The International Space Station

A home and laboratory in the sky

Right now, as you read this, there are people living and working in space. They are aboard the International Space Station, or ISS — a huge spacecraft the size of a football pitch, circling the Earth far above your head. It is the largest object humans have ever built in space, and it has been continuously occupied since the year 2000. For over two decades, there has not been a single day without humans living off the planet.

The ISS is part science laboratory, part home, and part symbol of what people can achieve when they work together. Let's explore how it stays up, why its crew floats, and what they actually do up there.

Orbiting the Earth at incredible speed

The ISS orbits about 400 kilometres above the Earth's surface — high above where aeroplanes fly, but far closer than the Moon. This region is called low Earth orbit.

To stay in orbit, the station must travel astonishingly fast: about 28,000 kilometres per hour. At that speed it loops all the way around the planet in just 90 minutes. This has a strange and beautiful result — the crew sees the Sun rise and set about 16 times every single day, racing through day and night in a constant cycle.

The station does not fall down because it is, in a sense, always falling — but it is moving sideways so fast that it keeps missing the Earth and circles instead. This is exactly how all orbits work, the same principle that keeps the Moon and satellites going around us.

Why astronauts float

Photos and videos from the ISS always show astronauts and their food and tools floating freely. Many people assume this is because there is no gravity in space. That is a myth.

At the height of the ISS, Earth's gravity is still nearly as strong as it is on the ground — about 90% as strong. If gravity vanished, the station would fly off into space instead of orbiting. So why do they float?

The answer is free fall. The whole station, and everyone inside it, is falling around the Earth together — the floor, the walls, the astronauts, the apples in storage, all falling at the same rate. When everything falls together, nothing presses against anything else, so the astronauts feel weightless and float. It is the same floating feeling you get for a split second at the top of a roller coaster drop, except on the ISS it never stops. Scientists call this condition microgravity.

Built by the world, together

What makes the station truly special is in its name: International. It was not built by one country. It is a partnership of many nations and their space agencies — including the United States, Russia, Japan, Canada and the countries of Europe — who each built different sections.

The pieces were launched separately on rockets and the space shuttle, then assembled in orbit by astronauts over many years, starting in 1998. Different parts had to fit together perfectly the first time, even though they were built in different countries. The ISS has hosted astronauts from around 20 nations, who must cooperate and often speak more than one language. In a world that is sometimes divided, the station is a powerful example of people achieving something extraordinary by working as one team.

What astronauts actually do up there

The ISS is first and foremost a laboratory. In microgravity, ordinary things behave in extraordinary ways — flames burn in round balls instead of teardrops, liquids form floating spheres, and crystals can grow more perfectly. Scientists use the station to run experiments that are impossible on Earth, studying everything from physics and biology to new materials and medicines.

A huge focus is the human body. By watching how astronauts' muscles, bones and hearts change in space, doctors learn things that help patients on Earth and prepare for longer journeys to the Moon and Mars. Astronauts also test the technology and life-support systems future explorers will need.

Beyond science, the crew must keep the station running: fixing equipment, exercising for about two hours a day to stay healthy, and sometimes putting on spacesuits for spacewalks to make repairs outside, tethered so they don't drift away.

Try it yourself: spot the station with your own eyes

Here is something amazing — you can see the ISS fly overhead without any equipment at all. It is one of the brightest objects in the night sky, because its huge solar panels reflect sunlight.

1. Find out when it passes over you. Space agencies run free "Spot the Station" services and apps that tell you the exact time, direction and how high it will appear from your location. Look for passes shortly after sunset or before sunrise, when the sky is dark but the station is still catching sunlight.
2. Go outside a few minutes early and look toward the direction the app tells you.
3. Watch for a steady, bright moving light. The ISS looks like a very bright "star" gliding smoothly and quickly across the sky. It does not blink like an aeroplane (planes have flashing lights) and it crosses the sky in just a few minutes.
4. Wave! Remember, real people are inside that bright dot, travelling at 28,000 km/h, hundreds of kilometres above you.

Want to learn more? Discover the daily challenges of weightlessness in Living in Space, and find out how spacecraft reach orbit in How Rockets Work.