What are Comets?

This week saw the peak of the Lyrid Meteor Shower and in the last post I mentioned that a comet called C/1861 G1 Thatcher was the source of the dust that causes the Lyrids. But what exactly is a comet? Comets are often described as dirty snowballs. They are small and are made mainly of ice and rock.

Comet 67P/Churyumov-Gerasimenko up close on March 14, 2015 by ESAs Rosetta spacecraft. Credit : NASA

Comets orbit the Sun just like the planets; however, they differ in the shape of the orbit. Planets, such as Earth and comets orbit in a shape called an ellipse. An ellipse is like a stretched circle. In the case of the planets the circle is not stretched very much so the orbit is close to a circle. This means that the distance from the Earth to the Sun doesn’t vary too much and therefore we get roughly the same amount of energy at all points in the orbit. In the case of a comet the ellipse is stretched much more so that for some of the orbit the comet is very close to the Sun and at other times it is very far away. The diagram below depicts the orbit of the Earth and the orbit of Comet Kohoutek. You can see how much closer the comet comes to the Sun compared with the Earth.

Orbit of Earth and comet Kohoutek. Credit: NASA

Some comets take only a few years to travel around the Sun while others can take hundreds. For example there is a comet called 2P/Encke which takes 3.3 years to orbit the Sun and a comet called C/1887 B2 which takes 999 years to complete one orbit. When these comets travel out into deep space, away from the Sun, there is no heat or light and the comet remains frozen. Once the comet starts to approach the Sun the comet heats up. It starts to release gas and dust as the ice and other materials in the comet melt and evaporate. The resulting gas and dust streaming away from the comet is called a tail. When a comet is bright the comet and tail can look stunningly beautiful in the sky but bright comets are rare.

Comet ISON, with a tail 8th November 2013. Credit:NASA

As the comet moves along it leaves the trail of dust along its path. Depending on the path is takes, sometimes the Earth will pass through the trail of dust left by the comet. This is how meteor showers happen. Sometimes a comet will pass so close to the Sun that it will get completely destroyed by the heat of the Sun. These comets are called Sun grazers.

So the next time time you look up and see a meteor streak across the sky remember that it possibly came from a comet the may have had a very long journey around the Solar System to get here. And fingers crossed we get a nice bright comet to look at soon!

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