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(meteorobs) Excerpts from "CCNet DIGEST, 03 Nov 98"


Note in particular the very LOOOONG article from Science News about
the Leonids below: The media barrage about this event has begun...
Here's hoping the cautions about unpredictability don't get lost in
the blizzard of coverage!

Lew

------- Forwarded Message

From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet DIGEST, 03/11/98
Date: Tue, 3 Nov 1998 13:43:08 -0500 (EST)


CCNet DIGEST, 3 November 1998
-----------------------------

[...]

(2) THE LEONIDS ARE COMING!
    Science News
    http://www.sciencenews.org/sn_arc98/10_31_98/bob1.htm

[...]

(4) THE DUST ENVIRONMENT OF COMET HALE-BOPP
    M. Fulle et al., ASTRONOMICAL OBSERVATORY TRIESTE

(5) HUBBLE SPACE TELESCOPE OBSERVATIONS OF COMETARY NUCLEUS & COMA
    P.L. Lamy et al., CNRS,MARSEILLE


--------------------------------------------------------------
(2) THE LEONIDS ARE COMING!

>From Science News
     http://www.sciencenews.org/sn_arc98/10_31_98/bob1.htm

October 31, 1998

A memorable light show or just a bracing shower?

By RON COWEN

The night of Nov. 12, 1833, might as well have been July 4. The skies =20
over the United States were ablaze with bursts of light surpassing the=20
most extravagant display of fireworks.=20

It was, one eyewitness said, as if "a tempest of falling stars broke =20
over the Earth . . . . The sky was scored in every direction with=20
shining trails and illuminated with majestic fireballs." Even some=20
souls asleep in their beds were awakened by the silent light show=20
streaking through their windows -- an unusually intense display of=20
shooting stars called the Leonid meteor storm.

Every year in mid-November, Earth encounters the Leonid storm, so=20
named  because it was thought to come from the constellation Leo. In=20
fact, this storm originates from a tenuous stream of dusty debris=20
expelled by Comet 55P/ Tempel-Tuttle during centuries of passes near=20
the sun. These particles, known as meteoroids, spread out along the=20
comet=92s orbit.

As the meteoroids from this comet or others burn up in Earth's =20
atmosphere, some 100 streaks of light an hour may grace the skies. =20
Such events, known as meteor showers, are common; our planet travels=20
through about 12 a year, each from a meteoroid stream spewed by a=20
different comet.

A more intense downpour, strong enough to qualify as a meteor storm,=20
is  rarer. A Leonid storm occurs roughly every 33 years, when the=20
planet passes through a dense trail of debris that lies close to the=20
comet (SN: 6/14/97, p. 371).=20

Chinese astronomers reported seeing the first storm in 902 A.D. The  
last one, in 1966, rivaled the spectacle of 1833 and was so intense=20
that viewers likened the profusion of shooting stars to snowflakes in=20
a snowstorm. This November 17, and perhaps the same time next year,=20
the planet will once again plunge into a dense concentration of the=20
Leonid stream.

Earth will pass three times farther from the comet than it did during =20
the 1966 blockbuster. It's not known whether this year's passage will=20
generate a mild storm, with thousands of shooting stars an hour, or=20
just an intensified shower, with the streaks of light appearing at=20
perhaps one-tenth to one-hundredth that rate.

Either way, East Asia is the prime location for witnessing the peak of=20
the encounter, predicted to occur about 2:20 p.m. eastern standard=20
time on Nov. 17 and to last at most a few hours. While it will be a=20
moonless night in East Asia, daylight will render even the most=20
intense fireworks invisible in North America. Sky watchers in the=20
United States, however, might still be treated to a better-than-average
spectacle in the wee hours of the morning on both Nov. 17 and Nov. 18,
says Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics
in Cambridge, Mass. (see sidebar).

There's a good chance the light show in 1999 will be at least as
lavish as the one this year, says Peter Brown of the University of
Western Ontario in London, Ontario. After that, thanks to Jupiter's
tug on Tempel-Tuttle, the comet won't get close to Earth for another
century.

That's one reason why Brown and his colleagues are setting up shop
next month in Australia and in the Gobi desert in Mongolia. By
bouncing radar beams off the meteoroids and using optical TV monitors
to track the streaks of light that the particles produce, the
researchers hope to gauge accurately the number of meteoroids striking
Earth's atmosphere and to test how well computer models have predicted
the intensity of the event.

A close encounter between Earth and Tempel-Tuttle happens to occur
just before or just after the icy comet passes closest to the sun's
warming rays, forcing the comet to vent fresh debris. Because it takes
time for the meteoroids to drift away from the comet, most of the
debris encountered by Earth is in fact composed of material spewed by
the comet during previous passes.

The models developed by Brown and his colleagues, in which they =20
simulate the release of debris from Tempel-Tuttle and track its path=20
over several hundred years, suggest that the material Earth will plow=20
through this year comes from dust expelled by the comet in 1899 and=20
1932. The team also finds that the 1999 event, best visible from=20
western Europe, might be about 50 percent stronger than the one this=20
November.

In contrast, Donald K. Yeomans of NASA=92s Jet Propulsion Laboratory in 
Pasadena, Calif., and his collaborators used a historical approach to=20
predict that the Leonids will be equally intense this year and next.=20

"We looked at all the showers from 902 A.D. through 1966 and asked,=20
When were major storms witnessed?" says Yeomans. Factoring in such=20
information as the proximity of the comet to Earth during a storm and =20
the time lag between the comet's closest approach to the sun and the
occurrence of a downpour, the team predicts that the events of 1998 and
1999 may be the most spectacular showers since 1966.

The historical record suggests that the passages this year and next=20
have characteristics in common with two previous sets of encounters --=20
those in 1866 and 1867, in which observers saw a maximum of 5,000=20
shooting stars, or meteors, an hour, and those of 1931 and 1932, in=20
which viewers counted at most about 200 meteors an hour. Yeomans thus=20
estimates that observers this year will see somewhere between 200 and=20
5,000 meteors an hour.

That's an admittedly wide range, he notes. "Nobody can predict these
things well, so it's a bit of a crapshoot," says Yeomans. "There's
nothing precise about predicting meteor storms."

Such uncertainties are proving particularly frustrating for technology
companies, the U.S. military, NASA, and others who own or operate any
of the 650 or so satellites in space. Researchers estimate that each
craft's chance of getting hit by a Leonid meteoroid is only about 0.1
percent.

The debris is mostly tiny, and scientists don't think any particles=20
will punch holes in satellites. They are worried, however, about the=20
electrical damage that may be wrought by these high-speed meteoroids.=20
Traveling at 72 kilometers per second relative to Earth, more than 200=20
times the speed of a 22-caliber bullet, Leonid particles have the=20
highest speeds of any group of meteoroids. That=92s because Earth and=20
the debris plow headlong into each other.

When a meteoroid no bigger than a grain of dust slams into a satellite=20
at such speeds, the kinetic energy it delivers can generate a cloud of=20
highly charged gas, or plasma. Depending on where the meteoroid hits=20
and the construction of the satellite, the plasma may generate an=20
electromagnetic pulse that could short-circuit or destroy delicate=20
electronic parts.

"It really comes down to our lack of understanding about what happens
when something the width of a human hair hits a satellite 30,000 km
from Earth at 72 km/second," says Brown. "The simple answer is that we
have only a vague idea. You can't accelerate particles [this large]
on Earth to that velocity."

The uncertainty breeds both apprehension and debate. The upcoming
Leonids "will represent the largest meteoroid threat to spacecraft in
history," claims David K. Lynch of the Aerospace Corp. in Los Angeles.
That assertion is "just nonsense," says Yeomans.. "That's just Chicken
Little stuff."

Some scientists are quick to point out that during the 1966 Leonid
storm, not a single satellite was damaged. However, there are more
than 10 times as many craft in space today as in 1966. In 1993, a
European Space Agency satellite spun out of control due to an
electrical disturbance generated by the impact of a particle from the
Perseid meteor shower (SN: 10/2/93, p. 217).

This year "is the the first time . . . since we have had a large number
of satellites that we've had a major, predicted meteor storm, and
this is likely the largest one since 1966," says S. Pete Worden of the
U.S. Air Force in Arlington, Va.

"If you're the owner of a spacecraft that's worth $500 million up
there, even if the probability of being hit is less than 0.1 percent,
which it is, you may want to [take precautions]," Yeomans notes. To
that end, NASA will not launch the space shuttle in mid-November, and
the Hubble Space Telescope will point its mirror away from the Leonid
meteoroids. Other satellites will be aligned edge on to the storm so
that the smallest possible surface area will be exposed to the debris.

In a few cases, says Worden, the military may shut power to components
of satellites, such as antennae, that may be particularly susceptible
to an electrical discharge. Some satellites might be switched off
entirely during the peak of the Leonid activity.

The craft that are at the greatest risk are those that reside in
extremely distant orbits, where Earth's tug balances that of the sun.
In such an orbit, about 1 million km closer to the sun than Earth is,
a craft is far more likely to plow into a concentrated part of the
Leonid trail. Ironically, these devices include the Solar and
Heliospheric Observatory, which just recovered from an unrelated
episode in which it lost power and spun out of control in June.

The only people in space, the cosmonauts in the Mir space station, are
planning to ride out the height of the storm in Mir's escape vehicle,
the Soyuz capsule. In an emergency, the crew could fire up Soyuz to
return to Earth.

Looking on the bright side, says Worden, "this is a really unique=20
opportunity to test out all the satellites" a few years in advance of=20
the solar maximum, a time when outbursts from the sun can hurl clouds=20
of high-speed, charged particles toward Earth and generate global=20
electrical storms. Astronomers predict that a solar maximum will occur=20
in 2000.

As the Leonid shower proceeds, observations by several teams,
including Brown's, will guide government agencies and businesses
concerned with the health of satellites. "The idea is to give an early
indicator if it [turns out to be] a bad storm," says Brown. "This is
the first real-time warning system" in place for a meteor storm.

Radar observations by Brown and his collaborators may test an=20
intriguing, and somewhat disturbing, hypothesis that suggests the=20
coming Leonids could pose a greater threat to orbiting satellites =20
than expected. In the October Astronomy and Geophysics, Duncan Steel=20
of Spaceguard Australia in Adelaide proposes that the number of Leonid=20
meteoroids in 1998 or 1999 could be 10 times greater than other=20
estimates.

Steel suggests that many of the freshest Leonid meteoroids are=20
composed entirely of organic, tar-like compounds -- key constituents=20
of comets. In contrast to compounds rich in silicates or iron, organic=20
compounds burn at relatively low temperatures high in Earth's=20
atmosphere and are therefore not detected.

Some of the radar systems that Brown uses to detect meteoroids do have=20
frequencies low enough to begin to detect Steel's proposed population,
Brown says. "I think, in broad terms, what Steel says is probably
true." He notes, however, that Steel's hypothesis holds true only if
the composition of meteoroids is 100 percent organic. Even trace
amounts of silicates would cause a meteoroid to vaporize lower down in
the atmosphere, where it could easily be detected by standard equipment.

"My own feeling is that it's not nearly as severe a problem as he's
presenting, but it underscores how much we don't understand," Brown
concludes. Come mid-November, sky watchers will have a chance to find
out how stormy the Leonids will be.

Looking at the Leonids

Although astronomers aren't at all sure that Earth's passage through
the Leonid meteor stream this November will prove a blockbuster, it's
a good bet that the annual light show will be more intense than usual
-- even in North America, where viewers won't see the peak of activity
that sky watchers in East Asia will witness.

Instead of staying up late, it's probably wiser to wake up early on
Nov. 17 and 18. That's because the radiant -- the point in the sky
from which the Leonids appear to originate -- doesn't rise until about
12:30 a.m. local time and the show will be best at about 3:00 a.m.,
when the radiant is about 30 degrees above the horizon. The radiant
lies within the constellation Leo's western portion, a group of stars
referred to as the sickle or backwards question mark.

The Leonid meteors are visible to the naked eye, and binoculars would
only cut down the field of view, notes Brian G. Marsden of the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Lie
down on a reclining chair, pointing your feet toward the east. For the
best view, don't gaze directly at the radiant; instead, look 30 or 40
degrees above or west of that point. The meteors should be visible,
weather permitting, through sunrise. Dress warmly and remember, if the
1998 event proves a dud, you'll have a second chance in 1999.

--  R.C.

>From Science News, Vol. 154, No. 18, October 31, 1998, p. 280.
Copyright 1998 by Science Service.

References:

Steel, D. 1998. The Leonid meteors: Compositions and consequences.
Astronomy & Geophysics (October).

Yeomans, D.K., K.K. Yau, and P.R. Weissman. 1996. The impending=20
appearance of Comet Tempel-Tuttle and the Leonid meteors. Icarus=20
124:407.

Further Readings:
Cowen, R. 1997. Leonids: The coming storm. Science News 151
(June 14):371.

Sources:

Peter Brown
University of Western Ontario
Department of Astronomy
London, Ontario N6A 3K7
Canada

Brian G. Marsden
Harvard-Smithsonian Center for Astrophysics
Smithsonian Astrophysical Observatory
60 Garden Street
Cambridge, MA 02138

Duncan Steel
Spaceguard Australia P/L
P.O. Box 3303
Rundle Mall
Adelaide SA 5000
Australia

Donald K. Yeomans
NASA Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91125

--------------------------------------------------------------
(4) THE DUST ENVIRONMENT OF COMET HALE-BOPP

M. Fulle*), G. Cremonese, C. Bohm: The preperihelion dust=20
environment of C/1995 O1 Hale-Bopp from 13 to 4 AU. ASTRONOMICAL=20
JOURNAL, 1998, Vol.116, No.3, pp.1470-1477

*) ASTRONOMICAL OBSERVATORY TRIESTE,VIA TIEPOLO 11,I-34131=20
   TRIESTE, ITALY

Two UK Schmidt plates of comet Hale-Bopp dust tail taken in=20
1996 May are analyzed by means of the inverse dust tail model.=20
The dust tail fits are the only available tools providing=20
estimates of the ejection velocity, the dust-loss rate, and the
size distribution of the dust grains ejected during years=20
preceding the comet discovery. These quantities describe the=20
comet dust environment driven by CO sublimation between 1993=20
and 1996, when the comet approached the Sun from 13 to 4 AU.=20
The outputs of the model are consistent with the available coma
photometry, quantified by the Af rho quantity. The dust mass=20
loss rate increases from 500 to 8000 kg s(-1), these values=20
being inversely proportional to the dust albedo, assumed here=20
to be 10%. Therefore, the mass ratio between icy grains and CO=20
results is at least 5. Higher values of the dust-to-gas ratio=20
are probable, because the model infers the dust-loss rate over=20
a limited size range, up to 1 mm sized grains, and because the=20
power-law index of the differential size distribution ranges=20
between -3.5 and -4.0, so that most of the dust mass was=20
ejected in the largest boulders that Hale-Bopp was able to=20
eject. The dust ejection velocity close to the observations,=20
between 7 and 4 AU, was close to 100 m s(-1) for grains 10 mu m
in size, much higher than that predicted by R. F. Probstein's=20
theory, thus confirming previous results of Neck-Line=20
photometry. This result is an indicator of CO superheating with
respect to a free sublimating CO ice, in agreement with the=20
high observed CO velocity. The fundamental result of the paper=20
is that such a high dust velocity remained constant between 13=20
and 4 AU, thus providing a strong constraint to all models of=20
the GO-driven activity of the comet during its approach to the=20
Sun: CO superheating must have been active since 13 AU from the
Sun. It might be provided by the abundant dust itself, or by=20
seasonal effects heating the subsurface layers, as was=20
suggested for comet 29P/Schwassmann-Wachmann 1. Another=20
similarity between the two comets is provided by the power-law=20
index of the time-averaged size distributions: -3.6 +/- 0.1 for
C/1995O1 and -3.3 +/- 0.3 for 29P/SW1. However, other=20
characteristics of the dust environments are very different, so
that, in general, it is impossible to distinguish a CO-driven=20
comet from a typical water-driven one. Copyright 1998, Institute for=20
Scientific Information Inc.

--------------------------------------------------------------
(5) HUBBLE SPACE TELESCOPE OBSERVATIONS OF COMETARY NUCLEUS & COMA

P.L. Lamy*), I. Toth, H.A. Weaver: Hubble Space Telescope=20
observations of the nucleus and inner coma of comet 19P 1904 Y2=20
(Borrelly). ASTRONOMY AND ASTROPHYSICS, 1998, Vol.337, No.3,=20
pp.945-954

*) CNRS,ASTRON SPATIALE LAB,BP 8,F-13376 MARSEILLE 12,FRANCE

The nucleus of comet 19P/Borrelly was detected using the=20
Planetary Camera (WFPC2) of the Hubble Space Telescope (HST).=20
During the time of our observations, the comet was 0.62 AU from
the Earth, 1.40 AU from the Sun, and had a solar phase angle of
38 degrees. The high spatial resolution of the HST images=20
allowed us to discriminate clearly between the signal from the=20
nucleus and that from the coma. The lightcurve of the nucleus=20
indicates that it is a highly elongated body rotating with a=20
synodic period of 25.0 +/- 0.5 hr. Assuming that the nucleus=20
has a geometric albedo of 4% and is a prolate spheroid with a=20
rotational axis pointing in the direction determined by=20
Sekanina (1979), we derive that its semi-axes are 4.4 +/- 0.3=20
km and 1.8 +/- 0.15 km. The corresponding fractional active=20
area of similar to 8% suggests a moderately active comet. The=20
highly anisotropic coma is dominated by a strong sunward fan,=20
and the dust production rate exhibited signs of temporal=20
variability throughout our observations. Copyright 1998, Institute=20
for Scientific Information Inc.

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