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(meteorobs) ESA's Experts Are Ready For A Storm Of Comet Dust
ESA Science News
http://sci.esa.int
6 Nov 1998
ESA's experts are ready for a storm of comet dust
When the Earth crosses the wake of Comet Tempel-Tuttle on 17 November,
European scientists will use the NASA-ESA Hubble Space Telescope to detect
impacts of cosmic dust. ESA's European Space Operations Centre (ESOC) has
sent out a warning to spacecraft operators about risks to all satellites in
the Earth's vicinity on that day, from the Leonid meteor storm. Countless
dust grains thrown out by the comet will slam into the Earth's atmosphere
at 71 kilometres per second. The Leonids, as they are called, may produce
the most spectacular shower of meteors, or "shooting stars", seen since
1966.
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
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.
The Leonids on the Internet
For the ESA/ESOC report "The Leonid 1998 Meteor Shower: information for
spacecraft operators":
http://www.estec.esadot nl/spdwww/leonids/
For a brief video clip of telescope images of meteor trails, see:
http://www.so.estec.esadot nl/planetary/meteors/animation/
For general information see the Leiden Leonid site (mirroring a NASA site)
and the home page of the International Meteor Organization, both of which
have leads to abundant material:
http://strw.leidenunivdot nl/~leonid/
http://www.imodot net
For updates on the Leonid event as it affects ESA science activities, watch
the ESA science site:
http://sci.esa.int
For further information please contact:
ESA Public Relations Division
Tel: +33(0)1.53.69.7155 Fax: +33(0)1.53.69.7690
At ESA's Satellite Operations Centre (ESOC), Darmstadt (Germany), you may
contact:
Walter Flury -- Tel: +49(0)6151.2270
The following specialists at ESA's Technical and Research Centre (ESTEC)
can give more detailed information in various languages:
Dutch: Hakan Svedhem -- Tel: +31(0)71.565.3370
English: Trevor Sanderson -- Tel: +31(0)71.565.3577
John Zarnecki -- Tel: +31(0)71.565.3423
French: Jean-Pierre Lebreton -- Tel: +31(0)71.565.3600
Bernard Foing -- Tel: +31(0)71.565.5647
German: Detlef Koschny -- Tel: +31(0)71.565.4828
Gerhard Schwehm -- Tel +31(0)71.565.3539
Italian: Francesca Ferri -- Tel: +31(0)71.565.5634
Spanish: Luisa Lara Lopez -- Tel: +31((0)71.565.4893
Swedish: Hakan Svedhem -- Tel: +31(0)71.565.3370
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USEFUL LINKS FOR THIS STORY
ESOC report on Meteor showers
http://www.estec.esadot nl/spdwww/leonids/
Meteor trail animation
http://www.so.estec.esadot nl/planetary/meteors/animation/
Leiden univ. Leonids site
http://strw.leidenunivdot nl/~leonid/
International meteor organisation
http://www.imodot net
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