Elliptical Orbits (CIE IGCSE Physics)

Extended tier only

• The orbits of planets, minor planets and comets are elliptical
  • An ellipse is a 'squashed' circle

• Planets and minor planets have slightly elliptical orbits
  
  • Their orbits are approximately circular, so the Sun is generally considered to be at the centre

• Comets have highly elliptical orbits
  • The Sun is not at the centre of the orbit, it is at one of the two foci of the ellipse

Elliptical orbit of a planet

Extended tier only

• An object in an elliptical orbit around the Sun travels at a different speed depending on its distance from the Sun 
• Although these orbits are not circular, they are still stable
  • For a stable orbit, the radius must change if the comet's orbital speed changes

• As the comet approaches the Sun:
  • the radius of the orbit decreases
  • the orbital speed increases due to the Sun's strong gravitational pull

• As the comet travels further away from the Sun:
  • the radius of the orbit increases
  • the orbital speed decreases due to a weaker gravitational pull from the Sun

Speed of a comet in an elliptical orbit

Comets travel in highly elliptical orbits, speeding up as they approach the Sun

Conservation of energy in elliptical orbits

• When an object moves in an elliptical orbit, energy must be conserved
• Throughout the orbit, gravitational potential energy is transferred to kinetic energy, and vice versa
  • When a comet travels closer to the Sun, it has greater kinetic energy
  • When a comet travels further from the Sun, it has greater gravitational potential energy

• As the comet approaches the Sun:
  • it loses gravitational potential energy and gains kinetic energy
  • the increase in kinetic energy causes it to speed up

• As the comet moves away from the Sun:
  • it gains gravitational potential energy and loses kinetic energy
  • the decrease in kinetic energy causes it to slow down