Comet ISON, an incoming sun-grazing comet

November 20, 2013

The formal (technical) name is C/2012 S1 (ISON)

Comet ISON is presently approaching the sun and being closely monitored by astronomers around the world. The reason for the special interest is that it will pass only 1.165 million kilometres above the solar surface on 29 November this year and has the potential to soon become very bright in the twilight sky.  Even now it is being heated by the sun and this is causing volatile material to evaporate into space creating a long tail and greatly increasing its total brightness. As it passes the sun, it will be subjected to temperatures estimated to be around 2,800°C.

“Currently the comet has brightened to the point where it is easy to see with binoculars in the dawn sky, and may be just visible to the naked eye depending on where you’re viewing it from.  At 5am this week the comet will be below the bright star Spica in Virgo. By next weekend the comet will be close to Saturn and Mercury but difficult to spot close to the eastern horizon just before sunrise.  It is very low in the eastern sky so a good unobstructed view of the eastern horizon is required, as is a very early start!” Says Dr Grant Christie, Stardome’s Honorary Astronomer.

CAUTION: Severe eye damage. If using binoculars near sunrise, great care is needed to avoid pointing them at the sun

As the comet get closer to the sun over the next week, it will become harder to see in the dawn light unless it undergoes a sudden increase in brightness.    If the comet remains intact after perihelion (the closest approach to the sun on 29 November)  it will follow its orbit that will see it tracking northwards along the eastern horizon at sunrise. This means it will continue to rise at about the same time as the sun for the next few weeks. If the comet develops a spectacular tail as some have suggested, the tail will be aligned along the eastern horizon before dawn. The tail may also be visible on the western horizon soon after sunset.

While it is not possible to predict with confidence the future displays of this comet, it has the potential to release a great deal more material over the next few weeks as the sun’s heating increases. If so, it could put on a spectacular display with some media even suggesting it could be the ‘comet of the century’. While experts consider this very unlikely, it still could provide a fine display.

The comet’s frozen nucleus is estimated to be between 400 and 1,200 metres across which means that it should survive this close encounter with the sun. If it is less than 400 metres across, it will most likely disintegrate after which its parts quickly completely evaporate.  Observations by the Hubble Space Telescope suggested that the comet’s nucleus was rotating with one pole pointing at the sun. If so, then the pole facing away from the sun has not yet been subjected to much heating and its store of volatile material, such as frozen water and carbon dioxide, remains intact. As the comet gets nearer to the sun and the shadowed side begins to be exposed to the direct solar rays, the release of a lot more volatile material could produce a sudden and spectacular increase in brightness. We will know over the next few weeks.

Comet ISON was discovered on 21 September 2012 by a small team of astronomers based in Russia called the “International Scientific Optical Network” – or ISON for short.  The comet’s orbit was soon determined at which point it was obviously going to be a “sun-grazer” and potentially capable of putting on a brief but spectacular display.

The comet’s orbit shows that it comes from the Oort Cloud, a reservoir of cometary bodies that orbit nearly a light year from the sun (between 30,000 and 60,000 times further than the Earth from the sun). It will be the first time this comet has entered the central region of the solar system since it formed 4.5 billion years ago.

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