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Catch a comet

by Bob McDonald

If you’re outside just after sunset this weekend (Jan 13/14) and the skies are clear, look to the west and you should see two bright objects in the twilight sky, one slightly above the other. The upper one is the planet Venus, the other is Comet McNaught. And if predictions are accurate, the comet will be the brighter of the two.

Australian astronomer Robert McNaught spotted the comet (his 31st) last August, when it was barely visible in a telescope. But since then, the icy object has been on an almost direct path towards the sun. And this Sunday, it will swing by at a distance halfway between the sun and Mercury. That close encounter with the sun will boil off water vapour, gas and dust from the comet nucleus at such a furious rate, it should grow the brightest head and tail that’s been seen in the last 30 years.

Unfortunately, that close approach to the sun also makes it difficult to see from Earth. Unlike comet Hale-Bopp, which passed high overhead in 1997 and was visible throughout the night, this comet is only visible for a shot time, low in the western sky, right after sunset. So while the comet itself will be very bright, much of the tail will be lost in the twilight colours of dusk. Still, it will appear as a bright object to the naked eye and a pair of binoculars will bring out much more detail in the tail.

One of the best views of the comet’s close flyby of the sun will be from an orbiting satellite called SOHO, which has a special camera permanently pointed at the sun. The comet will pass through the frame of the camera over several days beginning on the 12th. The result will be a movie of the comet’s closest approach to the sun, when activity in the tail will be at maximum.

Comets are the vagabonds of the solar system, often arriving by surprise, putting on a dazzling show as they whiz through our part of the solar system, then swing around the sun before heading back into deep space, sometimes never to be seen again. Extremely old, they hold the secrets to the early formation of the solar system as shown by the results of the Stardust Mission that returned the first samples of comet dust to Earth. It turns out these frozen ice balls that come from the coldest part of space contain minerals that were formed in the hottest region near the sun.

So while scientists are still scratching their heads over the nature of comets, the fire and ice dance between the comet and our star will be played out this weekend, just above the western horizon, for all to see.

- Bob McDonald

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