How fast do satellites go?
Put a satellite in a low orbit like the International Space Station, in a GPS orbit, or in a geostationary orbit used for TV. See how far it is from the Earth's centre, how fast gravity makes it go and how long one trip round the Earth takes.
Core concept
Gravity pulls a satellite towards the Earth and keeps it moving in a circle; in an orbit of radius r its speed is v = √(GM ÷ r), so higher satellites move slower and take longer to go round
Topic: Our planet and sky
A satellite is always falling towards the Earth, but it moves sideways so fast that the ground curves away beneath it as quickly as it falls: it goes round and round. The pull of gravity: The pull that makes things fall towards the Earth. All key words gets weaker further out, so a higher satellite needs less speed: How far something travels in a certain time, like kilometres per hour. All key words: in a circle of radius r (from the Earth's centre) the speed is v = √(GM ÷ r), where GM = 398 600 km³/s² for the Earth. The radius is the Earth's radius, 6371 km, plus the height. One orbit is a circle 2πr long, so it takes 2πr ÷ v. Far enough out, 35 786 km up, one orbit takes a day, so the satellite stays above the same place: a geostationary orbit.
What you will learn
- Explain why a satellite stays in orbit.
- Use v = √(GM ÷ r) and T = 2πr ÷ v for a circular orbit.
- Explain why TV satellites seem to stay still in the sky.
Before you start
- Know Newton's law of gravitation and the circumference of a circle, 2πr.
Materials
- A stone tied to a string (to feel a centripetal pull)
Equipment
- None
Procedure
- Record the orbit of the International Space Station.
- Record a GPS satellite's orbit.
- Record a geostationary orbit.
How this simulation is simplified
- Circular orbits; the Earth's radius is 6371 km and GM = 398 600 km³/s².
- The speed is rounded to 2 decimal places before the time for one orbit is worked out.