to commands or sending data confirming its health. While still routing power correctly, it was scheduled to be swapped out at the next available EVA. A spare MBSU was already on board, but a 30 August 2012 EVA failed to be completed when a bolt being tightened to finish installation of the spare unit jammed before the electrical connection was secured.[282] The loss of MBSU-1 limited the station to 75% of its normal power capacity, requiring minor limitations in normal operations until the problem could be addressed.

On 5 September 2012, in a second six-hour EVA, astronauts Sunita Williams and Akihiko Hoshide successfully replaced MBSU-1 and restored the ISS to 100% power.[283]

On 24 December 2013, astronauts installed 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.[284]

Mission control centres

The components of the ISS are operated and monitored by their respective space agencies at mission control centres across the globe, including RKA Mission Control Center, ATV Control Centre, JEM Control Center and HTV Control Center at Tsukuba Space Center, Christopher C. Kraft Jr. Mission Control Center, Payload Operations and Integration Center, Columbus Control Center and Mobile Servicing System Control.

Life aboard

Crew activities

Gregory Chamitoff peers out of a window
STS-122 mission specialists working on robotic equipment in the US lab

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.[285]

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 Space Shuttle mission.[286][287][288]

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.[289][290] The USOS quarters are private, approximately person-sized soundproof booths. The ROS 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.[291][292][293] 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.[294] 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.[291] During various station activities and crew rest times, the lights in the ISS can be dimmed, switched off, and colour temperatures adjusted.[295][296]

Food and personal hygiene

Nine astronauts seated around a table covered in open cans of food strapped down to the table. In the background a selection of equipment is visible, as well as the salmon-coloured walls of the Unity node.
The crews of STS-127 and Expedition 20 enjoy a meal inside Unity.
Fresh fruits and vegetables are also grown in the International Space Station

On the USOS, 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,[291] so efforts are taken to make the food more palatable, including 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, and liquid condiments are preferred over solid 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.[292] Drinks are provided as dehydrated powder that is mixed with water before consumption.[292][293] Drinks and soups are sipped from plastic bags with straws, while 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.[293]

Space toilet in the Zvezda service module
The main toilet in the US Segment inside the Node 3 module

Showers on space stations were introduced in the early 1970s on Skylab and Salyut 3.[297]:139 By Salyut 6, in the early 1980s, the crew complained of the complexity of showering in space, which was a monthly activity.[298] 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.[294][299]

There are two space toilets on the ISS, both of Russian design, located in Zvezda and Tranquility.[292] 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.[291] 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.[292][300] 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.[293]

Crew health and safety

On 12 April 2019, NASA reported medical results from the Astronaut Twin Study. One astronaut twin spent a year in space on the ISS, while the other twin spent the year on Earth. Several long-lasting changes were observed, including those related to alterations in DNA and cognition, when one twin was compared with the other.[301][302]

In November 2019, researchers reported that astronauts experienced serious blood flow and clot problems while on board the ISS, based on a six-month study of 11 healthy astronauts. The results may influence long-term spaceflight, including a mission to the planet Mars, according to the researchers.[303][304]

Video of the Aurora Australis, taken by the crew of Expedition 28 on an ascending pass from south of Madagascar to just north of Australia over the Indian Ocean

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) from the Earth's surface, depending on Solar activity, the magnetosphere begins to deflect solar wind around Earth and the space station. 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.[305][306]

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, ISS crews are exposed to approximately one millisievert each day (about a year's worth of natural exposure on Earth), resulting in a higher risk of cancer. Radiation can penetrate living tissue and damage the DNA and chromosomes of lymphocytes; being central to the immune system, any damage to these cells 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 medications may lower the risks to an acceptable level.[44]

Radiation levels on the ISS are about five times greater than those experienced by airline passengers and crew, as Earth's electromagnetic field provides almost the same level of protection against solar and other types of 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; this is only one fifth the rate experienced by an astronaut in LEO. Additionally, airline passengers experience this level of radiation for a few hours of flight, while the ISS crew are exposed for their whole stay on board the station.[307]

Cosmonaut Nikolai Budarin at work inside Zvezda service module crew quarters

There is considerable evidence that psychosocial stressors are among the most important impediments to optimal crew morale and performance.[308] 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 crewed missions began, was rekindled when astronauts joined cosmonauts on the Russian space station Mir. Common sources of stress in early US 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.[309] ISS crew flights typically last about five to six months.

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. Astronauts must speak English and Russian, and knowing additional languages is even better.[310]

Due to the lack of gravity, confusion often occurs. Even though there is no up and down in space, some crew members feel like they are oriented upside down. They may also have difficulty measuring distances. This can cause problems like getting lost inside the space station, pulling switches in the wrong direction or misjudging the speed of an approaching vehicle during docking.[311]

A man running on a treadmill, smiling at the camera, with bungee cords stretching down from his waistband to the sides of the treadmill
Astronaut Frank De Winne, attached to the TVIS treadmill with bungee cords aboard the ISS

The physiological 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.[44]

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

To prevent some of the adverse effects on the body, the station is equipped with: two TVIS treadmills (including the COLBERT); the ARED (Advanced Resistive Exercise Device), which enables various weightlifting exercises that add muscle without raising (or compensating for) the astronauts' reduced bone density;[312] and a stationary bicycle. Each astronaut spends at least two hours per day exercising on the equipment.[291][292] Astronauts use bungee cords to strap themselves to the treadmill.[313][314]

Hazardous moulds that can foul air and water filters may develop aboard space stations. They can produce acids that degrade metal, glass, and rubber. They can also be harmful to the crew's health. Microbiological hazards have led to a development of the LOCAD-PTS which identifies common bacteria and moulds faster than standard methods of culturing, which may require a sample to be sent back to Earth.[315] Researchers in 2018 reported, after detecting the presence of five Enterobacter bugandensis bacterial strains on the ISS (none of which are pathogenic to humans), that microorganisms on the ISS should be carefully monitored to continue assuring a medically healthy environment for astronauts.[316][317]

Contamination on space stations can be prevented by reduced humidity, and by using paint that contains mould-killing chemicals, as well as the use of antiseptic solutions. All materials used in the ISS are tested for resistance against fungi.[318]

In April 2019, NASA reported that a comprehensive study had been conducted into the microorganisms and fungi present on the ISS. The results may be useful in improving the health and safety conditions for astronauts.[319][320]

Space flight is not inherently quiet, with noise levels exceeding acoustic standards as far back as the Apollo missions.[321][322] For this reason, NASA and the International Space Station international partners have developed noise control and hearing loss prevention goals as part of the health program for crew members. Specifically, these goals have been the primary focus of the ISS Multilateral Medical Operations Panel (MMOP) Acoustics Subgroup since the first days of ISS assembly and operations.[323][324] The effort includes contributions from acoustical engineers, audiologists, industrial hygienists, and physicians who comprise the subgroup's membership from NASA, the Russian Space Agency (RSA), the European Space Agency (ESA), the Japanese Aerospace Exploration Agency (JAXA), and the Canadian Space Agency (CSA).

When compared to terrestrial environments, the noise levels incurred by astronauts and cosmonauts on the ISS may seem insignificant and typically occur at levels that would not be of major concern to the Occupational Safety and Health Administration – rarely reaching 85 dBA. But crew members are exposed to these levels 24 hours a day, seven days a week, with current missions averaging six months in duration. These levels of noise also impose risks to crew health and performance in the form of sleep interference and communication, as well as reduced alarm audibility.

Over the 19 plus year history of the ISS, significant efforts have been put forth to limit and reduce noise levels on the ISS. During design and pre-flight activities, members of the Acoustic Subgroup have written acoustic limits and verification requirements, consulted to design and choose quietest available payloads, and then conducted acoustic verification tests prior to launch.[323]:5.7.3 During spaceflights, the Acoustics Subgroup has assessed each ISS module's in flight sound levels, produced by a large number of vehicle and science experiment noise sources, to assure compliance with strict acoustic standards. The acoustic environment on ISS changed when additional modules were added during its construction, and as additional spacecraft arrive at the ISS. The Acoustics Subgroup has responded to this dynamic operations schedule by successfully designing and employing acoustic covers, absorptive materials, noise barriers, and vibration isolators to reduce noise levels. Moreover, when pumps, fans, and ventilation systems age and show increased noise levels, this Acoustics Subgroup has guided ISS managers to replace the older, noisier instruments with quiet fan and pump technologies, significantly reducing ambient noise levels.

NASA has adopted most-conservative damage risk criteria (based on recommendations from the National Institute for Occupational Safety and Health and the World Health Organization), in order to protect all crew members. The MMOP Acoustics Subgroup has adjusted its approach to managing noise risks in this unique environment by applying, or modifying, terrestrial approaches for hearing loss prevention to set these conservative limits. One innovative approach has been NASA's Noise Exposure Estimation Tool (NEET), in which noise exposures are calculated in a task-based approach to determine the need for hearing protection devices (HPDs). Guidance for use of HPDs, either mandatory use or recommended, is then documented in the Noise Hazard Inventory, and posted for crew reference during their missions. The Acoustics Subgroup also tracks spacecraft noise exceedances, applies engineering controls, and recommends hearing protective devices to reduce crew noise exposures. Finally, hearing thresholds are monitored on-orbit, during missions .

There have been no persistent mission-related hearing threshold shifts among US Orbital Segment crewmembers (JAXA, CSA, ESA, NASA) during what is approaching 20 years of ISS mission operations, or nearly 175,000 work hours. In 2020, the MMOP Acoustics Subgroup received the Safe-In-Sound Award for Innovation for their combined efforts to mitigate any health effects of noise.[325]

An onboard fire or a toxic gas leak are other potential hazards. Ammonia is used in the external radiators of the station and could potentially leak into the pressurised modules.[326]

Orbit

Graph showing the changing altitude of the ISS from November 1998 until November 2018
Animation of ISS orbit from 14 September 2018 to 14 November 2018. Earth is not shown.

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. This orbit was selected because it is the lowest inclination that can be directly reached by Russian Soyuz and Progress spacecraft launched from Baikonur Cosmodrome at 46° N latitude without overflying China or dropping spent rocket stages in inhabited areas.[327][328] 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).[2][17] The station's altitude was allowed to fall around the time of each NASA shuttle flight to permit heavier loads to be transferred to the station. After the retirement of the shuttle, the nominal orbit of the space station was raised in altitude.[329][330] Other, more frequent supply ships do not require this adjustment as they are substantially higher performance vehicles.[29][331]

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 Automated Transfer Vehicle is constructed with the possibility of adding a second docking port to its aft end, allowing 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.[331] Maintaining ISS altitude uses about 7.5 tonnes of chemical fuel per annum[332] at an annual cost of about $210 million.[333]

Orbits of the ISS, shown in April 2013

The Russian Orbital Segment contains the Data Management System, which handles Guidance, Navigation and Control (ROS GNC) for the entire station.[334] 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.[335] 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 (reaction wheels) and thrusters to turn itself around. Gyroscopes do not require propellant; instead 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 its extra mass. When gyroscopes 'saturate', thrusters are used to cancel out the stored momentum. In February 2005, 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, this can result in a rare 'force fight' where the ROS GNC computer must ignore the USOS counterpart, which itself has no thrusters.[336][337][338]

Docked spacecraft can also be used to maintain station attitude, such as for troubleshooting or during the installation of the S3/S4 truss, which provides electrical power and data interfaces for the station's electronics.[339]

Orbital debris threats

A 7-gram object (shown in centre) shot at 7 km/s (23,000 ft/s), the orbital velocity of the ISS, made this 15 cm (5.9 in) crater in a solid block of aluminium.
Radar-trackable objects, including debris, with distinct ring of geostationary satellites

The low altitudes at which the ISS orbits are also home to a variety of space debris,[340] including 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,[341] are a significant threat. Objects large enough to destroy the station can be tracked, and are not as dangerous as smaller debris.[342][343] 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. Spacewalking crew in spacesuits are also at risk of suit damage and consequent exposure to vacuum.[344]

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 depend on their predicted exposure to damage. The station's shields and structure have different designs on the ROS and the USOS. On the USOS, Whipple Shields are used. The US segment modules consist of an inner layer made from 1.5–5.0 cm-thick (0.59–1.97 in) aluminum, a 10 cm-thick (3.9 in) intermediate layers of Kevlar and Nextel,[345] and an outer layer of stainless steel, which causes objects to shatter into a cloud before hitting the hull, thereby spreading the energy of impact. On the ROS, a carbon fibre reinforced polymer 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.[citation needed]

Example of risk management: A NASA model showing areas at high risk from impact for the International Space Station.

Space debris is tracked remotely from the ground, and the station crew can be notified.[346] If necessary, thrusters on the Russian Orbital Segment can alter the station's orbital altitude, avoiding the debris. These Debris Avoidance Manoeuvres (DAMs) are not uncommon, taking place if computational models show the debris will approach within a certain threat distance. Ten DAMs had been performed by the end of 2009.[347][348][349] Usually, an increase in orbital velocity of the order of 1 m/s is used to raise the orbit by one or two kilometres. If necessary, the altitude can also be lowered, although such a manoeuvre wastes propellant.[348][350] 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 in order to 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.[351][352]

Sightings from Earth

Skytrack long duration exposure of the ISS

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.[353] 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 other satellite 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 16 times the brightness of Venus as sunlight glints off reflective surfaces.[354][355] The ISS is also visible in broad daylight, albeit with a great deal more difficulty.

Tools are provided by a number of websites such as Heavens-Above (see Live viewing below) as well as smartphone applications that use orbital data and the observer's longitude and latitude to indicate when the ISS will be visible (weather permitting), where the station will appear to rise, the altitude above the horizon it will reach and the duration of the pass before the station disappears either by setting below the horizon or entering into Earth's shadow.[356][357][358][359]

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.[360] The station is visible from 95% of the inhabited land on Earth, but is not visible from extreme northern or southern latitudes.[327]

The ISS on its first pass of the night passing nearly overhead shortly after sunset in June 2014
The ISS passing north on its 3rd pass of the night near local midnight in June 2014

Under specific conditions, the ISS can be observed at night on 5 consecutive orbits. Those conditions are 1) a mid-latitude observer location, 2) near the time of the solstice with 3) the ISS passing north of the observer near midnight local time. The three photos show the first, middle and last of the five passes on June 5/6, 2014.

The ISS and HTV photographed from Earth by Ralf Vandebergh

Using a telescope-mounted camera to photograph the station is a popular hobby for astronomers,[361] while using a mounted camera to photograph the Earth and stars is a popular hobby for crew.[362] The use of a telescope or binoculars allows viewing of the ISS during daylight hours.[363]

Composite of 6 photos of the ISS transiting the gibbous Moon

Some amateur astronomers also use telescopic lenses to photograph the ISS while it transits the Sun, sometimes doing so during an eclipse (and so the Sun, Moon, and ISS are all positioned approximately in a single line). One example is during the 21 August solar eclipse, where at one location in Wyoming, images of the ISS were captured during the eclipse.[364] Similar images were captured by NASA from a location in Washington.

Parisian engineer and astrophotographer Thierry Legault, known for his photos of spaceships transiting the Sun, travelled to Oman in 2011 to photograph the Sun, Moon and space station all lined up.[365] 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 transit the Sun or Moon and from what location those passes will be visible.

International co-operation

A Commemorative Plaque honouring Space Station Intergovernmental Agreement signed on 28 January 1998

Involving five space programs and fifteen countries,[366] the International Space Station is the most politically and legally complex space exploration programme in history.[367] The 1998 Space Station Intergovernmental Agreement sets forth the primary framework for international cooperation among the parties. A series of subsequent agreements govern other aspects of the station, ranging from jurisdictional issues to a code of conduct among visiting astronauts.[368]

Participating countries

End of mission

Many ISS resupply spacecraft have already undergone atmospheric re-entry, such as Jules Verne ATV

According to the Outer Space Treaty, the United States and Russia are legally responsible for all modules they have launched.[369] Natural orbital decay with random reentry (as with Skylab), boosting the station to a higher altitude (which would delay reentry), and a controlled targeted de-orbit to a remote ocean area were considered as ISS disposal options.[370] As of late 2010, the preferred plan is to use a slightly modified Progress spacecraft to de-orbit the ISS.[371] This plan was seen as the simplest, cheapest and with the highest margin.[371]

The Orbital Piloted Assembly and Experiment Complex (OPSEK) was previously intended to be constructed of modules from the Russian Orbital Segment after the ISS is decommissioned. The modules under consideration for removal from the current ISS included the Multipurpose Laboratory Module (Nauka), planned to be launched in spring 2021 as of May 2020,[97] and the other new Russian modules that are proposed to be attached to Nauka. These newly launched modules would still be well within their useful lives in 2024.[372]

At the end of 2011, the Exploration Gateway Platform concept also proposed using leftover USOS hardware and Zvezda 2 as a refuelling depot and service station located at one of the Earth-Moon Lagrange points. However, the entire USOS was not designed for disassembly and will be discarded.[373]

In February 2015, Roscosmos announced that it would remain a part of the ISS programme until 2024.[18] Nine months earlier—in response to US sanctions against Russia over the annexation of 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.[374]

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.[375] Igor Komarov, the head of Russia's Roscosmos, made the announcement with NASA administrator Charles Bolden at his side.[376] 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.[377]

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.[378]

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", with discussion to continue under the new presidential administration.[citation needed] There have also been suggestions that the station could be converted to commercial operations after it is retired by government entities.[379]

In July 2018, the Space Frontier Act of 2018 was intended to extend operations of the ISS to 2030. This bill was unanimously approved in the Senate, but failed to pass in the U.S. House.[380][381] In September 2018, the Leading Human Spaceflight Act was introduced with the intent to extend operations of the ISS to 2030, and was confirmed in December 2018.[22][382][383]

Cost

The ISS has been described as the most expensive single item ever constructed.[384] As of 2010 the total cost was US$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.[385]

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