LONDON, Nov 16: The space shuttle is being grounded this Sunday, and the Hubble Space Telescope will be steered to look away for the return of the Leonid meteors.

Although they pose no danger to us on the ground because the atmosphere shields them out, tiny meteoroids pose (rather literally) a bumper danger to satellites or astronauts in orbit. That’s why the space shuttle has not flown in mid-November for the past several years, and Hubble is re-oriented to point its rear end towards the direction from which these minute projectiles arrive. For the three inhabitants of the space station, it’s a matter of sit tight and keep your fingers crossed. Ditto commercial satellite operators with billions invested.

This will not go on forever. We are in a brief period of heightened activity. Although the Leonid meteor shower is seen every year on November 17-18, its prominence follows a regular cycle. Every 33 years there is a sequence during which the sky really lights up on the same date, and we have records dating back to AD 902 to prove it.

In recent centuries, phenomenal storms in 1799, 1833, 1866 and 1966 have occurred. While 1999 and 2000 were spectacular, the predictions for 2001 and 2002 are better still.

The regular period of 33 years results from the length of time the parent comet, on its elongated orbit, takes to circuit the Sun. This comet, named Tempel-Tuttle, was discovered by German and American astronomers during the 19th century.

Each time a comet returns and traverses the Earth’s orbit, it is heated sufficiently by solar radiation to make a thin layer of its surface, predominantly made of ice, evaporate into space. This produces a cloud of vapour, plus the characteristic tail, which both reflect a great deal of sunlight, making comets relatively easy to discover.

As that water vapour expands, it carries with it myriad small solid lumps of rock, and these meteoroids tend to follow much the same orbit around the sun as the parent comet. Therefore, although comet Tempel-Tuttle passed by a couple a years ago and can only be seen now with a powerful telescope, the Earth is still passing through clumps of trailing debris.

In fact, it is even possible to calculate when distinct clumps must have been released, and precisely when they should intersect the Earth. For example, the time on Sunday when meteoroids liberated by the comet way back in 1767 will arrive is known fairly accurately. Four separate groups of astronomers have done the calculations, and agree that within a few minutes of 10:00 (Greenwich Time/ 3.00pm Karachi) many thousands of meteoroids that have spent the past 234 years winging through space will end their lives in fiery oblivion as they enter the upper atmosphere. Unfortunately that’s daytime in Britain.

Similarly, two other major groups of meteoroids are expected: one cluster jettisoned by the comet in 1699 and another relatively youthful bunch, released in 1866. In both cases the constellation from which the meteors appear to emanate - Leo, hence their name - will still be far below the horizon, and so not visible from Europe.

This is why meteor enthusiasts are flying either east or west. One choice is the East Coast of the US, when it will still be dark with Leo high in the sky. Another is the Far East (ie, Japan or Australia), because the two outbursts occur at about 3am local time on November 19.

Not all the astronomers agree, however. Peter Brown, a Canadian meteor researcher, expects the greatest activity to come from a trail of debris shed by the comet in 1799, with heightened meteor rates occurring over several hours during November 18, giving viewers in the eastern Pacific region the best opportunity.

Neither do the teams agree on how many meteors to expect. David Asher, who works at the Armagh Observatory in Northern Ireland, is cagey on precise numbers, but thinks that the youngest meteoroids, from 1866, will produce a peak rate of up to 8,000 per hour. He is in Japan already, waiting to see whether his computer modelling is correct.

By contrast, Peter Jenniskens of Nasa-Ames Research Centre anticipates the peak due to the 1767 trail to be highest of all, maximising at 4,000 per hour. Certainly a phenomenal sight, if it turns out that way, although nowhere near the estimate of over 100,000 per hour reported in 1966.

Calculations have already been performed for 2002, when Europe is one of the favoured places, although one can expect them to be modified in line with what is observed this year. The rates may be high, but the moon will flood the sky with light.

High hopes, then, for a celestial spectacle on Sunday, even if we only see it on the TV news. We must also hope for no damage, especially to the International Space Station. In 1993 the European Space Agency lost a satellite during a meteor outburst: US dollars 250 million worth of hardware wiped out by a tiny fleck of cometary dust. Come Monday, a satellite roll call showing all present and correct would be good news indeed.—Dawn/The Guardian News Service.