Minute grains of dust create the glowing heads and tails that make comets famous. A trail of dust traces the orbit of each comet, and when the Earth encounters a comet trail the result is a meteor shower. Comet Tempel-Tuttle has just refreshed its dust trail on a visit to the Sun's vicinity, which it makes every 33 years.

The Leonids approach the Earth from the direction of the constellation Leo. As a precaution, the Hubble Space Telescope will turn its back on Leo for ten hours around the predicted peak of the Leonid event, which is at about 20:30 CET on 17 November. Astronomers will take the opportunity to look for undiscovered galaxies in the opposite direction in the sky. Any disturbances caused to the 11.6-tonne Hubble spacecraft by the Leonid dust impacts will be recorded for analysis by dust specialists. One of the teams chosen for this study includes ESA and UK scientists and is headed by John Zarnecki of the University of Kent.

Zarnecki comments: "It seems like doing an experiment with the crown jewels. But Hubble is a fantastically accurate star pointer, so we should detect wobbles due to quite small impacts. We hope to check our theories about the numbers of grains of different masses. But I'd hate to see any harm come to Hubble," Zarnecki adds. "Or any other spacecraft for that matter."

Taking account of the risk to spacecraft

This year Comet Tempel-Tuttle passed within 1.2 million kilometres of the Earth's orbit, which is very near by astronomical standards. Similar close encounters have produced widely differing results in the past. In 1932 the count of visible meteors in the Leonids reached an unremarkable rate of 240 per hour, compared with a normal background of about 10-20 sporadic meteors per hour at quiet times. Yet in 1966 the count-rate for the Leonids was 15,000 per hour, or 4 per second, and some observers reported even higher rates.

If the rate is again 15,000 per hour, a spacecraft presenting a target of 10 square metres to the Leonid storm is likely to receive one hit penetrating aluminium to a depth of 0.4 millimetre. A larger spacecraft has a greater chance of being hit by a more penetrating dust grain. Operators are therefore advised to turn their spacecraft to present as small a target as possible, and to try to ensure that sensitive parts do not face the meteor stream.

"Bullet-like damage caused by large particles is only part of the story," says Walter Flury of ESOC's mission analysis section. "Fine grains are far more numerous and can sand-blast optical systems, thermal blankets and solar cells. And in a cloud of charged particles created by the impacts, lightning-like discharges can cause faults in the electronic systems of the spacecraft. The very high speed of the Leonids aggravates that risk, so it may be advisable to switch off sensitive equipment. Damage due to electrical discharges may be the most serious hazard from the Leonids."

Predictions are very uncertain and effects are very chancy, so one recommendation is simply to reinforce the spacecraft operation teams on 17 November, to cope with any emergency that arises. The direction of arrival of the Leonids is favourable for satellites in one respect. The dust grains will come from a direction almost at right angles to the direction of the Sun. Flat solar panels in their normal orientation, facing the Sun, present only a narrow edge as a target for the Leonids.

Controllers of ESA's Earth observation satellites ERS-1 and ERS-2 will switch off the instruments during the hazardous period to reduce the risk of electrically-induced damage. ESA's solar spacecraft SOHO, stationed 1.5 million kilometres out in space, is likely to experience an even stronger storm of Leonids than satellites in the Earth's vicinity. Measures to reduce the hazard may include rotating the spacecraft to screen vital equipment, and switching off scientific instruments.

The view from the ground in Europe

When the Leonids are at their peak, Leo will just be rising on Europe's eastern horizon. Nevertheless, observers in Europe watching out between midnight and dawn, on 17 and 18 November, may see unusual numbers of meteors. The best view will be from east Asia, where Leo will be high in the night sky at the time of the expected maximum. ESA has joined with other space agencies in sponsoring a Canadian expedition to Mongolia to observe the Leonids with video cameras equipped with image intensifiers. The same Canadian initiative will use radars in northern Australia to detect the meteors. Real-time information on the intensity and duration of the dust storm will help spacecraft operators to judge when the risk has passed.

Next year's appearance of the Leonids, in November 1999, will be best seen from Europe, and it could be bigger than this year's event. For the same reason, the risk posed by the Leonids to spacecraft will recur at that time. ESA scientists will be rehearsing this year for ground-based observations of the Leonids next year, from southern Spain.

Historical note on dust damage ESA has brutal experience of cosmic dust storms. In March 1986, its Giotto spacecraft flew deep into the dusty head of Halley's Comet, where it obtained amazing pictures of the nucleus. A dust particle no bigger than a grain of rice slammed into the spacecraft at 68 kilometres per second with the force of a hand grenade, and set it wobbling. A sand-blast of smaller grains, recorded as a continous drumbeat by dust detectors on Giotto, disabled the camera and caused other damage. Nevertheless the ESA operations team recovered control of the spacecraft and even managed to fly Giotto on an extended mission that took it to Comet Grigg-Skjellerup six years later.

Controllers were less lucky in August 1993 when a dust grain from Comet Swift-Tuttle, in the Perseid meteor stream, was probably to blame for knocking out ESA's Olympus telecommunications satellite after four years of operation. Although it remained intact, Olympus lost so much thruster fuel in trying to correct its attitude that it became unmanageable. More direct knowledge of dust impacts on spacecraft came from examining part of the original solar array of the Hubble Space Telecope, provided by ESA, which was returned to Earth in the first refurbishment mission in December 1993. The solar cells were pitted by many small dust impacts.

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