Elliptical Orbits (Cambridge (CIE) IGCSE Physics)
Revision Note
Written by: Katie M
Reviewed by: Caroline Carroll
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Elliptical orbits
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
Planets, minor planets and comets travel in elliptical orbits where the Sun is not at the centre
Examiner Tips and Tricks
You will not be asked to do any calculations with elliptical orbits. If you are asked to calculate the time period, orbital speed or radius of an orbit, it can be assumed that it is circular.
Comet speed
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
Examiner Tips and Tricks
Remember that an object's kinetic energy is defined by: 
where m is the mass of the object and v is its speed. Therefore, if the speed of an object increases, so does its kinetic energy. Its gravitational potential energy therefore must decrease for energy to be conserved.
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