Orbital Speed & Radius (Oxford AQA IGCSE Combined Science Double Award): Revision Note

Orbital Speed

Extension Tier only

• Orbital speed is the speed at which an object travels whilst in orbit around another object

• The centripetal force due to gravity decreases as the separation of the orbiting objects increases

  • This means objects orbiting further away have lower orbital speeds

• Objects with a larger orbital radius take more time to make an orbital revolution

Planets orbiting at different distances from the Sun

Orbital Radius

Extension Tier only

• Orbital radius is the separation distance between the object in orbit and the centre of the object it is orbiting

• When two masses have a particular separation, the centripetal force results in a particular orbital speed

  • To maintain a stable orbit, a satellite or planet must move at a particular orbital speed

• A satellite needs to travel at a specific speed to maintain a circular orbit at a particular distance from the object

• If the speed of the satellite is too fast:

  • The radius of the orbit will increase and the satellite will spiral into space

  • This is because the gravitational attraction cannot provide enough force to keep it in orbit

• If the speed of the satellite is too slow:

  • The radius of the orbit will decrease and the satellite will be pulled towards the object it should be orbiting

  • This is because the gravitational attraction is too strong to maintain a constant orbital radius

Maintaining a stable satellite orbit

• If an artificial satellite or a moon is to change the radius at which it is orbiting then the speed at which it is travelling must change

• To maintain a stable orbit:

  • If the speed increases the radius must decrease

  • If the speed decreases the radius must increase