Solar System

Comet Orbits

11.1 - Be able to use data about the names and relative locations of bodies in the Solar System, including:
d) comets

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11.3 - Understand the orbits of short-period comets and their likely origin in the Kuiper Belt

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11.4 - Understand the orbits of long-period comets and their likely origin in the Oort Cloud

Planets orbit a line between Earth and the Sun called the ecliptic. Comets have what is called INCLINED 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 elliptical 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.

Short-Period / Long Period

Short-period comets are thought to orbit from the Kuiper Belt, an area in the region of Pluto comprising of small random rocky bodies, frequently these will have orbits of tens or hundreds of years. Long-period 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.

Astronomers deduce the orbit of a comet by studying its trajectory over a period of time. Comets that return to orbit the Sun are said to have elliptical comets. These move toward and away from the Sun at a speed less than the escape velocity needed to leave the Solar System. Comets at escape velocity will be captured by the Sun or thrown out of the solar system. These follow a parabolic orbit. If it is moving faster than the escape velocity it will travel past the Sun and will not return to it.

Comet orbits can be disturbed by other orbiting bodies such as planets and may even impact them. Their orbits will also be slightly disturbed by their own gases escaping their surface and moving the comet. Otherwise comets will end their lives by exhausting their material and orbiting as a space rock.