Comet Orbits

Describe cometary orbits and distinguish them from planetary orbits

Planets orbit a line between Earth and the Sun called the ecliptic. Comets have what is called INLCINED orbits, meaning they orbit at a higher (or lower) angle to the ecliptic. Essentially this means they can be observed in any point in the sky.

Comparing their orbits to planets

Planets also orbit the Sun in a prograde motion. This means they would appear to orbit the Sun anti-clockwise if you were looking at them kilometres above the north pole of the Sun.

Comets may orbit the Sun in a retrograde motion, meaning they orbit the Sun the opposite direction to the planets.

All objects orbit the Sun in an ellipse, that is not a perfect circle. Planets have a low eccentricity, that is they are very circle-like. They are not very elliptical. Comets have highly ellipitcal orbits - an oval with the Sun near the very end of one ellipse. They often have parabolic, or open, orbits meaning they may not return to the same position again.

Nature of Orbits

Short term comets are thought to orbit from the Kuiper Belt, an area in the region of Pluto comprising of small random rocky bodies. Long term comets have periods of thousands or millions of years and are thought to come from the Oort Cloud, at the very furthest region of the Solar System.

Many comets end their lives by exhausting their material and orbiting as a space rock or by hitting or being destroyed by the Sun or a planet. Some are 'thrown' out of the Solar System never to return by the gravity of the Sun.


  • What is unusual about the orbits of comets compared to planets?
  • Describe 3 features about the orbit of a comet.
  • Describe the areas of the Solar System that comets are thought to orbit from.
Did you know?

Spacecraft have flown close to 7 comets - Tempel 1, Hartley 2, Borrelly, Wild 2, Halley, Giacobini-Zinner, and Grigg-Skjellerup