The International Space Station, or ISS, is a crewed spacecraft and research laboratory that orbits Earth about every 90 minutes. It is one of the largest and most complex machines ever built in space, with solar arrays, living modules, laboratories, docking ports, and robotic systems. The ISS matters because it lets scientists study physics, biology, Earth, and technology in microgravity.
It also shows how many nations can work together on a long-term engineering project beyond Earth.
Key Facts
- Typical ISS altitude: about 400 km above Earth's surface.
- Orbital speed: about 7.66 km/s, or about 27,600 km/h.
- Orbital period: about 90 minutes per orbit.
- Centripetal acceleration relation: a = v^2/r.
- Gravitational force in orbit: F = Gm1m2/r^2.
- The ISS is about 109 m wide across its solar arrays and usually supports about 7 crew members.
Vocabulary
- Microgravity
- Microgravity is the condition in orbit where objects appear nearly weightless because they and their spacecraft are falling around Earth together.
- Orbit
- An orbit is the curved path of an object moving around a planet, moon, star, or other body due to gravity.
- Docking port
- A docking port is a sealed connection point where visiting spacecraft attach to the space station.
- Solar array
- A solar array is a group of solar panels that converts sunlight into electrical energy for the station.
- Robotic arm
- A robotic arm is a remotely controlled mechanical system used to move equipment, capture spacecraft, and support repairs outside the station.
Common Mistakes to Avoid
- Saying there is no gravity on the ISS is wrong because Earth's gravity is still strong at that altitude, but the station and crew are in continuous free fall.
- Treating the ISS as stationary above one point on Earth is wrong because it circles Earth about 16 times per day and passes over many regions.
- Confusing altitude with distance from Earth's center is wrong because orbital radius equals Earth's radius plus the altitude.
- Assuming astronauts float because there is no air resistance is wrong because floating inside the ISS is mainly caused by shared free fall, not the absence of air drag.
Practice Questions
- 1 The ISS orbits at about 7.66 km/s. How far does it travel in 90 minutes? Give your answer in kilometers.
- 2 Assume Earth's radius is 6371 km and the ISS altitude is 400 km. What is the orbital radius of the ISS measured from Earth's center?
- 3 Explain why astronauts on the ISS appear weightless even though Earth still pulls on them with gravity.