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How does the International Space Station stay in orbit?

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Quick answer

The ISS is in continuous free fall: gravity pulls it toward Earth, but it moves sideways at about 28,000 km/h, so the planet's surface curves away beneath it just as fast. This balance of speed and gravity produces a stable orbit roughly 400 km above Earth, completed every 90 minutes.

The International Space Station stays in orbit because it moves sideways fast enough that, as gravity pulls it toward Earth, the curve of the planet falls away beneath it at the same rate. It is essentially in continuous free fall, circling Earth roughly every 90 minutes.

Step by step

  1. A rocket launches the station's modules to about 400 km above Earth's surface.
  2. The station is accelerated sideways to roughly 28,000 km/h, parallel to Earth's surface.
  3. Gravity pulls it downward, but its forward speed carries it past the curve of the planet.
  4. The combined motion forms a closed loop, so the station circles Earth instead of hitting it.
  5. Thin atmospheric drag slowly lowers the orbit, and docked spacecraft periodically boost it back up.

Key things to understand

  1. Gravity at the station's altitude of about 400 km is still almost as strong as on the ground.
  2. The station travels at about 28,000 km/h sideways, fast enough to keep missing Earth as it falls.
  3. Forward motion and downward pull combine into a curved path that traces Earth's curvature.
  4. Because everything inside falls at the same rate, astronauts float rather than feel gravity pulling them down.
  5. Without occasional reboosts, thin air drag would slow the station and eventually bring it down.

Labelled diagram

  1. Launch

    Rocket lifts station modules to about 400 km altitude above Earth.

  2. Sideways speed

    Station accelerates to roughly 28,000 km/h parallel to the surface.

  3. Gravity pull

    Earth's gravity pulls the station downward toward the planet's centre.

  4. Curved path

    Forward motion and downward pull combine into a circular orbit.

  5. Reboost

    Docked spacecraft fire engines to counter drag and raise the orbit.

How the ISS stays in orbit

Real-world examples

  • The ISS orbits at about 400 km altitude, close enough that its bright dot is visible to the naked eye from the ground.
  • Astronauts on the station exercise daily because their bodies, like the station, are in free fall and lose muscle and bone without resistance.
  • The Hubble Space Telescope orbits higher, a little over 500 km, where drag is weaker; it has no engines, so its orbit slowly decays.
  • Satellites in low Earth orbit, such as those for imaging and internet, follow the same free-fall principle as the ISS.

Common mistake

✗ Myth: There is no gravity in space, which is why the ISS stays up and astronauts float.

✓ Fact: Gravity at the ISS altitude is nearly as strong as on Earth's surface. The station stays up because it is falling around Earth, and astronauts float because they fall at the same rate as the station.

People also ask

What keeps the international space station in orbit?
The station's high sideways speed combined with Earth's gravity keeps it in orbit. Gravity pulls it down, but it moves forward fast enough that it keeps missing the ground, tracing a circle around the planet. This is the same balance that keeps the Moon in orbit, just much closer.
How high is the iss and how fast does it go?
The ISS orbits at roughly 400 km above Earth and travels at about 28,000 km/h. At that speed it circles the planet about every 90 minutes, meaning astronauts experience around 16 sunrises and sunsets per day. Its altitude varies slightly as drag and reboosts change its path.
Will the iss eventually fall back to earth?
Yes, eventually. Thin air at its altitude creates drag that slowly lowers the orbit, and without regular reboosts the station would spiral down. Engineers plan to deorbit it safely over an ocean when its mission ends, rather than let it fall uncontrolled.
Why do astronauts float on the ISS if gravity is still strong there?
They float because they and the station are falling together at the same rate, so nothing pushes them against a floor. This is free fall, not the absence of gravity. The sensation is the same as in a falling elevator.
How fast does the ISS travel, and how long is one orbit?
It moves at roughly 28,000 km/h and completes one orbit in about 90 minutes, so astronauts see around 16 sunrises and sunsets each day. That speed is what keeps it from falling back to Earth.
What keeps the ISS from drifting down over time?
A tiny amount of atmosphere at that altitude creates drag, slowly lowering the orbit. Cargo spacecraft docked to the station fire their engines to raise it back up periodically. Without these reboosts, the station would spiral down within a few years.
Why is the ISS in orbit rather than simply hovering above Earth?
Hovering would require constant upward thrust to cancel gravity, which is impractical. Orbiting uses sideways speed so gravity curves the path into a circle. No fuel is needed to maintain the motion itself, only to correct drag.

Part of the guide: Space & Astronomy →