Come mid-November over recent years, keen sky watchers have pulled out the warm clothes and headed for dark-sky sites to watch the Leonid meteors. Only clouds could disappoint them: if they could see the skies, the shooting stars were spectacular.
The Leonid meteors get their name from the fact that they appear to emanate from the constellation Leo, but this is simply a perspective effect. They are, in fact, tiny pieces of debris from a comet named Tempel-Tuttle, which returns to our part of the solar system every 33 years.
Its daughter meteoroids, however, are spread along its path, producing temporary annual intersections of our planet's trajectory. Tremendous showers have been seen since 1998, and a similar event is expected this year, perhaps the last until 2006 if' forecasts are correct.
The best view is from orbit high above the Earth. Last year, astronauts on the International Space Station reported a phenomenal display.
"It looked like we were seeing UFOs approaching the Earth flying in formation, three or four at a time," Nasa astronaut Frank Culbertson enthused. "There were hundreds per minute!"
There have been few such Leonid meteor sightings by astronauts, because space shuttle launches have been scheduled deliberately to miss the shower's returns. The European Space Agency's Olympus satellite was put out of action by a meteor shower in 1994.
A Russian cosmonaut in orbit with Culbertson, Vladimir Dezhurov, reported hearing pings on the outside of a space station module during the short-lived Leonid outburst last year.
Meteors have also been detected by surveillance satellites in their fiery death throes. During one shower, the MSX satellite - an experimental probe operated by the US military - was oriented to look across the limb of the Earth, and it picked up dozens of bright fireballs.
When a tiny Leonid meteoroid, perhaps the size of a grape, plunges into the atmosphere at a speed of 72 kilometres per second (160,000mph), it releases energy equivalent to a large stick of dynamite. No wonder we see a vivid streak across the sky (meteors typically burn up at an altitude of about 60 miles).
But at least our atmosphere protects us from direct hits. The moon has no such shield. In the late 1990s, amateur astronomers claimed to have seen flashes on the moon due to meteoroid strikes, but few people believed them, despite video evidence. The matter was taken up by Professor Jay Melosh, an impact specialist at the University of Arizona, who was able to show that such flashes would be detectable from Earth providing the projectiles were above about one kilogram in mass. Such lumps are seen to enter our atmosphere during showers, producing phenomenally bright fireballs.
It is inevitable that they must occasionally strike the lunar surface, too. The difference is that on the moon they explode instantaneously. Last year, several teams detected coincidental lunar flashes. Flashes will be out in force again on Tuesday morning. Although the Leonids have peaked on November 17 or 18 in recent decades, a variety of factors (including the effect of our leap year cycle) means that this year, these natural fireworks are expected to arrive early on the 19th.
To see them, you need to be out under clear, dark skies after 2am. From then through to dawn, one every couple of minutes should be seen, but a sudden outburst is anticipated around 4am. Such forecasts are fraught with difficulty, but we can be fairly sure of a sudden increase to several meteors per minute at around that time, lasting for perhaps half an hour.
The near-full moon in the west will be making the sky too bright to see the larger number of faint meteors, but plenty of brighter ones should be visible. Look broadly towards the north-east and midway between the horizon and the zenith. As the sun rises, the baton is passed across the Atlantic. A second strong outburst of the shower, not detectable from Britain due to daybreak, is expected to tantalise American aficionados.
Culbertson's stint on the space station ended earlier this year, but he still recalls vividly what he saw, and how glad he was that his orbiting home was protected from the cosmic detritus that whizzed by.
"It makes you think how crowded it is in space sometimes," he mused, "and why it's so important to have those shields on the outside."
