Flying some 240 miles above Earth's surface at 17,500 miles an hour, the International Space Station (ISS) is a science laboratory dedicated to helping humans learn how to live in space. Crucially, that means figuring out how the space environment affects biology–and human bodies, especially. Other onboard experiments are aimed at better understanding how the cosmos works, from the highest-energy particles that streak through our solar system to the faraway, extremely dense corpses of former stars.
Continuously occupied by rotating crews since November 2000, the space station is the work of five space agencies: NASA, Russia’s Roscosmos, the European Space Agency, the Japanese Aerospace Exploration Agency, and the Canadian Space Agency. Hundreds of spacefarers have visited the ISS—primarily professional space travelers, although a handful of space tourists have also made the journey. Here’s how the ISS came to be and what scientists hope to learn from experiments conducted there.
What is the ISS?
In the mid-1980s, President Ronald Reagan directed NASA to build an international space station within a decade, declaring that it would “permit quantum leaps” in science research. First, the U.S. partnered with Europe and Japan; it then invited Russia into the enterprise in 1993 because that nation had the most extensive experience operating orbital space stations. By 1998, all five space agencies were on board with the project.
The ISS was conceived as a series of linked, cylindrical modules that are solar powered and cooled by loops that radiate heat. These are divided among the station’s two larger segments: the Russian Orbital Segment, operated by Russia, and the U.S. segment, which includes contributions from many countries.
Construction began in November 1998, when the first piece of the eventual ISS structure—the Russian Zarya Control Module—was autonomously delivered to orbit by a Proton rocket. Named using the Russian word for “sunrise,” the Zarya module originally provided power, communications, and altitude control functions and is now primarily used for storage and propulsion. Two weeks later, astronauts aboard the space shuttle Endeavour delivered one of the major U.S. components, Unity, a module that now connects the Russian and U.S. segments of the station. The first crew to inhabit the station arrived on November 2, 2000, and included cosmonauts Yuri Gidzenko and Sergei Krikalev, and NASA astronaut Bill Shepherd.
The station today spans the area of a U.S. football field and is typically occupied by at least three astronauts, or as many as six. Construction is still ongoing, with Russia getting ready to send a new science module to the station.
Both the U.S. and Russian segments of the ISS generate power and host laboratories, living spaces, and docking ports. Astronauts can move between the segments, which are connected to a larger structural truss that holds the station’s solar arrays and thermal radiators. Also attached to that truss is Canadarm2—a Canadian-built apparatus that functions as a large, remote-controlled space crane used to do a range of tasks, from moving equipment to capturing inbound spacecraft.
On the U.S. side, the single largest module is Kibo, a Japanese science laboratory space with an external “porch” that is used for experiments in the vacuum of space. The nearby Node 3, or Tranquility module, houses the European-built cupola that affords astronauts unforgettable views of planet Earth. In 2016, NASA attached an inflatable habitat—the Bigelow Expandable Activity Module—to the space station for the first time, perhaps paving the way for future inflatable space hotels and tourist destinations.
Why the ISS matters
Everything on Earth evolved to thrive in its home environment and not the alien surroundings of space, which can challenge lifeforms in surprising ways. The space station is by far the best place to practice living and working in these unfamiliar conditions, and to better understand how space affects our complex biology.
Most importantly, surviving in space means dealing with microgravity (the near-absence of gravity) and increased radiation exposure— two conditions that can dramatically impact biological functions. Life in orbit also means tolerating a small, closed environment, limited human contact, and high-pressure situations that might require rapid teamwork to survive.
Crews must adapt to an existence in which the sun rises and sets as many as 16 times a day. Getting a good night’s sleep requires securing themselves to a non-floating object. Intense daily exercise is crucial for maintaining healthy bones and combatting microgravity-induced muscular atrophy. And astronauts have to contend with a variety of conditions that arise when there’s literally no up or down—problems with balance and orientation, changes in blood circulation and fluid distribution, and a strange and mostly unexplained deterioration in eyesight.
Increased radiation exposure means a higher risk of developing various cancers. Teams on the ground are currently designing spacesuits to shield long-duration space travelers from potentially destructive cosmic particles.
The crew is also occupied with a full suite of scientific experiments designed to help humans figure out how to live in space permanently. These include growing plants and human tissues in microgravity, seeing how microbes respond to the space environment, studying how space affects DNA and gene expression, and learning whether normal reproduction is even possible (the jury is still out).
Although the bulk of research done on the station is focused on learning more about surviving in space, experiments outside of the space station are peering into the cosmos and attempting to learn more about the environment in which spacefaring humans might live. One of these instruments, called NICER, is studying neutron stars, stellar corpses that are the densest objects in the known universe. Another, the Alpha Magnetic Spectrometer, is a particle physics experiment that is attempting to snare and analyze cosmic rays, the most fundamental components of the cosmos, to better understand the origins of the universe.
In recent years, U.S. astronauts have typically spent between three and six months on board the ISS, but the longest continual stay lasted nearly one year—a milestone achieved in 2016 by NASA astronaut Scott Kelly and his Russian colleague, cosmonaut Mikhail Kornienko. Back on Earth, teams are simulating these environments and studying the psychological challenges associated with isolation, in an attempt to better understand who might be best suited for space flights of even longer duration.
Modern journeys to the ISS
Until 2011, astronauts were ferried to the station by the U.S. space shuttles and the Russian Soyuz spacecraft. After the U.S. retired the space shuttle program, Soyuz became the only ride into orbit—until private company SpaceX successfully flew a crewed mission to the station in May 2020.
Piloted by NASA astronauts Doug Hurley and Bob Behnken, this crucial test flight departed the station in early August, ending in a safe and successful splashdown off the western coast of Florida. Now, astronauts can once again launch into orbit from U.S. shores; and NASA’s new reliance on private companies could, in theory, foster competition that might drive down prices and open up travel to the ISS to a broader swath of humanity.
For now, plans include operating the space station through at least 2024, although that timeline could be extended.