The International Space Station and how astronauts live there
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Since 2 November 2000 there have always been people living in space. They live about 400 km up, on the International Space Station (ISS), which circles the Earth roughly every 90 minutes. On a clear evening you can watch it cross the sky without a telescope.
Built in pieces
The station was far too large to launch in one go. It was assembled in orbit from separate modules, carried up one or a few at a time and joined together.
- November 1998: the first module, the Russian-built Zarya, was launched.
- December 1998: the American module Unity was attached to it.
- 2 November 2000: the first long-stay crew moved in.
- The following years: more modules, a long metal backbone called the truss, and huge solar arrays were added, many of them carried up by the Space Shuttle.
Five space agencies run the station together: NASA (USA), Roscosmos (Russia), ESA (Europe), JAXA (Japan) and CSA (Canada). Astronauts from many other countries have visited. The British ESA astronaut Tim Peake lived on board from December 2015 to June 2016 and made a spacewalk during his stay.
The numbers
| Approximate value | |
|---|---|
| Height above the Earth | about 400 km (it varies) |
| Speed | about 28,000 km/h |
| Time for one orbit | about 90 minutes |
| Length along the truss | about 109 m, close to the length of a football pitch |
| Mass | about 420 tonnes |
| Usual crew | around 7 |
Worked example: sunrises per day
- There are 24 × 60 = 1,440 minutes in a day.
- One orbit takes about 90 minutes.
- 1,440 ÷ 90 = 16.
The crew pass through about 16 sunrises and 16 sunsets every 24 hours. They keep to a fixed timetable on UTC rather than following the Sun outside the window. Batteries, charged by the solar arrays, keep the power on during each pass through the Earth's shadow.
Worked example: distance per orbit
The station circles at a radius of about 6,400 km (the Earth's radius) + 400 km = 6,800 km. One lap is 2 × π × 6,800 ≈ 42,700 km, further than the 40,000 km around the Equator, every 90 minutes.
Why astronauts float
At 400 km up, gravity is still about 90% as strong as on the ground. If it vanished, the station would fly off in a straight line instead of circling the planet.
The crew float because the station and everything in it are falling around the Earth together, all the time. Nothing presses them against the floor, because the floor is falling at exactly the same rate. This condition is called free fall, or microgravity. Our guide to how rockets work explains why moving sideways fast enough turns a fall into an orbit.
The station is not quite outside the atmosphere either. It orbits in the very thin thermosphere, which drags on it gently, so engines boost it back up every so often; see the layers of the atmosphere.
A day on board
Exercise. Without the load of gravity, muscles and bones weaken. Each astronaut exercises for about two hours a day, using a treadmill (with a harness holding them down), an exercise bike and a resistance machine that works like weightlifting.
Food. Meals come in pouches, cans and sealed packets; some are eaten as they come, others need water added. Floating crumbs can get into equipment, so tortillas often replace bread, and salt and pepper are supplied as liquids.
Water. Launching water is expensive, so the station recycles most of it, including moisture from the air, sweat and urine, which is cleaned until it is safe to drink.
Sleep. Each astronaut has a small private cabin, about the size of a phone box, with a sleeping bag fixed to the wall. With no "up", any direction will do.
Work. The crew run experiments, from how the human body changes in space to how flames and fluids behave in free fall, and maintain the station, sometimes going outside on spacewalks.
How to see the ISS
You need clear skies and the right time, not a telescope. Sunlight reflecting off the station's solar arrays makes it one of the brightest objects in the night sky.
- It looks like a bright, steady point gliding smoothly across the sky, taking a few minutes to cross.
- It does not flash. That is the easiest way to tell it from an aircraft.
- Look in the hours after sunset or before sunrise, when the sky where you stand is dark but the station, high above, is still lit by the Sun.
NASA's Spot the Station service lists passes for your area. Go out with an adult, and see our stargazing guide for telling satellites from stars and planets.
Activity: plan a day on the ISS
Draw a 24-hour timetable for one astronaut. Include 8 to 9 hours of sleep, about 2 hours of exercise, three meals, at least 6 hours of work split between experiments and maintenance, and one short video call home.
Then work out how many sunrises happen during a 9-hour working day. (Answer: 9 × 60 = 540 minutes, and 540 ÷ 90 = 6.)
Frequently asked questions
How do astronauts get to and from the ISS?
In crew capsules launched on rockets, such as the Russian Soyuz and American commercial capsules. A capsule stays docked during each crew's stay, ready to act as a lifeboat in an emergency.
What will happen to the ISS in the end?
NASA and its partners plan to retire the station around 2030. A specially built spacecraft is planned to steer it down over a remote part of the Pacific Ocean, so that any pieces that survive re-entry fall far from people.
Is the ISS the only space station?
No. China's Tiangong station was completed in 2022 and has had crews on board since.
Sources and further reading
- International Space Station – NASA – NASA's hub for the station, its crew, research and spacewalks.
- International Space Station – European Space Agency – Europe's part in the station and its astronauts.
- Spot the Station – NASA – find out when the ISS will pass over where you live.
