What is the International Space Station? Life in orbit

The International Space Station (ISS) is a crewed laboratory orbiting about 400 km above Earth, circling the planet every 90 minutes. Five space agencies, from the United States, Russia, Europe, Japan and Canada, built and run it. Its first piece launched in 1998, and people have lived aboard without a break since 2 November 2000. NASA plans to retire it and bring it down into the ocean after 2030.

In short

• The ISS is about as long as an American football field and weighs about 420 tonnes; it is the largest object people have put in space.
• Astronauts float not because there's no gravity up there but because the station is falling around Earth, constantly missing it.
• Its main job is research in near-weightlessness, especially on what long spaceflight does to the human body.

What is the ISS, and who runs it?

The station is a set of pressurized modules, the rooms people live and work in, joined to a long truss that carries its solar arrays and radiators. According to NASA's facts and figures, it is 109 m from end to end and has a mass of about 419,725 kg. Its eight solar arrays make 75 to 90 kilowatts of power. Inside, its pressurized volume is about 1,005 cubic meters, roughly that of a large house.

It was built in orbit, piece by piece, over more than a decade. The Russian module Zarya went up first, on 20 November 1998, and 42 assembly flights by American space shuttles and Russian rockets delivered the major parts. It belongs to a partnership of five agencies, NASA, Roscosmos of Russia, the European Space Agency, Japan's JAXA and the Canadian Space Agency, representing 15 countries. The station has two halves that work together: a Russian segment and a US-led segment that includes the European and Japanese laboratories.

Crews usually number about seven, staying around six months each. They arrive on SpaceX Crew Dragon and Russian Soyuz spacecraft, while uncrewed freighters bring food, water, fuel and experiments. In October 2026, NASA counted 298 people from 26 countries who had visited, according to its list of visitors by country.

Why do astronauts float if there's gravity in orbit?

Astronauts float because they and the station are falling together, not because gravity has gone away. At 400 km up, Earth's gravity is still strong. Here is the arithmetic, then the trick.

1. How strong gravity is up there. Gravity weakens with the square of the distance from Earth's center. Earth's radius is about 6,371 km, so the station is about 6,771 km from the center. (6,371 ÷ 6,771)² ≈ 0.89. Gravity at the station is about 89% of what it is on the ground.
2. How fast it goes. The station moves at about 7.7 km every second, roughly 28,000 km per hour.
3. How far it falls in one second. At 89% of ground gravity, anything dropped falls about 4.3 m in its first second.
4. How far Earth curves away. Over 7.7 km of travel, the curve of a sphere 6,771 km in radius drops away by about 7.7² ÷ (2 × 6,771) km, roughly 4.4 m.

So in each second, the station falls about 4.3 m toward Earth while the surface curves about 4.4 m away beneath it, the same distance within rounding. It keeps falling and keeps missing. That is what an orbit is, as Newton's laws predicted. Everything inside falls at the same rate as the station, so nothing presses on the floor, and people and objects float. It is the same feeling as the top of a roller-coaster drop, lasting for months.

At that speed, one lap of the planet takes about 90 minutes, so the crew sees about 16 sunrises and 16 sunsets every day. In a day, the station travels about 7.7 km × 86,400 seconds ≈ 665,000 km, most of the way to the Moon and back.

What do astronauts do on the ISS?

Mostly science and maintenance. The station is a laboratory where gravity's pull is effectively switched off, which changes how flames burn, fluids flow, crystals grow and cells behave. Crews run experiments for researchers on the ground, maintain the station's air, water and power systems, and sometimes go outside on spacewalks to repair or install equipment.

The biggest subject is themselves. Living in near-weightlessness weakens bones and muscles, shifts fluids toward the head, can change eyesight, and exposes crews to more radiation than on Earth. Astronauts exercise about two hours a day on a treadmill, a bike and a resistance machine to limit the loss. What researchers learn from long stays is groundwork for longer journeys, such as crewed missions to Mars, which would mean years away from Earth.

Daily life is engineered. Water is recycled from humidity and urine. Crews sleep in small cabins in sleeping bags tethered to the wall, and eat food that is mostly prepackaged.

How can you see the ISS from the ground?

Often, with no telescope. The station looks like a bright, steady star moving smoothly across the sky, with no flashing lights, and crosses from horizon to horizon in a few minutes. It is easiest to see within a few hours of sunset or sunrise, when the sky around you is dark but the station, high up, is still lit by the Sun. NASA's Spot the Station service lists when it will pass over a given place. Its orbit is tilted 51.6° to the equator, so it never passes directly overhead for people farther north or south than 51.6° latitude, although it can still be seen low in the sky.

When will the ISS be retired?

The United States, Europe, Japan and Canada have committed to running the station through 2030, and NASA plans to bring it down after that. In June 2024, NASA chose SpaceX to build a US Deorbit Vehicle, in a contract worth up to $843 million, to push the station into a controlled fall over a remote stretch of ocean, according to NASA. At the time, Russia had committed to operations through at least 2028. NASA wants commercially owned stations to take over, with NASA as a paying customer rather than owner.

Still argued

The end date isn't fixed. In June 2026, the US Government Accountability Office reported that NASA had about a year to decide whether to extend the station beyond 2030, that its life could be stretched into the late 2030s if needed, and that air leaks from cracks in a Russian tunnel had not been fully fixed. Supporters of retiring it on time point to its age and cost, and to the money it would free for commercial stations and for missions beyond Earth orbit; supporters of extending it worry that commercial replacements won't be ready, and the GAO warned that without one the United States risks losing its continuous human presence in orbit.

The ISS is the successor to earlier stations, from the Soviet Salyut and Mir to the American Skylab. More than half a century after the Moon landings, it is where people learn to live off Earth. For where it sits in the bigger picture, see the solar system and the universe.

Check yourself

• Why do astronauts float on the ISS even though gravity there is almost as strong as on the ground?
• Why is the station easiest to see shortly after sunset or before sunrise?
• Who runs the station, and what happens to it when it retires?

Sources

• Space station facts and figures — NASA
• Space station visitors by country — NASA, 2026
• NASA selects International Space Station US Deorbit Vehicle — NASA, 2024
• The next outpost: NASA's plans for replacing the aging International Space Station — U.S. Government Accountability Office, 2026
• Spot the Station — NASA
• International Space Station — Wikipedia