Halley's was hugely disappointing, Hale-Bopp was striking in a limited kind of way, but Comet McNaught is breathtakingly beautiful.
After two nights of playing "catch me if you can" with stargazers while dodging behind clouds, mist and rain, the latest of the "Great Comets" finally decided to grace the sky along the Atlantic seaboard with wonderful views on Wednesday night.While those inland have been disappointed, a group of comet watchers waiting on the slopes of Table Mountain got what they had been waiting for: a world-class view of the comet.
By 8.15pm, Venus was clearly visible, but Comet McNaught - named after the Australian astronomer who discovered it from Siding Spring on August 7 last year when it was still 200 000 times too faint to see with the unaided eye - remained elusive.'This may be Comet McNaught's only visit to Earth'
Then, just five minutes later, University of Cape Town physics lecturer Gregor Leigh called out excitedly: "I can see it!"
In the next few minutes, all the others in the party also picked it up: a bright, shining head, pointing down towards the horizon, with a broad stubby tail streaming out behind it - just slightly to the left of where the sun had set and only marginally lower in the sky than Venus.
For the next 50 minutes or so, as the sky grew darker, they all watched entranced as the head glowed and the star-speckly tail increased in intensity and length and appeared to develop a slight curve as the comet "dropped" towards the horizon.
"I've seen three or four comets and I think this was the best," Leigh said.
"It's clear and it looks like the comets traditionally drawn in medieval pictures. It's an extremely beautiful thing."
Although best viewed through binoculars, the comet was clearly visible with the naked eye, because unlike previous much-hyped comets - Halley's and Shoemaker-Levy come to mind - McNaught is massive: just more than half a hand long.
It made its closest approach to the sun last weekend, and should be visible until Sunday, appearing higher in the sky each night but also getting dimmer.
Dave Laney of the SA Astronomical Observatory, who has been watching McNaught, said this comet, like others, was proving unpredictable. It was supposed to have been visible until about Friday.
He explained that most comets were found in the Oort Cloud part of the outer solar system beyond the orbit of Pluto (recently downgraded to "dwarf planet").
They are often called "dirty snowballs" because they consist largely of chunks of ice and dust.
They are survivors from the early solar system, and are also small - usually only a few kilometres across.
"If their orbits are disturbed only slightly, they can plunge into the inner solar system, where we see the glowing cloud of evaporated gas and dust around the tiny solid lump, while the pressure of sunlight and of particles streaming outward from the sun can push some of the cloud, or 'coma', into the classic 'tail', which always points away from the sun," said Laney.
Comets' tails can reach enormous lengths, many million of kilometres long, but the amount of gas in each of them is minute - it could fit inside a soccer ball!
This may be Comet McNaught's only "visit" to Earth.
University of Manchester astronomer Albert Zijlstra, who is currently visiting the SA Astronomical Observatory, explained that comets would normally be expected to return to the outer solar system, although on an orbit that would eventually return to the sun.
"We do not yet know the precise orbit of Comet McNaught, but current estimates indicate it may be travelling on an orbit that will allow it to escape the sun altogether.
"This can happen if a comet picked up some extra speed while falling towards the sun, from coming too close to a planet - spacecraft travelling to the outer planets use the same trick, normally using the gravity of Jupiter.
"If this is correct, Comet McNaught will never come back to Earth. Instead it will leave the solar system, and may eventually, after millions of years, fall towards another star."
Although other comets are expected to become visible - typically, there are a few each decade - it may be decades before one as bright as McNaught comes along.
"Or we may be lucky and get another one this year - but don't bet on it," Zijlstra said.
Original Source: John Yeld, IOL.co.za
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