Comet ISON

Tuesday 18th July, 2017

Our visiting speaker was Dave Eagle who had come to talk about "Comets, Enigmatic and Beautiful Visitors — What do we know about them?". He is a fellow of the Royal Astronomical Society and has been interested in astronomy since he was a young boy. He is very active in astronomy outreach, giving talks and workshops as well as being an author of many practical astronomy books.

He began by saying that the word "comet" derives from the Greek word "kometes" meaning "long-haired"; reflecting the long tail of gas and dust that appears as a comet approaches the Sun. Out in the cold dark reaches of the Solar System a comet will appear quite inert but this can all change as its highly elliptical orbit brings it nearer the Sun.

Mr Eagle then explained that comets consist of a mixture of rock, dust, water ice and frozen gases such as carbon dioxide or ammonia. In the outer reaches of the Solar System these stay in a solid stable state but as they get nearer the Sun its heat begins to turn much of the solid material straight into gas. The resulting jets of gas can blast dust and small bits of rock off the comet's surface sending a trail of material off into space that we see as the comet's tail.

He continued by saying that sometimes two tails are observed: a dust tail and a tail made up of ionized gases (the ion tail). The dust tail consists of small particles no bigger than cigarette smoke and forms when sunlight exerts pressure on them. As this pressure is relatively weak the dust particles form a curved diffuse structure that can extend for millions of kilometres.

In contrast the ion tail always points directly away from the Sun in a roughly straight line that is also much narrower than the dust tail. This ion tail is formed when charged particles ejected from the Sun (known as the solar wind) interact strongly with the ionized gases in the ion tail.

Mr Eagle stressed that it is very difficult to predict the behaviour of a comet and referred to a quote from the Canadian astronomer David H Levy who said: "Comets are like cats: they have tails, and they do precisely what they want". He then mentioned one comet that some had hoped would turn out to be an amazing spectacle but did not live up to expectations. This was Comet C/2012 S1 (ISON) which was discovered in September 2012 by Russian astronomers when it was very dim and located out past the orbit of Jupiter.

Comet ISON was a new visitor to the inner Solar System and so astronomers had hoped it would put on a bright enough show to be visible to the naked eye. However, it was for good reason that this comet was called a "sungrazer" as it would be travelling through the Sun's outer atmosphere.

The first bit of bad news came when observers realised that the comet's central region or nucleus appeared to be crumbling as it approached the Sun. It was soon lost in the Sun's glare to any observers on Earth but some NASA spacecraft could still track it. Both amateur and professional astronomers alike were very disappointed when images showed that it had disintegrated and all that emerged from its close call with the Sun was a fan-shaped trail of debris.