Article written by: Helen McLoughlin, Education Officer
In a nutshell, comets are small bodies of ice and dust in orbit around the Sun. When they pass near the Sun, they start to vaporise creating long tails of dust and gas. Even this small amount of information makes us ask so many questions about these members of our solar system. Who discovered them? Where are they formed? How do I spot one? In this article, I hope to expand on these questions and hopefully show that they are so much more than ‘dirty cosmic snowballs’.
What are Comets?
As mentioned in the introduction they are balls of frozen gases, rock and dust that orbit the Sun. When they are frozen, they can be the size of a small town! If a comet’s orbit brings it close to the Sun, it heats up and throws out dust and gases into a large glowing ball larger than most planets. The dust and gases form a tail that stretches away from the Sun for millions of miles.
If we look at a comet in more detail, we can see that it is more complex. Each comet has a tiny frozen part which is called the nucleus. This is the only solid part of a comet and is usually just a few kilometres across. A comet warms up as it gets closer to the Sun and develops an atmosphere called the coma, which engulfs the nucleus so that it is hidden from view. The Sun’s heat causes the comet’s ices to sublimate into gases, so the coma gets larger. The pressure of sunlight and high-speed solar winds can then blow the coma dust and gas away from the Sun, this is the long, bright tail that we see. Comets have not one, but two tails. The first is a dust tail, which is yellowish-white and is the broader of the two tails. The second is a gas tail, which is characteristically blue and narrow. These tails can stretch for over a 100 million km into space. It’s not hard to believe by this description that in the past people were both awed and alarmed by these long-haired stars that appeared in the sky unannounced!
Where are comets found?
‘Short- period’ comets can be found in the Kuiper Belt. Gerard Kuiper, an American astronomer, first proposed the existence of this “belt” in 1951. The belt is on the plane of the ecliptic and extends out around 500 AU (astronomical units; 1 AU is the Earth-Sun distance) from the Sun. Astronomers estimate that the Kuiper Belt contains at least 200 million comets! These comets can take typically two hundred years to orbit the Sun. Their passages can be predicted, for they do regularly pass by, even if encounters with the inner Solar System are a long time apart.
On the other hand, there are also ‘long period’ comets that are found further out in the Oort Cloud. The Oort Cloud lies far beyond Pluto and the most distant edges of the Kuiper Belt. It is thought to form a giant spherical shell surrounding the Sun, planets and Kuiper Belt Objects. Several billion comets are believed to exist in this cloud. While short period comets take less than 200 years to complete one journey around the Sun, long period comets take more than 200 years (sometimes as long as 30 million years!) With their longer orbits, long period comets are also less predictable than short period ones; when we find one it is generally the first time that humanity has ever witnessed it.
Ever wondered where Halley’s Comet got its name? Well, the first person to realize that comets orbit the Sun was Edmond Halley. In 1705, Halley realized that the comets that had appeared in the sky in 1531, 1607 and 1682 were actually just different appearances of the same comet. With these observations he predicted this comet orbited the Earth every 76 years and he was correct. The last time we observed Halley’s comet was in 1986, so we will be waiting until 2061 to see it again.
Unfortunately, 2019 doesn’t promise a wealth of comets. However, you may have been lucky enough to see the creatively named 46P/ Wirtanen comet in December 2018. It was most prominent at the end of last year but continued to move through the constellations Camelopardalis and Ursa Major during January. It will continue to be a binocular object and remain well placed through February and in to March for northern hemisphere observers as the small icy rock fades further.
So there you have it! Maybe not everything you need to know about comets but hopefully it’s inspired you to find out more and look out into the skies to see if you can spot one!