Deborah Smith -Apr 1, 2012
Skimming the sun … Comet Lovejoy visible near the Earth's horizon was discovered by the amateur astronomer Terry Lovejoy.
You've got as much chance as a snowball in hell. It's an old saying, but take heart. Some snowballs can survive the cosmic equivalent of hell - a close encounter with the fiery surface of the sun.
Comet Lovejoy - named after the amateur Australian astronomer who found it - was declared a wonder last year when it withstood an hour-long death dive through the sun's atmosphere.
Now scientists have worked out which comets can achieve this seemingly impossible feat and which ones will just slowly fizzle out or explode.
John Brown, Astronomer Royal for Scotland, said the key was whether the comet's orbit brought it within 7000 kilometres of the sun's surface.
His team studied how these giant balls of dusty ice lose mass and energy at different altitudes, and he presented the results at an astronomy conference in Manchester this week.
''In modelling how comets behave in this extreme environment, we really are starting to understand what happens to these supersonic snowballs in hell,'' Professor Brown said.
Below 7000 kilometres, they become ''sunplungers'' and are destroyed in a few seconds as they crash into the dense solar atmosphere, he said.
Above 7000 kilometres, they become ''sunskimmers'' and slowly fizzle out, as sunlight and friction vaporise their icy cores. This may take a few minutes or much longer, depending on their size, which can range from 10 metres to tens of kilometres across.
For decades, astronomers have watched thousands of comets fall towards the sun, but the solar glare made it impossible to see the snowballs on close approach.
Last year, however, a NASA satellite, the Solar Dynamics Observatory, made the first direct observations of two sunskimmers on both their intrepid journeys.
In July, Comet SOHO disintegrated during a 15-minute period as it passed within 100,000 kilometres of the surface.
In December, the larger Comet Lovejoy survived its hour-long trip at an altitude of 140,000 kilometres, although it lost a large amount of its mass.
Professor Brown said these two events matched their predictions and he hoped the flare of an exploding sunplunger would soon be detected as well.