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(meteorobs) Excerpts from "CCNet DIGEST, 10 November 1998"


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From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST 10/11/98
Date: Tue, 10 Nov 1998 15:22:07 -0500 (EST)

CCNet DIGEST, 10 November 1998
------------------------------

(1) ESA EXPERTS ARE READY FOR LEONIDS
    Andrew Yee <ayee@nova.astro.utorontodot ca> 
...
(3) SCALING ANALYSIS OF METEORITE SHOWER MASS DISTRIBUTION
    L. Oddershede et al., TECHNICAL UNIVERSITY DENMARK

(4) THE COMETARY BOMBARDMENT ON THE PRIMITIVE ASTEROID BELT
    A. Brunini & R.G. Huton, ASTRONOMICAL OBSERVATORY LA PLATA

(5) ALBEDO MEASUREMENTS ON METEORITE PARTICLES
    J. Piironen et al., COMMISS EUROPEAN COMMUNITIES
...
======================
(1) ESA EXPERTS ARE READY FOR LEONIDS

>From Andrew Yee <ayee@nova.astro.utorontodot ca> 

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 nowledge 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
 
- ----------------------------------------------------------------------------
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

================
(3) SCALING ANALYSIS OF METEORITE SHOWER MASS DISTRIBUTION

L. Oddershede*), A. Meibom, J. Bohr: Scaling analysis of meteorite 
shower mass distributions. EUROPHYSICS LETTERS, 1998, Vol.43, No.5, 
pp.598-604

*) TECHNICAL UNIVERSITY DENMARK, DEPT PHYS 307, DK-2800 
   LYNGBY, DENMARK

Meteorite showers are the remains of extraterrestrial objects  which 
are captivated by the gravitational field of the Earth. We have 
analyzed the mass distribution of fragments from 16 meteorite 
showers for scaling. The distributions exhibit distinct scaling 
behavior over several orders of magnitude; the observed scaling 
exponents vary from shower to shower. Half of the analyzed showers 
show a single scaling region while the other half show multiple 
scaling regimes. Such an analysis can provide knowledge about the 
fragmentation process and about the original meteoroid. We also 
suggest to compare the observed scaling exponents to exponents 
observed in laboratory experiments and discuss the possibility that 
one can derive insight into the original shapes of the meteoroids.
Copyright 1998, Institute for Scientific Information Inc.

====================
(4) THE COMETARY BOMBARDMENT ON THE PRIMITIVE ASTEROID BELT

A. Brunini & R.G. Huton: The cometary bombardment on the primitive 
asteroid belt. PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.8, 
pp.997-1001

*) ASTRONOMICAL OBSERVATORY LA PLATA,PASEO BOSQUE S-N,RA-1900 LA 
   PLATA,ARGENTINA

In this paper, we estimate the number of catastrophic collisions 
between asteroids and comets scattered from the Uranus-Neptune zone 
during the process of planetary accretion. We found that the change 
in slope at approximate to 75 km in the size distribution for 
non-family asteroids could be produced by an intense bombardment of 
scattered comets in a short period of time after the accretion of 
Uranus and Neptune, provided that any asteroid with radius less than 
or equal to 130-150 km received a catastrophic collision by 
scattered comets. Larger asteroids have probably not received any 
catastrophic collision, due to the small number of large comets. We 
also estimate that the total amount of mass in the primitive 
asteroid belt, before the depletion produced by dynamical and 
collisional processes, was not less than approximate to 1800 times 
the present one. (C) 1998 Elsevier Science Ltd. All rights reserved.

==============
(5) ALBEDO MEASUREMENTS ON METEORITE PARTICLES

J. Piironen*), K. Muinonen, T. Nousiainen, C. Sasse, S. Roth, 
J.I. Peltoniemi: Albedo measurements on meteorite particles.
PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.8, pp.937-943

*) COMMISS EUROPEAN COMMUNITIES,JOINT RES CTR,SPACE APPLICAT 
   INST,ADV TECHNOL UNIT,I-21020 ISPRA,VA,ITALY

We have measured single-particle albedos for several chondrite and 
achondrite meteorites and for carbonaceous material. We have 
introduced a new, relative method for measuring the albedo, 
including a new method for evaluating the particle cross-section. We 
determined albedos of 0.50 +/- 0.25 for ordinary chondrite, 0.15 +/- 
0.02 for C2 carbonaceous chondrite, 0.64 +/- 0.13 for achondritic 
shergottite, 0.33 +/- 0.14 for enstatite chondrite meteorites, and 
0.21 +/- 0.03 for carbon particles. Making use of the measured 
single-particle albedos, we have predicted geometric albedos for 
atmosphereless solar system bodies covered with regoliths of such 
particles. The geometric albedos are consistent with the values 
derived for C- and S-type asteroids in earlier works. However, for 
enstatite chondrite, the geometric albedo is much lower than that 
accepted for E-type asteroids. The predicted geometric albedos are 
0.04-0.07 for C2 chondritic material and 0.13-0.24 for chondritic 
material. These ranges describe the usually accepted values for C- 
and S-type asteroids.. Our results put important constraints on the 
radiative transfer models for the regoliths of asteroids and comets. 
(C) 1998 Elsevier Science Ltd. All rights reserved.


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