Monday, 18 November 2013

Comet ISON begins final stage of its fiery plunge towards the sun

Guest blog: Dr Mark Gallaway, Bayfordbury Observatory

When it was discovered at end of 2011, the comet known as C/2012 S1 aka ISON caused quite a stir in the astronomical community.  ISON had a very similar orbit to the Great Comet of 1680 which was very bright - reputedly visible even in daytime – and was noted for its spectacularly long tail.
ISON’s characteristic green colour - thought to be caused by gases escaping, most likely cyanogen and diatomic carbon, and fluorescing in the sunlight. Image credit: David Campbell, Bayfordbury Observatory, University of Hertfordshire
Astronomers thought that ISON could become a daylight comet, or at least a very bright twilight object, with a large arching tail. However, there was always a caveat. Comets are fickle things especially one that has not been observed before.

Living in the outer reaches of the solar system, comets are balls of rock and frozen gases, often described as a dirty snow ball and are the remnants of the formation of the solar system. Occasionally a comet will be disturbed and begin the long fall towards the Sun. As it reaches the orbit of Mars, the light from the Sun begins to warm the comet enough to begin to melt the trapped ices (which turn straight to a gas).

At first this gas forms an atmosphere, known as the coma, around the comet. As the comet gets nearer the Sun, the solar wind begins driving the coma back like a wind sock at an airport. This forms the tail, which always points away from the Sun, and may, for a while, be the largest object in the Solar System.

In many ways ISON has been a disappointment being nowhere near as bright or active as astronomers hoped. Already past the Earth and nearing its close approach to the Sun (perihelion) on the 28th November, ISON is just visible with the naked eye before sunrise but is an easy target for a pair of binoculars.

ISON will pass within 1,165,000 km of the surface of the sun before swinging back round and into interstellar space, never to return. However, the stresses and temperatures of such a close pass to the Sun may very well be too much for ISON and it might break up; only time will tell. If it survives it may yet put on a spectacular show.

ISON has been imaged a number of times by the staff at the University of Hertfordshire’s Bayfordbury Observatory. The latest was taken last week by David Campbell and shows ISON’s characteristic green colour. This is thought that this is caused by gases escaping, most likely cyanogen and diatomic carbon, and fluorescing in the sunlight.

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