Ison: The comet of the century

Ison Comet Sun Corona

If we're lucky, then in just a few days time we could witness one of the most spectacular sights in the night sky for a generation or more.

Astronomers hope that on 3 December a comet will appear on the eastern horizon - Comet Ison.

For the whole month of December, millions of people across the northern hemisphere should be able to see its tail, which is several millions of kilometres long, stretching across the dawn sky.

Ison has come from the Oort cloud, a belt of comets on the very edge of the Solar System, where it has been for the last 4.6 billion years.

What makes Ison so special is that it is a "sungrazer". Many comets pass through the Solar System every decade, but very few go through the corona of the Sun. Ison will do just that.

A so-called "sungrazer", it approaches our star at a distance of just 1.2 million km from the surface

Ison brushes past the Sun on 28 November; the heat at "perihelion" is expected to exceed 2,000C

Its passage through the corona, which happens on 28 November, will be watched with great interest by astronomers across the world.

It's not known exactly what effect the great heat and gravitational force of our Sun will have on the comet.

Dr Matthew Knight, from the Lowell Observatory in Arizona, has been watching the comet for the last year and has worked out three scenarios to describe what Ison might do in the coming week.

Scenario one: It could suffer the same fate as Comet Lovejoy, which went around the Sun in autumn 2011.

The gravity of the sun pulled one side of the comet's nucleus more strongly than the other, stretching it apart.

As Lovejoy emerged from the corona it exploded. Whether this will happen to Ison depends on its size. A nucleus of two kilometres or under puts it at great risk. Astronomers estimate that Ison is almost exactly two kilometres, so it's right on the borderline ( via ).

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