The International Space Station
The International Space Station (ISS) is a giant science laboratory in orbit around the Earth, about 400 km above our heads. It is the biggest single object humans have ever built in space, the result of a partnership between the United States, Russia, Europe, Japan and Canada. The ISS has been continuously occupied by crews of astronauts since November 2000, meaning at this exact moment, somewhere above you, there are people living and working in space, and there have been continuously for almost a quarter of a century.
- Altitudeapprox. 400 kmAbove Earth's surface
- Speedapprox. 28,000 km/hOne orbit every approx. 93 minutes
- Crew sizeapprox. 6 to 10 peoplePlus extra during crew changeovers
- Continuously crewed since2 November 2000Over 24 years
- Massapprox. 420 tonnesAbout the weight of a Boeing 747
- Lengthapprox. 109 metresWingtip to wingtip, larger than a football pitch
What is the ISS?
The International Space Station is a series of interconnected pressurised modules, plus a long truss running through the middle that holds the solar panels and external science experiments. It is roughly the size of a football pitch, wraps in 8 huge sets of solar panels that give it a distinctive look, and has been built up piece by piece since 1998 by dozens of separate launches.
The ISS is a collaboration between five space agencies:
- NASA (United States)
- Roscosmos (Russia)
- ESA (European Space Agency)
- JAXA (Japan)
- CSA (Canada)
How it was built
The first piece of the ISS, a Russian module called Zarya, was launched in November 1998. The American Unity node joined it weeks later. Over the next 13 years, dozens of separate Space Shuttle and Russian launches added new modules: living quarters, science laboratories, docking ports, an airlock for spacewalks, and the giant truss with the solar panels. Major construction finished in 2011, with the retirement of the Space Shuttle.
Building the ISS in space was an enormous engineering effort. Astronauts performed over 100 spacewalks to bolt modules together, run cables, and install equipment. Pieces from different countries had to fit together perfectly even though they were manufactured years apart in different parts of the world.
Who lives on the ISS?
The ISS is normally home to a crew of 6 or 7 astronauts at a time, although the number rises briefly to 10 or more during crew changeovers. Most stay for about 6 months. The longest single stay was by Russian cosmonaut Frank De Winne and others on the ISS, but the record for total time in space is held by Russian cosmonaut Oleg Kononenko, who has spent over 1,100 days in space across multiple missions.
As of 2025, more than 270 individuals from 23 countries have visited the ISS. Most are professional astronauts, but the ISS has also hosted a small number of private "space tourists" and crew from commercial companies like SpaceX and Axiom Space.
What do they do up there?
The ISS is mainly a science laboratory. Astronauts spend most of their working day running experiments that take advantage of weightlessness (or what scientists call microgravity). You cannot do these experiments anywhere on Earth. Recent research has included:
- Growing crystals more perfectly than is possible under gravity, to study disease.
- Studying how plants grow without gravity (including the first chilli peppers grown in space, in 2021).
- Investigating how the human body changes during long stays in space (bone loss, muscle loss, vision changes, immune system) to prepare for future trips to Mars.
- Burning materials in microgravity to study fire and combustion.
- Astronomy and Earth observation from outside the atmosphere.
Astronauts also spend about 2 hours per day exercising, because the lack of gravity weakens muscles and bones surprisingly quickly. They have treadmills (with bungee cords to hold them down), a stationary bike, and a special weights machine.
How does the ISS stay up?
The ISS is not really "floating": it is in orbit, which is just the right combination of falling and moving sideways fast enough to keep missing the Earth. At 400 km up the ISS travels at about 28,000 km/h: it goes all the way around the planet every 93 minutes, which means astronauts see 16 sunrises and sunsets per day.
At that altitude there is still a tiny bit of leftover atmosphere, so the ISS gradually slows down and drifts lower over time. To stop it eventually falling out of orbit, supply ships periodically fire their engines while docked to push the whole station to a slightly higher orbit. This is called a "reboost" and happens several times a year.
What does daily life look like?
The ISS has six "bedrooms" (small soundproof closets where each crew member has a sleeping bag attached to the wall), two toilets (with vacuum suction; weightlessness makes this complicated), a small kitchen, a gym, and several science labs. There is no shower; astronauts wash with damp cloths and special no-rinse shampoo.
Food is shipped up from Earth: most of it is freeze-dried or vacuum-packed, although some fresh fruit arrives now and then. Astronauts have to be careful not to release crumbs (which would float around and clog air filters). There is no laundry; dirty clothes are eventually packed into a departing cargo ship and burned up in the atmosphere along with the ship. Crew members each get a tiny bit of personal time, often spent looking out of the spectacular 7-window cupola at the Earth turning below.
Deeper dive: what microgravity does to the human body
The human body evolved over millions of years to cope with Earth's gravity. Take away that constant pull, and a lot of things start to go wrong, slowly but surely.
Astronauts on the ISS lose roughly 1 to 2% of their bone density per month in zero gravity, because their bones no longer have to support body weight and start dissolving the minerals. Muscle mass shrinks for the same reason: without gravity, you do not have to use your legs to stand or your back to hold yourself up. To fight back, ISS crew exercise for around 2 hours every day.
Other strange effects include: body fluids redistributing upwards (giving astronauts a slightly puffy face called the "moon face"), reduced ability to taste food, eyesight changes from pressure on the eyeball, weakened immune system, reduced quality of sleep, and longer space-flights have been linked to small changes in the brain. After a long mission astronauts often need help walking when they first get back to Earth and take weeks or months to recover.
All of this is one of the biggest obstacles for future Mars trips. A round trip to Mars and back would take 2 to 3 years, far longer than any ISS mission. NASA and other agencies are using the ISS to test exercise routines, drugs and possibly artificial gravity systems that might help astronauts survive the trip to the Red Planet and back in reasonable condition.
For the rockets that get astronauts there see SpaceX. For other historic crewed missions see the Apollo missions.