Between 10pm and 2.30am tomorrow the sky over Europe and western Asia should be lit by a celestial firework display as a cloud of interplanetary dust blazes briefly in the upper reaches of the atmosphere. Weather permitting, it should be visible in Britain but in cities street lights will diminish the display.
According to the Armagh planetarium, 'watchers will have an excellent chance to see as many as 20 to 30 shooting stars per second'. This would be not just a shower, but a veritable storm, and others are more conservative, with the International Meteor Organisation noting that 'a genuine meteor storm is very rare indeed, and cannot be predicted in advance with certainty'.
The Perseid shower happens every year. Ancient Chinese, Japanese, and Korean records extend back nearly 2,000 years, but this time astronomers expect an exceptional display - the Perseids will not have been as strong for more than a century. Although the dust grains are microscopic, the American space agency Nasa is sufficiently concerned at the prospect of a storm to have delayed launching a space shuttle mission.
Meteor streams appear when the Earth sweeps in its orbit through a cloud of dust left by a comet. Not only does the Earth move, it moves around the Sun fast, with a speed of 30 km/s (108,000 kph or 67,000 mph). Dust particles from the Perseids hit the air at twice that speed and friction causes them to burn up in a brief incandescent flash. Most evaporate before reaching 90km (56 miles) above the surface. None is expected to be big enough to reach the ground.
In the case of the Perseid swarm, the parent comet is Swift-Tuttle. The meteors are so named because a trick of perspective makes them appear to come from the constellation of Perseus, low in the North-east sky this month, although they have nothing to do with the distant stars.
Comet Swift-Tuttle returned to the Earth's vicinity last year, but tonight's dust dates from its last approach in 1862. This dust clump is thicker than any the Earth has encountered and should provide a spectacular display - optimists hope it may surpass the 1966 Leonids (visible mainly from the US). Last year, there was a foretaste as the Earth caught part of the cloud, although the maximum display then lasted half an hour and was visible only in Asia.
Dr Jacqueline Mitton of the Royal Astronomical Society says the serious meteor observers are gathering at Puimichel in the Alpes de Haute Provence this time. The best may be to come. Two astronomers from Queen Mary and Westfield College in London believe the Earth may plough through the thickest part of the cloud in 1994.