PLEASE NOTE:
*
CCNet DIGEST, 10 November 1998
------------------------------
(1) ESA EXPERTS ARE READY FOR LEONIDS
Andrew Yee <ayee@nova.astro.utoronto.ca>
(2) INFLUENCE OF COSMIC RAYS ON EARTH'S CLIMATE
Rolf Sinclair <rsinclai@nsf.gov>
(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
(6) VELOCITY-MASS RELATION IN CATASTROPHIC IMPACTS
I. Giblin, BEACHWOOD
(7) DYNAMICAL BEHAVIOUR OF NEAR EARTH ASTEROIDS
P. Michel, COTE AZUR OBSERVATORY
(8) ARE THERE CORONAE ABOUT ASTEROIDS?
T.H. Morgan*) & R.M. Killen, NASA
(9) THE YARKOVSKY EFFECT ON ASTEROIDAL FRAGMENTS
D. Vokrouhlicky*) & P. Farinella, CHARLES
UNIVERSITY
(10) DYNAMICS OF EROS
P. Michel et al., ASTRONOMICAL
OBSERVATORY TORINO
======================
(1) ESA EXPERTS ARE READY FOR LEONIDS
From Andrew Yee <ayee@nova.astro.utoronto.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.esa.nl/spdwww/leonids/
For a brief video clip of telescope images of meteor trails, see:
http://www.so.estec.esa.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.leidenuniv.nl/~leonid/
http://www.imo.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.esa.nl/spdwww/leonids/
Meteor trail animation
http://www.so.estec.esa.nl/planetary/meteors/animation/
Leiden univ. Leonids site
http://strw.leidenuniv.nl/~leonid/
International meteor organisation
http://www.imo.net
================
(2) INFLUENCE OF COSMIC RAYS ON EARTH'S CLIMATE
From Rolf Sinclair <rsinclai@nsf.gov>
Date: Mon, 9 Nov 1998 14:30:13 -0500 (EST)
From: physnews@aip.org (AIP
listserver)
To: physnews-mailing@aip.org
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 401 November 9, 1998 by Phillip F. Schewe and
Ben
Stein
INFLUENCE OF COSMIC RAYS ON EARTH'S CLIMATE. Do small changes in
solar activity translate into climate change on our planet?
One
possible linkage is the sun's influence over the local flux
of
galactic cosmic rays (GCR); as the solar magnetic field gets
stronger, fewer cosmic rays are able to penetrate to the inner
solar
system and Earth. And because the GCR are the biggest
ionizer of
air molecules in the lower atmosphere, they might play a role in
processes like cloud formation. Henrik Svensmark, a
physicist now
at the Danish Meteorological Institute (011-45-3-536-2475,
hsv@dsri.dk), has studied the
connection between GCR flux, solar
activity, and climate on Earth. He finds that during the
past
11-year solar cycle, Earth's cloud cover was more closely
correlated
with the GCR flux than with other solar activity parameters, such
as
solar radiance, the main energy emitted by the sun.
Svensmark
concludes that climate seems to be influenced by solar activity
via
the GCR-cloud connection. In other words, climate is partly
affected by processes in deep space. (Physical Review
Letters, 23
November 1998; see figure at www.aip.org/physnews/graphics.)
================
(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.
================
(6) VELOCITY-MASS RELATION IN CATASTROPHIC IMPACTS
I. Giblin*): New data on the velocity-mass relation in
catastrophic
disruption. PLANETARY AND SPACE SCIENCE, 1998, Vol.46, No.8,
pp.921-928
*) BEACHWOOD,FILMER LANEOTI 2,SEVENOAKS TN14 5AG,ENGLAND
The relationship between fragment velocity and mass following a
disruptive impact is of great,importance when modelling
populations
of small bodies such as the asteroid main belt or the more
recently,observed Edgeworth-Kuiper belt where mutual collisions
play
an important role in their dynamic evolution. The velocity-mass
relation following these mutual collisions strongly affects not
only
the collisional lifetime of large (gravity dominated) asteroids,
but
also the rate at which mass is ejected from the belt into
resonances, providing a source of resupply for the Earth-crossing
asteroid population and, in the case of the Edgeworth-Kuiper
Belt,
the short period comets. Although considerable work has been done
on
the subject of the relationship between velocity and mass of
fragments from cratering and catastrophic disruption events, it
has
recently become apparent that there may not be a valid general
relationship between these quantities. In this paper I present a
summary of size-velocity data obtained from single- and
twin-camera
films of hypervelocity, highly catastrophic impacts into
spherical
21 cm targets of artificial rock with strength and density
similar
to basalt. The 2D velocities of at least 951 fragments larger
than
approximately 10 mm have so far been measured in 8 similar
experiments. Of these, 69 have been studied in 3D in the recent
experiments using two cameras at 60 degrees. The data collected
here
suggest that in general there is only a weak correlation between
mass and velocity, and that the best-fitting exponent varies
between
0 and -1/6 with an average value of approximately - 1/13. (C)
1998
Elsevier Science Ltd. All rights reserved.
===============
(7) DYNAMICAL BEHAVIOUR OF NEAR EARTH ASTEROIDS
P. Michel: Dynamical behaviour of Near-Earth asteroids in the
terrestrial planet region: the role of secular resonances.
PLANETARY
AND SPACE SCIENCE, 1998, Vol.46, No.8, pp.905-910
COTE AZUR OBSERVATORY, BP 4229,F-06304 NICE 4,FRANCE
In this paper, we present the last results on the dynamics of
Near-Earth asteroids with orbital semimajor axes smaller than 2
AU
and we concentrate,particularly on the dynamics of the secular
resonances with planets. Many numerical integrations of NEA
orbits
show that the secular resonances are present in this region and
that
their effects can play a major role in the orbital evolutions and
dynamical lifetimes of small bodies. They can either provide a
protection mechanism from close approaches, or a transport
mechanism
from a region of the phase space to another. Moreover the recent
location of all the linear secular resonances, by means of a
semi-analytical method, confirms that all of them are present in
this region and Some can even overlap. This shows that the region
of
the inner planets is dynamically very complex and therefore, that
a
dynamical model considering only the effects of close encounters
with planets to study the evolution of NEAs may be oversimplified
and should take into account the secular perturbations and
resonances which appear to be numerous and relevant in the region
where NEAs evolve. (C) 1998 Published by Elsevier Science Ltd.
All
rights reserved.
=============
(8) ARE THERE CORONAE ABOUT ASTEROIDS?
T.H. Morgan*) & R.M. Killen: Production mechanisms for faint
but
possibly detectable coronae about asteroids. PLANETARY AND SPACE
SCIENCE, 1998, Vol.46, No.8, pp.843-850
*) NASA,RES PROGRAM MANAGEMENT DIV,CODE SR,300E ST
SW,WASHINGTON,DC,20546
Asteroidal surfaces are exposed to the same processes which
create
observable exospheres about such refractory, volatile-free
surfaces
as those of the Moan and Mercury. Are there modest coronae about
asteroids? We have chosen to examine this problem for two
species,
sodium and water vapor. Sodium was chosen because NaD emission is
easily observed from the Earth's surface and emissions are
observed
in the exospheres of both the Moon and Mercury. In addition, the
spatial distribution of emission in the sodium D lines about an
asteroid would be a direct measure of the velocity distribution
of
the sodium source, which cannot be measured directly in the case
of
Mercury or the Moon. This provides a new technique for assessing
the
possible water content of asteroids. There is now very good
evidence
that water is present in hydrates and other aqueous alteration
products. A detection of an hydroxyl corona about one of these
asteroids would be proof of the presence of these aqueous
bi-products; the amount of the water production would a direct
measure of the water content of the asteroidal regolith.
Water-rich
asteroids are of course a potential resource and hold important
clues to the evolution of the planets. We have calculated the
production of sodium and water (hydroxyl) coronae about asteroids
due to just two production mechanisms: sputtering and impact
vaporization. We find that for realistic assumptions an asteroid
with perihelion near 2 AU would have a small (about 3R) OH
corona,
while the likely sodium coronae would be about half as bright.
These
emissions will be extremely difficult to detect from the ground,
but
they may be detectable from space. (C) 1998 Elsevier Science Ltd.
All rights reserved.
=====================
(9) THE YARKOVSKY EFFECT ON ASTEROIDAL FRAGMENTS
D. Vokrouhlicky*) & P. Farinella: The Yarkovsky seasonal
effect on
asteroidal fragments: A nonlinearized theory for the
plane-parallel
case. ASTRONOMICAL JOURNAL, 1998, Vol.116, No.4, pp.2032-2041
*) CHARLES UNIVERSITY*), ASTRON INST, V HOLESOVICKACH 2,CZ-18000
PRAGUE, CZECH REPUBLIC
The ''seasonal'' Yarkovsky force is due to radiation pressure
recoil, which acts on anisotropically emitting rotating bodies,
heated by sunlight to different temperatures at different
latitudes
on their surfaces. This force gives rise to a significant
draglike
effect on rapidly spinning asteroid fragments approximate to
1-100 m
in size. Here we present a new treatment of this effect, based on
the numerical solution of the heat transfer equation with no
linearization in the ratio between the peak temperature
difference
and the average temperature on the body's surface. Our treatment
is
restricted to the large-body (plane-parallel) case, valid for
radii
larger than the penetration depth of the seasonal thermal wave
(approximate to 1-20 m depending on the conductivity of the
surface
layer). Also, we solve numerically the Gaussian perturbation
equations for the evolution of the orbital eccentricity, as well
of
the semimajor axis under the seasonal Yarkovsky force. We find
the
results to be in broad agreement with the linearized model of D.
P.
Rubincam, with two main discrepancies: (i) for the same thermal
and
optical parameters and near-circular orbits, the semimajor axis
decay rate predicted by the improved, nonlinearized theory is
some
15% lower, and (ii) for some directions of the spin axis relative
to
the perihelion direction, the Yarkovsky force can cause a secular
growth of the eccentricity. When gravitationally induced
perihelion
precession, spin axis precession, and collisional reorientations
are
accounted for, however, the eccentricity on average is found to
decrease. We also show that the theory can be easily generalized
to
bodies of spheroidal shapes, with typical discrepancies of a
factor
of 2 in the semimajor axis decay rate with respect to the
spherical
case. Copyright 1998, Institute for Scientific Information Inc.
===============
(10) DYNAMICS OF EROS
P. Michel*), P. Farinella, C.. Froeschle: Dynamics of Eros.
ASTRONOMICAL JOURNAL, 1998, Vol.116, No.4, pp.2023-2031
*) ASTRONOMICAL OBSERVATORY TORINO,STRADA OSSERVATORIO 20,I-10025
PINO TORINESE,ITALY
We have investigated the dynamical evolution of asteroid (433)
Eros,
soon to be explored by the Near-Earth Asteroid Rendezvous (NEAR)
probe, by performing 16 numerical integrations of ''dynamical
clones'' of Eros's chaotic orbit over a timespan of 5 Myr. By
analyzing the results of these integrations we have found the
following: (1) In six cases a clone becomes an Earth crosser,
typically because of eccentricity increases caused by the nu(3)
and
nu(4) secular resonances; two clones become Venus crossers, and
one
eventually collides with the Sun. (2) Some of the Earth-crossing
clones go back to the Mars-crossing state after some time, and
several have their inclination affected by the nu(13) and nu(14)
nodal resonances. (3) Nine clones have a slow evolution dominated
by
Mars encounters, and one of them is temporarily trapped into the
25:24 mean motion resonance with Mars, providing effective
protection from close encounters over more than I Myr. (4) From
the
number of planetary encounters recorded during our integrations,
Eros's lifetime versus a collision with Earth and Mars can be
estimated to be about 1.84 and 2.9 Gyr, respectively. (5) On the
other hand, it is impossible to estimate even as an order of
magnitude the past or future mean impact rate onto Eros's
surface.
These findings have the following implications: Eros's dynamical
lifetime is probably of the order of 50-100 Myr, and it has
approximate to 5% probability of eventually hitting Earth. Its
shape
may have been affected by tidal forces during past Earth
encounters.
Its birth location in the main belt cannot be traced back with
certainty, but if Eros comes from a family-forming catastrophic
breakup near one of the main resonances, this must have been one
of
the last such events to occur in the main belt. More likely,
Eros's
orbit became Mars crossing by slowly diffusing from the
high-eccentricity portion of the main belt; then, Mars and Earth
encounters led it to its current state. Unfortunately, the
forthcoming observations of Eros's cratering record by the NEAR
probe will not be useful to constrain its age or collisional
lifetime. Copyright 1998, Institute for Scientific Information
Inc.
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