Throw a ball hard enough and it lands further away. Throw it so hard that the Earth curves away beneath it as it falls, and it never comes down: that is an orbit. The higher up, the slower a satellite needs to go — and the longer a lap takes.
In practiceThe ISS at ± 400 km races round the Earth in an hour and a half and is visible to an observer on the ground for only a few minutes. A communications satellite at 35,786 km takes exactly one day and therefore seems to hang still: a dish can stay fixed on it.
Three quantities are tied together: altitude, speed and orbital period. Choose the altitude and the other two are fixed (for a circular orbit). Lower means faster and a shorter lap; higher means slower and a longer one.
| Altitude | Speed | Period | Example |
|---|---|---|---|
| 400 km | ± 7.7 km/s | ± 92 min | ISS |
| 550 km | ± 7.6 km/s | ± 95 min | Starlink |
| 800 km | ± 7.5 km/s | ± 101 min | Earth observation (sun-synchronous) |
| 20,200 km | ± 3.9 km/s | ± 12 hours | GPS |
| 35,786 km | ± 3.1 km/s | 23 h 56 min | Geostationary (GEO) |
Besides altitude, the inclination — the angle between the orbital plane and the equator — determines which part of the Earth a satellite passes over. An orbit inclined at 53° never goes above 53° north or south; a polar orbit (± 90°) goes everywhere. An elliptical orbit has a lowest point (perigee) and a highest point (apogee); near apogee the satellite moves slowest and dwells over the same area for a long time.
Try it yourself: pick an orbit or slide the altitude, and see what happens to speed, period, the visible part of the Earth and the delay of a signal.