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Comet ISON

There has been, and will be a lot more, publicity about a comet called ISON, (see Fig 1). It was discovered on 12 September 2012 by Vitali Nevski and Artyom Novichonok. The discovery was made using the 0.4-meter reflector of the International Scientific Optical Network, (ISON) near Kislovodsk, Russia, hence the name Comet ISON. It was predicted to become the comet of the century, possibly visible in the daytime. But comets are remarkably fickle things and behave in a most unpredictable way.

To date it has under-performed in that it hasn’t brightened as much as expected, but it might yet surprise us all! On Thursday, 28 November, it will reach what astronomers call perihelion, its closest to the Sun, when it will be only 1.3 million km above the Sun’s surface, which for a comet is extremely close. So it is still possible for ISON to shed a great deal of material, as it gets really, hot and the light reflected off the debris of gas and dust, could make it very bright. There is also a possibility that the comet breaks apart due to the high temperature and tidal forces.
What is disappointing for us is that it will really only be visible from the northern hemisphere, so we down here at the foot of Africa, will be unable to see it. However, should it become spectacular there will no doubt be huge media coverage, so I think it might be an opportune time to say what comets are!
Astronomer Fred Whipple called them “Dirty Snowballs” which is in fact an accurate description. They are the remnants of the formation of the solar system and consist of conglomerate of carbon dioxide (dry ice) and water ices, gases like hydrogen, dust and rocky rubble and assorted traces of other chemicals and compounds. As the approach the Sun they get heated and the ices start giving off gas and dust, and small pieces might break off and form a cloud around the comet itself. However the Sun’s radiation will push the cloud away from the comet and start to form the famous tail. As the comet gets closer to the Sun, the radiation pressure increases and the tail gets larger and always points away from the Sun (see Fig. 2).
Quite often there are two tails, one pointing directly away from the Sun and bluish in colour, called the ion tail: made up of charged particles. The other is often fan shaped, curved and yellowish on colour and made up of dust and gas. (see Fig. 3)

 
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