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Comet doomsday nonsense

Artist's impression of a comet

Artist's impression of a comet. Comet C/2010 X1 (Elenin) will make a distant fly-by of Earth in October 2011.

I WAS ASKED THE OTHER NIGHT (by a caller while I was a guest on Tony Delroy’s Nightlife show on ABC radio), whether there is any truth to the rumour that Earth is going to be buzzed—perhaps even hit—by a comet later this year.

I must admit I hadn’t been keeping my eye on cometary matters lately, so I wasn’t able to give the caller a detailed reply. What I did say was that if a very close approach by a comet were on the cards, let alone a collision, I was sure I would have heard about it.

But I promised to investigate the matter and post something about it on

Hunting around, it seems the caller had heard about Comet C/2010 X1 (Elenin), which was discovered on 10 December 2010.

Elenin is a long-period comet (ie. with an orbital period longer than 200 years) that is estimated to have a solid icy core, or nucleus, about 3 to 4 kilometres wide. Quite average for a comet.

As far as visibility for Earth-bound observers is concerned, there are two points that matter in a comet’s orbit—the point of its closest approach to the Sun, called perihelion, and its closest approach to Earth, called perigee.

A comet shines by light reflected from the gas and dust cloud that builds up around its icy nucleus—gas and dust that has been liberated from its frozen surface by the Sun’s heat. The point around perihelion is important, as this is where the most gas and dust can be expected to be liberated. (Technically speaking, the ice does not evaporate, which involves a liquid phase—it sublimates, which is when ice turns directly into gas.) The more gas and dust, the more reflected sunlight, and the brighter the overall comet will appear to be.

The time around perigee is important too, as being closest to Earth the comet will appear larger (and therefore brighter).

In the case of Elenin, perihelion will occur on 10 September 2011, and perigee will be reached on 16 October 2011 at a distance of about 34 million kilometres.

Is that close? No, it’s a long way away. It’s about 100 times further than the Moon, which means Elenin poses no threat to Earth at all.

So if you come across any Elenin doomsday stories on the Internet, please disregard them—they are nonsense.

South Australian amateur astronomer Ian Musgrave has a great set of Elenin questions and answers on his Astroblog site.

Story by Jonathan Nally.

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