IN THE Hollywood movie Armageddon, an asteroid the size of Texas is on a collision course with Earth, and Bruce Willis helps out by nuking the rock to bits. But even Willis couldn鈥檛 help if the cosmic baddies heading our way were invisible 鈥� and that is apparently a distinct possibility.
Astronomer Bill Napier, recently retired from the Armagh Observatory in Northern Ireland, and colleagues from Cardiff University in Wales were trying to work out why only a handful of comets have been seen in the inner solar system when theory predicts there should be hundreds.
It could be because some comets are black enough to be invisible. The researchers calculate that such comets would reflect less than 0.1 per cent of the light striking them, whereas even the darkest patches on the moon reflect 7 per cent of incident light.
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These comets would have a porous carbonaceous coating, formed as ice from the dusty core evaporated. This layer would trap light, making them invisible to optical surveys of the sky. 鈥淚f these things are very black, it will be like looking for black cats in coal-holes,鈥� says David Hughes from the University of Sheffield in the UK. Only observations in the infrared will reveal them.