On Friday and Saturday, our planet will once again pass through the minefield of cosmic dust that is the Leonid meteoroid stream.

When the dust, left behind by Comet Tempel-Tuttle, burns up in our atmosphere it forms the sometimes spectacular Leonid meteor shower.

Astronomers who predicted last year's display say there will be only a modest outburst this year - possibly a prelude to more powerful showers in 2001 and 2002.

The best place to see them this week is from Europe, Africa and North America. The flurries of shooting stars should be seen between local midnight and dawn on 17 and 18 November.

Peak rates

Every 33 years, Comet Temple-Tuttle, when it swings through the inner Solar System, lays down a fresh trail of dust. It was last here in 1988.

Most years, the Leonids are a minor shower but when our planet passes directly through a dense dust stream the rate at which shooting stars appear in the night sky can soar.

"We're very, very confident of the storms coming in 2001 and 2002," says David Asher of the Armagh Observatory. Some estimates suggest that next year peak meteor rates may reach 10,000 meteors per hour.

This week's display will be much more modest, about 100 meteors an hour - with luck. This is because the Earth is only crossing the outer fringes of the dust stream.

Naked eye

The Moon could be a problem, however. At its last-quarter phase, the satellite is in the same constellation, Leo, from which the meteors appear.

This means that the fainter streaks of light will be hard to spot.

The constellation of Leo rises above the eastern horizon just after midnight local time. Finding Leo will be easy because the waning Moon will be inside its boundaries.

The meteors will come from the general direction of Leo and can appear anywhere overhead. The best way to view them will be with the naked eye. They will be easiest to spot about 90 degrees from Leo.

Moon strike

A team of astronomers at Nasa's Marshall Space Flight Center (MSFC) will again launch a weather balloon outfitted with special sensors to observe the shower.

The balloon is intended to reach an altitude of 30,500 metres (100,000 ft).

Scientists will also watch the Moon's encounter with the Leonid meteors to see if any of the dust particles impact on the satellite's surface.

The Moon's passage through the stream will take place at about 0500 GMT on 17 Nov. This means that the Moon will be closer to the densest part of the dust stream than the Earth.

Because the Moon is airless, the Leonid meteoroids that strike the satellite will not cause shooting stars. Instead, they will strike the lunar soil at 225,000 km/h (140,000 mph). Last year, for the first time, astronomers captured video film of at least six meteoroids hitting the Moon's surface.