International Space Station

• 1. Introduction

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  The International Space Station travels at 7.67 kilometres every second, circling the Earth roughly every 93 minutes at about 400 kilometres up. It hangs below the Van Allen radiation belts and most space debris, tracing a path between 51.64 degrees north and south. Since the 2nd of November 2000, people have lived aboard it without a single day's break. That is the longest continuous human presence in space. Measuring 358 feet across with its solar arrays unfurled, it is as large as a full-sized football field, with a pressurised interior comparable to a Boeing 747. Five space agencies run it together: NASA, Roscosmos, ESA, JAXA, and CSA. It is the first station built, maintained, and crewed through international cooperation, and the largest human spacecraft ever constructed. But a station this size did not begin as one plan. It grew from two rival projects, was bolted together across continents, and now faces a deadline. How do nations that were once bitter rivals share a single home in orbit? What gets discovered when you put a laboratory beyond gravity? And what happens to a 500-ton structure when its time runs out?

• 2. Two Rivals, One Station

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  The ISS combines two stations that were never built separately. One was the United States' Space Station Freedom. The other was the Soviet Union's Mir-2. The Zvezda module that became the core of the Russian segment was first manufactured in 1985 as a component for Mir-2, a station that never launched. When the first crew finally moved in, the older Russian station Mir was still inhabited. The 1998 Space Station Intergovernmental Agreement set the primary framework for cooperation among the parties. Later agreements covered everything from jurisdictional issues to a code of conduct among visiting astronauts. Brazil was invited too, the only developing country to receive such an invitation, and was to supply six pieces of hardware in exchange for utilisation rights. Brazil could not deliver any of the elements, citing a lack of funding and political priority, and officially dropped out in 2007. The fragility of the partnership surfaced after the 2022 Russian invasion of Ukraine. Roscosmos Director General Dmitry Rogozin warned in a series of tweets that a Russian withdrawal could send the station into an unguided de-orbit. He asked who would save the ISS from impact on the United States, Europe, India, or China, claiming the station does not fly over Russia. That claim was untrue. The ISS passes over all parts of Earth between 51.6 degrees north and south, roughly the latitude of Saratov. NASA noted that the US corporation Northrop Grumman had offered a reboost capability to keep the station in orbit if needed. On the 26th of July 2022, Yury Borisov, Rogozin's successor, submitted plans to President Putin for withdrawal after 2024. NASA's Robyn Gatens replied that no formal notices had arrived.

• 3. Assembling A Field In Orbit

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  On the 20th of November 1998, a Russian-built module called Zarya rode a Proton rocket into orbit and began the station's life. Zarya provided propulsion, attitude control, communications, and electrical power. Its name means the beginning of a new era of international cooperation. Two weeks later, on the 4th of December 1998, Space Shuttle Endeavour carried the American-made Unity module up on STS-88 and joined it to Zarya. These two pieces, built on different continents by former rivals, lacked life-support systems, so the station stayed unmanned for two more years. The turning point came in July 2000 with the launch of Zvezda, which brought living quarters and life-support and made continuous human presence possible. The Expedition 1 crew arrived that November aboard Soyuz TM-31. Modules in the Russian segment launched and docked autonomously, with the exception of Rassvet. Other components rode the Space Shuttle and had to be installed by astronauts using robotic arms or during spacewalks. By the 5th of June 2011, astronauts had made over 159 spacewalks to add components, totalling more than 1,000 hours outside. Tragedy struck in 2003 with the loss of Space Shuttle Columbia, which grounded the fleet and halted construction. Assembly resumed in 2006 with STS-115 and Atlantis, delivering the second set of solar arrays. By March 2009, STS-119 completed the Integrated Truss Structure with the fourth and final set of arrays. The Cupola arrived in February 2010, and with it assembly reached 85 percent completion. The last pressurised module of the US segment, Leonardo, came in February 2011 on the final flight of Discovery. Russia's Nauka research module docked in July 2021, and the Prichal docking hub followed that November. As of June 2025, NASA states there are 43 different modules and elements installed.

• 4. Inside The Modules

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  The Cupola takes its name from the Italian word for dome, and its central window measures 80 centimetres across. Its seven windows are used for experiments, dockings, and observations of Earth. The station is divided into two main sections: the Russian Orbital Segment, developed by Roscosmos, and the US Orbital Segment, built by NASA, ESA, JAXA, and CSA. The Integrated Truss Structure ties the solar panels and radiators to 16 major pressurised modules. Unity, also called Node 1, is the connection between the Russian and US segments, built of steel by Boeing at the Marshall Space Flight Center in Huntsville, Alabama. It measures 4.57 metres in diameter and 5.47 metres in length, and has six berthing locations. Destiny, the primary US research facility, was NASA's first permanent orbital research station since Skylab. It was attached to Unity over five days in February 2001. The Quest Joint Airlock, launched in July 2001, lets astronauts step outside using either the US Extravehicular Mobility Unit or the Russian Orlan suit. Before Quest, the two suit types could not share an exit. Columbus, the largest single ESA contribution, was built in Turin, Italy by Thales Alenia Space, with functional equipment designed by EADS in Bremen, Germany. The European Space Agency spent 1.4 billion euros, about 1.6 billion US dollars, on building it. Japan's Kibo is the largest single module on the station, assembled in space over three Space Shuttle missions. Poisk, the Russian segment's airlock, has two inward-opening hatches. That was a deliberate change from Mir, where an outward-opening hatch once opened too quickly because of leftover air pressure. The Leonardo module was named for Italian polymath Leonardo da Vinci, and the Bartolomeo external platform was named for Christopher Columbus's younger brother.

• 5. A Laboratory Beyond Gravity

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  On the 3rd of April 2013, scientists reported that hints of dark matter may have been detected by the Alpha Magnetic Spectrometer. They wrote that the first results confirmed an unexplained excess of high-energy positrons in Earth-bound cosmic rays. According to NASA, the AMS is as important as the Hubble Space Telescope. It is docked on the station because its power and bandwidth needs could not be easily met on a free-flying satellite. Crews fly expeditions of several months, providing roughly 160 man-hours of labour per week with a crew of six, though much of that time goes to station maintenance. Research spans astrobiology, astronomy, physical sciences, materials science, space weather, meteorology, and human medicine. In 2006, data on bone loss and muscular atrophy suggested astronauts could face fractures and movement problems after a long cruise, such as the six-month trip required to reach Mars. Medical studies run on behalf of the National Space Biomedical Research Institute. One, the Advanced Diagnostic Ultrasound in Microgravity study, has astronauts perform ultrasound scans guided by remote experts, since there is usually no physician aboard. In August 2020, scientists reported that Deinococcus radiodurans bacteria survived for three years in outer space, supporting the idea of panspermia, the hypothesis that life exists throughout the universe. Researchers also study how fluids behave when almost completely combined in microgravity, and how reactions slowed by low gravity and temperature can improve understanding of superconductivity. The space environment itself is hostile, exposing anything unprotected to intense radiation, vacuum, extreme temperatures, and microgravity. Yet extremophiles and tiny invertebrates called tardigrades can endure it in a dry, desiccated state.

• 6. Keeping People Alive

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  Normal air pressure on the ISS is 101.3 kilopascals, the same as sea level on Earth. Earth-like conditions have always been maintained on Russian and Soviet spacecraft, while American spacecraft used pure oxygen at 5 psi after launch. Oxygen comes from the Elektron system aboard Zvezda and a similar system in Destiny, both splitting water by electrolysis. The 1-kilowatt Elektron uses about one litre of water per crew member per day, water that is brought from Earth or recycled. Mir was the first spacecraft to recycle water for oxygen production. Backup oxygen comes from bottled supplies and chemical canisters. The Russian Vika cartridges each take 5 to 20 minutes to decompose at 450 to 500 degrees, producing 600 litres. Carbon dioxide is scrubbed by the Vozdukh system in Zvezda, while activated charcoal filters remove methane and ammonia. Power flows from double-sided solar arrays that collect direct sunlight on one face and light reflected off Earth on the other. The US segment's arrays form four wing pairs producing 75 to 90 kilowatts, each array about 375 square metres in area and 58 metres long. A Night Glider mode aligns them parallel to the ground at night to cut aerodynamic drag at the station's low altitude. The station originally used nickel-hydrogen batteries to power the 45 minutes of every 90-minute orbit spent in Earth's shadow. Those batteries lasted 6.5 years across over 37,000 charge cycles. Starting in 2016, they were replaced with lithium-ion batteries expected to last to the end of the program. Excess heat is carried off by an external loop of liquid ammonia pumped to radiators that emit it as infrared, a system that can reject up to 70 kilowatts.

• 7. Living And Visiting

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  Oleg Kononenko of Roscosmos holds the record for the longest time spent in space, accumulating nearly 1,111 days across five long-duration missions. He commanded the station three times. Peggy Whitson of NASA and Axiom Space has spent more time in space than any other American, over 675 days across three expeditions and two Axiom missions. Expeditions run up to six months. The first six carried three-person crews, but after the loss of Columbia, Expeditions 7 to 12 shrank to two-person caretaker crews who could be resupplied by the small Russian Progress craft. Crews returned to three with Expedition 13, grew to six around 2010, and reached seven after larger US commercial spacecraft began arriving in 2020. That is the number the station was originally designed to hold. As of June 2023-13 individuals have paid for their own travel to the station. Many object to being called space tourists, since they undergo professional training, so Roscosmos and NASA classify them as spaceflight participants. Early private access came only through Roscosmos, with seats marketed by Space Adventures at about 40 million US dollars. NASA resisted training Dennis Tito, the first person to pay for a stay. In 2021, NASA began authorising Private Astronaut Missions, which require a NASA-certified US vehicle and a commander who is a former NASA astronaut. The first, Axiom Mission 1, launched in 2022; Axiom Mission 2 followed in 2023 carrying two astronauts from the Saudi Space Agency. In total, 294 individuals from 26 countries have visited, with the United States accounting for 172 and Russia for 65.

• 8. Closing The Hatch

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  The station is expected to remain operational until the end of 2030. Future plans include the Payload Power Thermal Module by Axiom Space, which would form the commercial segment. Axiom expects to launch that module no earlier than 2027, where it would stay until the launch of its Habitat One module about a year later, then detach to join it. After 2030, parts of the ISS are meant to live on through Axiom Station and the Russian Orbital Service Station, with Canadarm2 planned to keep working on Axiom Station. The US Deorbit Vehicle, built by SpaceX under a contract NASA awarded in June 2024, is intended to perform a controlled de-orbit and demise after the station's operational life. NASA plans to de-orbit only once it has the minimum capability in orbit: the deorbit vehicle and at least one commercial station. Not everyone accepts that ending. Critique of the de-orbit plan, along with a proposal to park the station at a higher, more stable orbit, has gathered congressional support. Whatever the outcome, the record set on the 2nd of November 2000 will keep growing until the day someone closes the hatch for the last time.