Dazzling tail lights

There is a new Moon on Tuesday, so the sky should be at its darkest. That will be perfect for viewing one of the most dazzling phenomena the heavens can offer - if it turns up.

At about 10pm that night the Earth is due to intercept the heart of the Leonid meteor stream, a 100,000km-wide river of material deposited along the orbit of comet Tempel-Tuttle, which orbits the Sun about every 33 years. In 1833 and 1866 this created spectacular showings of up to 100,000 meteors in the sky for the hour the Earth takes to traverse the meteor stream. Although the particles weigh on average just about a milligram, they enter our atmosphere at 70 kilometres a second, which makes them unusually bright.

David Hughes, an astronomer at Sheffield University, plans to take a student observing party onto the nearby moors in the hope of a spectacular Leonid sighting. But he says observers in East Asia will have the best chance of seeing a mass influx: at the expected peak time, the meteors' apparent source in Leo will be high in the sky in Asia but below the horizon for Europeans. By 6am Wednesday morning when Leo is at its highest for Europe, it may be all over.

Dr Hughes says his attempt to see a Leonid special in 1966 was a disappointment, though observers in the western United States saw a massive shower.

He points out that the spectacular Leonid meteor shower can mislead by giving the impression that the Earth is traversing a dense flow of comet matter. Not only are the particles tiny, they are on average a kilometre apart. Only the high relative velocity with which they intercept the Earth makes them seem close together. Dr Hughes says the Giotto spacecraft that went to Halley's Comet in 1986 was deliberately sent where it might be expected to encounter abundant cometary material, but only one particle struck it.

Despite this, the operators of the Hubble Space Telescope are taking no chances, turning it away from the direction from which the Leonids are approaching.