Meteorites to bombard the Earth

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The skies will be alive this weekend with a meteor shower - the same phenomenon that brought a piece of rock, possibly bearing traces of life, from Mars, 13,000 years ago.

For the next three days the Earth will be bombarded by meteorites from the Perseid shower, as it makes its annual passage through a region of space containing pieces thrown off Comet Swift-Tuttle. These will produce bright, short-lived streaks in the night sky, at a predicted peak rate of 100 per hour.

The ideal conditions for viewing will be at a dark site, away from city lights, and with a clear sky. Anyone should be able to see the meteorites, which will show up as bright streaks in the sky. The best view will come from looking east-north-east. The highest number are expected tomorrow, but the display is expected to start tonight.

There will be little light from the new moon, which should raise the chances of seeing the meteorites' trails. But the Meteorological Office in Bracknell said yesterday that the weather would not be ideal for watchers. "There is a messy low-pressure front, with lots of clouds," said a spokesman. "Some areas will get good views and others won't." Best conditions will probably be in north-west Scotland.

However, those hoping to catch a Martian in their backyard will be disappointed. The majority of meteorites making up the Perseid shower weigh about 50 milligrams and measure just 5 millimetres in diameter. They hit the Earth's atmosphere at 133,000 mph, where friction causes them to burn up before they reach the planet's surface.

"The intensity is likely to be back to normal this year," said Professor Mark Bailey, the director of Armagh Observatory. "But you will see them if there is a clear sky, and you have a dark site."

Observers can watch the meteorites with binoculars, and take a picture of them with a camera set at a long exposure with high-speed film, or a video camera set to its largest aperture, with the focus at infinity.

The annual Perseid shower has been known about since 830 AD. It was called the "Tears of St Lawrence", after a saint who was burned at the stake in 258 AD. The intensity of the showers has intensified since 1992, when the parent comet, which follows a fixed, hyperbolic orbit, passed close by the Sun.

Jacqueline Mitton, of the Royal Astronomical Society, said: "The peak of the stream can be very concentrated. The Earth can pass through it in one hour or less, with not much activity on either side. It's not totally predictable."

The meteorites' name is derived from the constellation from which they seem to emanate - Perseus - which can usually be seen low in the north-east after sunset. The best time for viewing will probably be between midnight and 2am on Sunday night.