The International Space Station, or ISS, is a large research laboratory that orbits Earth about 400 km above the surface. It matters because it lets astronauts study physics, biology, Earth, and technology in long-term microgravity. The station is not weightless because gravity is gone, but because it is continuously falling around Earth.
Its orbit, solar power system, life support, and international design make it one of the most important engineering projects ever built in space.
The ISS works by balancing many systems at once: motion, power, temperature, air, water, communication, and docking. Solar arrays turn sunlight into electricity, while radiators remove excess heat from equipment and crew areas. Pressurized modules provide living and lab space, and visiting spacecraft bring supplies, experiments, and new crew members.
Small thrusters and docked spacecraft occasionally boost the station because thin atmospheric drag slowly lowers its orbit.
Key Facts
- Average altitude of the ISS is about 400 km above Earth.
- Orbital speed is about 7.66 km/s, or about 27,600 km/h.
- One orbit takes about 90 minutes, so the ISS sees about 16 sunrises per day.
- Circular orbit speed is v = sqrt(GM/r), where r is distance from Earth's center.
- Orbital centripetal acceleration is a = v^2/r and is supplied by gravity.
- Microgravity occurs because the ISS, its crew, and everything inside are all in continuous free fall together.
Vocabulary
- Orbit
- An orbit is the curved path of an object moving around a planet, moon, star, or other body under gravity.
- Microgravity
- Microgravity is a condition in which objects appear nearly weightless because they are falling together with their surroundings.
- Solar array
- A solar array is a set of solar panels that converts sunlight into electrical energy for the station.
- Radiator
- A radiator is a surface that releases unwanted heat into space by emitting infrared radiation.
- Docking port
- A docking port is a connection point where spacecraft attach to transfer crew, cargo, and equipment.
Common Mistakes to Avoid
- Saying there is no gravity on the ISS is wrong because Earth's gravity is still strong at its altitude, about 90 percent of surface gravity.
- Thinking astronauts float because there is no air resistance is wrong because floating happens mainly from continuous free fall, not from the absence of air.
- Assuming the ISS stays in orbit without any energy changes is wrong because thin atmospheric drag slowly removes orbital energy and periodic reboosts are needed.
- Confusing solar arrays with radiators is wrong because solar arrays generate electrical power, while radiators remove waste heat.
Practice Questions
- 1 The ISS orbits at an altitude of 400 km. If Earth's radius is 6370 km, what is the ISS distance from Earth's center in kilometers?
- 2 The ISS travels at about 7.66 km/s. Estimate how far it travels in one 90 minute orbit, in kilometers.
- 3 Explain why astronauts inside the ISS appear to float even though Earth's gravity is still acting on them.