CCNet, 16 November 1999 - LEONIDS SPECIAL III


     "The scene was truly awful and indescribably sublime; ... it
     carried to the bosoms of many terror and consternation. Some
     imagined the world was coming to an end and began to pray;
     and a gentleman from the counrty states that such was the
     alarm produced in the neighborhood where he was that the
     welkin every where around him resounded with cries of
      - Reporting the 1833 Leonids, Raleigh Star, 15 November 1833

     "Such scientific matters are of little interest to the
     increasing numbers of apocalyptic Christian groups gathered
     in Israel and the United States. They have been moved to
     paroxysms of excitement by recent earthquakes in Turkey and
     Greece, which they believe are portents of a coming
     Armageddon. Their doomsday predictions, which involve mass
     destruction and the "rapture", when believers ascend to
     heaven, will be further encouraged by the meteor storm,
     which, like the star that led the Three Wise Men to the
     birthplace of Christ, will rise in the east over Jerusalem."
            -- The Times, 16 November 1999

     Andrew Yee <>

     The Times, 16 November 1999

     Andrew Yee <>

     Andrew Yee <>

     Andrew Yee <>

      John Bluck <>

     Andrew Yee <>

     SpaceViews, 16 November 1999


From Andrew Yee <>

European Space Agency
Press Information Note Nr. 17-99
Paris, France 15 November 1999


Will the night sky be illuminated with thousands of 'shooting stars' --
known to scientists as the Leonid meteors -- on the night of 17-18
November? No one knows for sure, but some experts are predicting a
dramatic display over Western Europe which will rival any millennium

What are the Leonids?

Each year, the night sky is illuminated by dozens of meteor showers.
During these showers, pieces of cosmic debris -- mostly coming from
giant dirty snowballs called comets -- leave glowing trails as they are
incinerated during entry to the Earth's upper atmosphere. We see them
as short-lived trails of light streaking across the sky.

One of the most famous meteor showers is known as the Leonids,
so-called because their light trails all seem to originate from the
constellation of Leo. The Leonid meteors are associated with dust
particles ejected from Comet P/55 Tempel-Tuttle, which pays periodic
visits to the inner Solar System once every 33.25 years.

Actually, the Leonids appear every year between November 15-20, when
the Earth passes very close to the comet's orbit. However, the numbers
on view vary tremendously. In most years, observers may see a peak of
perhaps 5-10 meteors per hour around 17 November.

But, roughly every 33 years, the Leonids generate a magnificent storm,
when thousands of them illuminate the night sky: they are renowned for
producing bright fireballs which outshine every star and planet. Their
long trails are often tinged with blue and green, while their vapour
trains may linger in the sky like enormous smoke rings for 5 minutes or

Although the incoming particles are small, ranging from specks of dust
to the size of small pebbles, the Leonids glow brightly because they
are the fastest of all the meteors. A typical Leonid meteor, arriving
at a speed of 71 km/s (more than 200 times faster than a rifle bullet),
will start to glow at an altitude of about 155 km and leave a long
trail before it is extinguished.

The Unpredictable Leonids

Based on past behaviour, a meteor storm was predicted for 1998 or 1999.
Last year, some very bright fireballs appeared unexpectedly 18 hours
before the predicted maximum. They were associated with a previously
unknown dust band which had been shepherded into a narrow stream by
Jupiter's gravity. Unfortunately, although there was also a peak in
meteor activity at the predicted time, their trails were not very
bright and hard to see with the naked eye.

"In hindsight people found out that actually in 1965 we saw a similar
display ... these fireballs the night before and then not quite the
activity that was expected at the real maximum and you might know that
in 1966, that was when the real storm occurred" said Detlef Koschny,
scientific coordinator of the ESA's Leonids observation campaign. "So
that was one year after. So some scientists say, 'Ok, for this year we
really expect a big storm or we hope; you can never be sure."

So what about this year? In 1999, although the Earth will reach Tempel-
Tuttle's orbit 622 days after the comet passed by, the distribution of
its dust ribbons means that a notable display is still possible. One
encouraging sign is that the 1998 shower was similar to that of 1965,
the year before the storm of 1966. Most astronomers are not expecting a
comparable display in 1999, but a spectacular show cannot be ruled out.

Activity will probably reach a peak on the night of 17-18 November,
though earlier fireballs are always a possibility. Nothing will be
visible until the 'sickle' of Leo rises above the eastern horizon
around 22.30 GMT. At first, the fainter meteors will be swamped by
light from the first quarter Moon, but once this sets soon after
midnight, conditions should be ideal as long as the sky is cloud free.

The maximum activity should occur around 02.00 GMT, at the time when
the Earth passes closest to the comet's orbit. At this time, Leo will
be well above the horizon over Western Europe.

Some scientists predict that 2000 or 2001 may provide even better
viewing opportunities for the Leonids, but no one can be sure if these
unpredictable cosmic travellers will live up to expectations.

"We just know from past history that, in the two years after the
perihelion of Comet Tempel-Tuttle, there is enhanced activity," said
Dr. Walter Flury of the European Space Operations Centre (ESOC).

"A storm is possible, but these things are very uncertain," he added.
"Predictions are based on models of the way material is distributed
along the comet's orbit. But the models are quite inaccurate. We just
don't have enough information."

There are two main reasons why scientists study meteors: the clues they
hold about the formation of the planets and the potential threat they
pose to Earth-orbiting satellites:

1. ESA Scientists Seek to Study the Leonids

Since the Leonids ejected from a comet and since comets are thought to
be left-overs from the formation of the planets, studying the Leonids
helps scientists to learn more about the physical and chemical
characteristics of their 4.5 billion year-old parents.

If the storm does materialise, scientists from ESA's Space Science
Department intend to be ready. Armed with a variety of equipment,
including image-intensifier video cameras, CCD cameras with wide-angle
lenses and a spectrograph, they are planning an observational campaign
at two observatories in southern Spain (Calar Alto and Sierra Nevada)
from 11 to 19 November.

The main science goals are:

* To determine the varying rates in the number of meteors and their
  magnitudes (visual brightness).

* To study the physical properties of individual meteors by measuring their
  light output and changing velocity, then compare these to other meteor

* To use the 1.5 m telescope at the Sierra Nevada Observatory to perform
  spectroscopy of persistent trains and so determine their composition.

There will also be an ESA scientist with a meteor camera on board an
aircraft operated by the American SETI (Search for Extraterrestrial
Intelligence) Institute. Results from the meteor count experiments will be
sent to ESOC in Germany so that spacecraft operators can determine the
level of threat posed by the space dust.

2. Hazards to spacecraft: how ESA is preparing

Although they are very small, the tremendous speed of the Leonids means
they pack a mighty punch. Apart from knocking a spacecraft off alignment
or causing physical damage in the form of an impact crater, such collisions
can also generate a cloud of plasma (gas composed of neutral and
electrically charged particles) which may cause electrostatic discharges
or damage a spacecraft's sensitive electronics.

This threat is not simply theoretical. In 1993, an ESA's satellite called
Olympus spun out of control due to an electrical disturbance caused by the
impact of a particle from the Perseid meteor shower.

The situation is further complicated by the fact that there are
currently more satellites in orbit around the Earth than ever before,
all of which pose a tempting target for one of nature's miniature
missiles. Despite this spacecraft population explosion, few, if any,
satellites are likely suffer significant problems from meteors, even
during a storm. Researchers estimate that the chance of one getting hit
by a Leonid meteor is only about 0..1 percent.

This low hit rate was born out by an absence of damage during the 1998
Leonids event. Nevertheless, driven by uncertainty over the future of
their high-tech hardware, satellite operators will once again be taking
precautions to protect their multi-million Euros charges this November.

One of the largest targets, the NASA-ESA Hubble Space Telescope will
be manoeuvred so that its mirrors face away from the incoming meteors
and its solar arrays are aligned edge on to them. These precautions
will continue for several Earth orbits, a duration of seven hours,
during the Leonids' predicted peak.

Apart from reducing the exposed area of giant solar arrays, operators
may shut off power to vulnerable electrical components of satellites,
or switch them off entirely during the peak of the Leonid activity.
Even spacecraft located some distance from the Earth may be at risk.
ESA's Solar and Heliospheric Observatory (SOHO) studies the Sun from a
vantage point 1.5 million kilometres away, but it, too, will roll so
that its main navigational aid, the star tracker, is pointing out of
harm's way.

"There could be a lot of activity, but we just don't know for sure,"
commented Walter Flury. "It's better to take precautions now than be
sorry later."

ESOC issued a warning to all satellite operators in August, explaining key
protective measures to be taken such as switching off the payload and
modifying the angle of the solar panels in relation to the Leonids

By November 17, 1999 ESOC will have secured both earth observation
satellites ERS-1 and ERS-2 by switching off the payload. The risk does
not only lie in the damage caused by direct impact but rather in plasma
discharge that could disrupt the functioning of the on-board
electronics. Teams of operations experts will be on shift throughout
the night to assess any potential damage and take necessary action.

The press is welcome to attend the press briefing that will take place on
Thursday 18th November at 8:00 am at ESOC after a night's watch. Please
contact Jocelyne Landeau-Constantin, Public Relations Office,
Tel +496151902696, Fax +496151902961 for accreditation. Possibility
for interviews is available as from 17th November 1999.

More information, including live images and reports, and an 'Ask the
experts' forum, can be found on the ESA Science Leonids 99 website at:

Information on the Leonids flux and events at ESOC is available at:

For further information please contact:

ESA Public Relations Division
Tel: +33 (0)
Fax: +33 (0)

Further information on the ESA science programme and a subscriber
service for news can be found on the Worldwide Web at:

More information about ESA at


From The Times, 16 November 1999


TWO specially equipped Nasa aircraft will arrive in Israel tomorrow as
part of an international effort to monitor the Leonid meteor storm and
to help to protect satellites from debris entering the Earth's upper

Up to 100 scientists from Britain and elsewhere will join Israeli
astronomers in the effort to warn satellite operators around the globe
to shut down or move their systems. The satellites are in danger of
being hit by debris travelling at 150,000mph, as well as from plasma, a
cloud of electronically charged particles that follows the meteors.

"Air Force satellite operators are reviewing contingency plans to
protect and recover, if necessary, military communications,
missile-warning and navigation satellites that could be affected by the
storm," the US Air Force said.

The meteors, which form the tail of the Temple-Tuttle comet, could also
damage the expensive optical equipment of spy satellites, which are
likely to be temporarily or even permanently blinded by the celestial
display. Scientists say it could generate up to 1,200 visible
flashes an hour, far higher than the last storm in 1966. The shower
will be seen best in Israel, but it will also be visible in Canada,
Florida and the Canary Islands tomorrow night.

Aby Har-Evan, director-general of the Israeli Space Agency, said
yesterday that in addition to the scientists on board the aircraft,
others in the Negev Desert would link to amateurs working at
observatories across Israel to collect data. "There is a great danger
of collision to satellites from particles no bigger than a grain of
sand," he said. "The Canadians will be able to alert systems
operators to shut down their equipment, or change its orientation so
that the smallest face is turned towards the meteor storm. We hope that
this event will give us some insights into how the universe was
created. There is a possibility that one day we will collect organic
matter from a comet tail of this kind, which would unlock a lot of
secrets by giving some indication to the foundation of the universe."

Such scientific matters are of little interest to the increasing
numbers of apocalyptic Christian groups gathered in Israel and the
United States. They have been moved to paroxysms of excitement by
recent earthquakes in Turkey and Greece, which they believe are
portents of a coming Armageddon.

Their doomsday predictions, which involve mass destruction and the
"rapture", when believers ascend to heaven, will be further encouraged
by the meteor storm, which, like the star that led the Three Wise Men
to the birthplace of Christ, will rise in the east over Jerusalem.

Ed Daniels, 54, a former surveyor from Denver, Colorado, who emigrated
to Israel two months ago, said that he had seen signs of the "end time"
so frequently during the past year that he was convinced that
Armageddon was fast approaching.

"I don't think it, I know it," he said. "The end time is approaching,
and we are expecting the rapture."

Copyright 1999, Times Newspapers Ltd.


From Andrew Yee <>

[From Cooperative Research Centre for Satellite Systems (CRCSS)
Space Industry News, Issue 84, September 1999]

Avoiding Leonid storms

Leonid meteor storms, particles from Comet Tempel-Tuttle, reach a peak
every 33 years. In Novembers near this peak, the meteoroids threaten
satellites. New predictions can pinpoint the storms to five minutes;
they will help agencies protect their satellites. Robert McNaught

Research in meteor science was well established before the space age. A
big worry was the potential threat meteoroids could pose to satellites
or humans in space. But experience has shown the threat was mostly

However, several times each century, meteor activity has increased to
intense levels, called storms. Although storms are related to the dust
released from specific comets, their occurrence has been largely
unpredictable. With their potential catastrophic consequences for
satellites, much attention is now being focused on meteor storms. 

Meteor storms result from Earth passing through dense narrow streams of
debris from comets. Comet Temple-Tuttle produces the Leonid meteor
storms. Storms have been largely unpredictable, but closely related to
the orbital period of the parent comet; they occur within a few years
of the comet passing Earth. The most intense Leonids occurred 1833 and
1966. The 1966 storm was the only one within the space age. No
satellite damage occurred, but few satellites were in orbit then. 

As comets approach the Sun, the increasing solar radiation sublimates
ices which are swept away by the solar wind. Fine dust grains are
released with the gasses. 

The size of particles in a comet's visible dust tail are too small to
produce naked-eye meteors. Particles causing visible meteors are only
slightly affected by solar radiation wind and begin in orbits close to
the comet's. The particles eventually scatter around the orbit.  

The traditional approach to predicting meteor storms involved examining
the correlation between comets' orbital geometry, and the historical
dates and times of storms. The best this approach could do was suggest
likely storm-years, and estimate peak intensity to within a day, or
maybe a few hours. This is inadequate to assess the potential risk or
organise evasive action. 

New theories

To improve, it's necessary to look at how particles evolve from comet-
ejection to collision with Earth's atmosphere. Dust ejected as a
spherical cloud each time the comet is nearest the Sun, becomes a
narrow dust trail, elongated along the original comet orbit. The
precise position of the dust trail in relation to the Earth can be
calculated for each year, a direct hit with the a trail being necessary
to produce a storms. 

We know the solar wind has some influence on the particles producing
visual meteors. The radiation counteracts the Sun's gravitational
attraction, so particles orbit the Sun more slowly. This is why storms
occur in years shortly after passage of the parent comet. The exact
time lag for the main bulk of the particles depends on the range of
masses and ejection velocities, and the number of orbital revolutions
before encounter. 

David Asher and I have checked Earth's predicted times of approach to
these dust trails. We could back-predict the peak times to within 5
minutes of the observed times, for all years when the maxima were timed
(1866, 1867, 1869, 1966 and 1969). We also developed a model of dust
trail density; our model closely fits the observed rates for all
storm years (1833, 1866, 1867, 1869 and 1966). 

We believe we can predict storms to within 5 minutes, and perhaps 50%
in visual rates. Our model predicts the main activity will occur between
1999 and 2002, with the highest rates in 2001 and 2002. We predicted no
activity from dust trails in 1998. 

Predictions & countermeasures

Although no storm occurred in 1998, we still saw impressive Leonid
fireballs (bright meteors). The explanation came in a separate study; as
comet Tempel-Tuttle is in an orbital resonance with Jupiter, any particles
ejected at low enough velocity (generally big ones), also inhabit this
resonance zone. The zone only covers about 1/30 of the comet's orbit,
so encounters with Earth only occur within a year of the comet passing.
Detailed calculations showed the Earth had a direct encounter with large
resonant meteoroids ejected from Comet Tempel-Tuttle in 1333. 

The highest activity over the next few years will be due to encounters
with young dust trails:
   * 1999, Nov. 18 02:08 UT 3-rev ZHR 1,200
   * 2001, Nov. 18 18.19 UT 4-rev ZHR 13,000
   * 2002, Nov. 19 10.36 UT 4-rev ZHR 25,000
Additional dust trail encounters in these years will be at different times
with lower rates. 

The geometry of encounter with the dust trails results in the more
southerly parts of Earth experiencing the peak first. It then moves
roughly northwards at around 600 km/min., leaving the northern extreme
about 22 minutes later. 

Given this model, and the small uncertainty in the predicted maximum
time, there are specific consequences in mitigating the threat to

For a satellite that can be maneuvered to any point in its orbit at the
maximum time, and having the appropriate orbital geometry, the two
obvious strategies are to place the satellite in the "Leonid shadow"
produced by the Earth, or at the point in the orbit furthest from the
center of the Leonid dust trail. With the maximum of a Leonid storm
being around 30 minutes, the period in shadow can be very significant. If
the satellite orbit is oriented perpendicular to the dust trail, the rates
furthest from the dust trail can be a small fraction of those closest to
the dust trail. Combinations of these, and additional strategies, can
result in a substantial reduction of the overall risk.

[Robert McNaught works in the Research School of Astronomy and
Astrophysics at the Australian National University, studying the
discovery and tracking of Near-Earth objects. He previously worked in
optical satellite tracking.]


From Andrew Yee <>

Centre for Research in Earth and Space Technology
Toronto, Ontario

Andre Bellefeuille
Director, Communications
office (416) 665-5464, cell (416) 707-9120,

Press Release - November 12, 1999

Elusive Meteor Storm Offers Last Glimpse

Toronto -- The debris of Comet Tempel-Tuttle, better known as the
Leonid Meteor Storm, is once again on Earth's radar screen for the
night of November 17 and likely for the last time until roughly the
year 2032.

The mysterious debris field is of great concern to scientists and
satellite owners. As Earth passes through the path of Tempel-Tuttle's
highly elliptic orbit, satellites within Earth's gravitational field
will face their greatest risk of physical damage since the huge Leonids
storm of 1966.

The Centre for Research in Earth and Space Technology (CRESTech) is
once again leading a global observation and risk-reduction program to
try and build a comprehensive model of the Leonid meteor storm. Though
the Leonids only appear every 32-33 years, they offer immense insight
toward building forecasting technology that could help protect the
world's satellite fleets from similar astral activity.

CRESTech's clients for the 1999 program include the Canadian Space
Agency (CSA), the Department of National Defence (DND), NASA, the
United States Air Force Space Command (USAF) and the European Space
Agency (ESA).

Last year's program surprised many scientists when instead of seeing a
massive quantity of "shooting stars," many viewers in the Far East were
treated to a show of large fireballs, few in number but highly visible.
As a result of travelling through a different "slice" of the meteor
field, this year's event is anticipated to result in massive quantities
of shooting stars, but few large fireballs.

Through the involvement of researchers at the University of Western
Ontario and other Canadian and international institutions, the
CRESTech-led team will set up electro-optical camps in Israel's Negev
desert and the La Palma Observatory on the Canary Islands, considered
to be the two best viewing points globally. Additional optical data
will be collected by the USAF in Florida, Hawaii and the Kwajalain
Atoll. Radar data will be collected in Northern Canada at a station in
Alert, Nunavut.

A full backgrounder on the Leonids program is available at
"" as are links to NASA and ESA descriptions of
the event.

CRESTech is made possible through the Ontario government's Centres of
Excellence Program.

For more information, please contact:
Andre Bellefeuille
Director, Communications
(416) 665-5464, cell (416) 707-9120


From Andrew Yee <>

European Space Operations Centre
European Space Agency
Darmstadt, Germany

November 1999

Leonids Warning 1999

Earth orbiting spacecraft are at risk of being hit by Earth orbiting
debris but to some extent also by meteoroids. For example, the demise
of ESA's Olympus spacecraft may be attributed to the Perseid meteoroid
stream. The Leonid meteoroid stream is expected to pose a particular
risk in the coming years around November 17-18, the time in the year
when the Earth passes closely the orbit of comet Tempel-Tuttle, which
is the progenitor of this stream.

ESOC Leonids Document

For more information about Leonid and the risks it poses, feel free to
download this PDF report.

The objective of this note is to provide background information about
the expected Leonid meteor shower around 18 November 1999 and to give
advice for spacecraft operators what kind of precautions can be taken
to minimize the risk of damage to operational spacecraft. In this note
updated predictions for the expected time and intensity of the Leonid
1999 meteor shower are given.


From John Bluck <>

Nov. 15, 1999
Kathleen Burton
NASA Ames Research Center, Moffett Field, CA
(Phone: 650/604-1731, 650/604-9000)

Laura Lewis
NASA Ames Research Center, Moffett Field, CA
(Phone:  650/604-2162, 650/604-9000)

RELEASE:  99-71

NOTE TO EDITORS/NEWS DIRECTORS: You are invited to observe video
footage of the Leonid meteors from the mission on Wednesday, Nov. 17,
from 2 p.m. to 6 p.m. (PT) in the main auditorium, N-201, at NASA Ames
Research Center, Moffett Field, CA.  To gain entry, show press
credentials and photo ID at the Visitor Badging Office, located at the
Main Gate, Moffett Federal Airfield.


To gain a better understanding of the way life may have evolved on
Earth, a team of scientists has begun a multinational airborne mission
to study the Leonid meteors.

The Astrobiology mission began when two U.S. Air Force planes, the ARIA
and the FISTA, lifted off from Edwards Air Force Base, Edwards, CA, on
Nov. 13 at 11:15 a.m. (PT) enroute to Royal Air Force Mildenhall
Airbase in the United Kingdom. During the mission, an international
cadre of scientists will point their instruments towards the sky to
study the Leonid meteors from the unique vantage-point of the aircraft.

"The planes provide a perfect platform for viewing the meteors," said
Peter Jenniskens, chief scientist for the Leonids mission. "They lift
us above the weather to ensure a fantastic view.  By flying over 35,000
feet in the air, we are above most of the atmospheric water vapor, and
our instruments get the best data possible."

The Leonid meteor showers occur each November when the Earth passes
through the debris shed from periodic comet 55P/Temple-Tuttle.  The
meteors, named the Leonids because they appear to stream from the
constellation Leo, are about the size of a grain of sand.  Studying
comets and meteors, which are made from ice and dust that existed when
the universe was formed, may help scientists develop a better
understanding of how life began on Earth.

"Comets and meteors are fascinating to study because they are a frozen
record from the time when the universe formed," explained
astrobiologist Dr. Scott Sandford of NASA's Ames Research Center,
Moffett Field, CA. "Due to geological activity, all of Earth's
materials have been reformed several times over, and we must study
comets, meteors and meteorites to get an early view of our universe."

Most years, observers with ideal viewing conditions can see 10 to 20
meteors per hour during the Leonid showers.  Every 33 years when the
parent comet Temple-Tuttle passes particularly close to the Earth, as
it did in 1998, meteor storms with hundreds or thousands of meteors per
hour are possible. In 1998, following Temple-Tuttle's pass by Earth,
counts of 250 meteors per hour were recorded.  Predictive models have
indicated that, in 1999, it may be possible to see 200 to 5000 meteors
per hour around the longitudes of Europe and the Middle East.  The 1999
Leonid Multi-instrument Airborne Campaign (MAC), a mission jointly
funded by NASA and the United States Air Force, has been designed to
fly over these longitudes for three consecutive observation nights,
Nov. 16-18.

Both aircraft being used for the mission have been specially outfitted
with a variety of instruments, including spectrometers and cameras, to
study the meteors. The FISTA, an NKC-135 aircraft, has been modified
with 20 upward-facing viewing ports.  The ARIA, an EC-18 airplane, has
telemetry equipment that will allow researchers to send images and near
real time data regarding comet flux, or counts, to the ground.

Research objectives for the mission involve taking many measurements
that have never been done in airborne astronomy, including real-time
meteor counts, spectroscopy (mid-infrared, near-infrared, ultraviolet
and visible) and stereoscopic viewing of meteors using intensified
high-definition television cameras. The stereoscopic view, obtained
when instruments on both aircraft image a meteor, will provide the
first-ever three-dimensional model of meteor trajectories.

About half of the scientists on the current mission participated in the
1998 Leonid MAC mission that flew over Japan.  That highly successful
mission is credited with observing more than 3,200 meteors, obtaining
the first differential spectrometry data from meteors as they burned
through the sky, and obtaining the first stereoscopic images of a
persistent meteor train.

After departing Edwards Air Force Base Nov. 13, the planes flew to
Mildenhall Airbase in the United Kingdom.  During the night-time
crossing of the Atlantic Ocean, scientists tested and calibrated their
instruments and completed initial observations, including taking
measurements of the Aurora Borealis.

The mission will begin Nov. 16 when the planes depart England and fly
the scientists overnight to Tel Aviv.  The following night, Nov. 17,
during the expected peak of the storm, the scientists will fly from Tel
Aviv to Lajes Air Base in the Azores.  The final night, Nov. 18, the
planes will fly from the Azores to Patrick Air Force Base in Florida.

The peak of the storm is expected to occur at 0200 (UT) Nov. 18 (9:00
p.m. ET, Nov. 17) over Europe and the Middle East.  While the best
viewing of the storm will be in these locations, it may be possible to
see the Leonid meteors in the United States, particularly in the
predawn hours of November 17 and 18.

For current information about the Leonid MAC Astrobiology mission,


From Andrew Yee < >

ESA Science News

11 Nov 1999

SOHO prepares to weather the Leonid's meteor storm

The Earth will have another close encounter with Comet Tempel-Tuttle's
dust trail in the early hours of 18 November, and the resulting meteor
storm, called the Leonids, could be spectacular.

But the storm so eagerly awaited by astronomers is also making
spacecraft controllers take precautions. Like a ship caught in a
tempest, ESA's Solar and Heliospheric Observatory (SOHO) will try to
stay as safe as possible during the meteor storm.

The scintillating effect produced by the friction between the comet's
dust particles and the upper layers of our atmosphere should be visible
in North America, Europe, Africa, and part of Asia.

With the Earth remaining at a distance of about 1 million kilometres
from the comet's path, the danger for SOHO and other satellites may be
considered minimal, say scientists. But given that the flow of dust
particles is hundreds of thousands of kilometres wide, space agencies
and satellite operators have decided the risk should not be

The loss of ESA's Olympus communications satellite in the early 1990s
was thought to have been caused by the Perseid meteor shower.

The relative velocity between our planet and the particles left behind
by comet Tempel-Tuttle will be about 70 km/s, that is more than 250 000
km/h. "At that speed, if you had a hit by a discrete particle, it would
pierce right through the spacecraft like a bullet," says Bernhard
Fleck, SOHO Project Scientist.

During the coming peak of activity of the Leonids, as many as 1000
meteors per hour are expected to strike the Earth's atmosphere.

To avoid the danger of a sandblasting effect on the optical systems of
its instruments, the protective doors on SOHO will be commanded to

In 1986, the camera on ESA's Giotto spacecraft suffered -- as expected
-- severe damage as it passed in front of Halley's comet nucleus. Since
cometary dust could cause similar problems to SOHO's vital star
trackers, ground controllers will command the spacecraft to rotate 120
degrees around its axis. The star trackers are used by the spacecraft
to determine its position in space as it observes the Sun from its
vantage point 1.5 million kilometres from Earth.

Ten of SOHO's 12 scientific instruments will also be turned off, to
minimise the risk that any instrument hit by a particle would be
severely damaged by the combination of incandescent debris from the
impact and the high-voltage power supply.

During the peak of the storm, SOHO will be placed in a safety
configuration, known as the Coarse Roll Pointing mode. The 10
instruments will remain switched off for a period of almost 70 hours
starting 16 November.


* ESA Leonids99 page
* More about SOHO
* SOHO near-real time image screen saver

[NOTE: An image supporting this release ia available at ]


From SpaceViews, 16 November 1999

Craters on some asteroids may be caused by the compression of porous
material, rather than the excavation of material as previously assumed,
scientists reported last week.

The results, presented in the November 11 issue of Nature, could
explain how small asteroids like Mathilde have such large craters
without breaking apart, and could make it harder to protect the Earth
from such asteroids.

Kevin Housen, a scientist at Boeing, and colleagues at the University
of Washington, performed laboratory tests to see how craters form in
low-density materials. By creating small impacts at high accelerations,
they were able to scale up their results to much larger craters at low
accelerations, as on the surface of an asteroid.

To their surprise, they found that craters formed not by digging out
material, as is the general case for impact craters, but by compressing
the material. The porous material effectively absorbs the energy
of then impact, compressing and increasing its density as a result.

This would explain who asteroids like Mathilde, which was observed up
close by the Near-Earth Asteroid Rendezvous (NEAR) spacecraft during a
June 1997 flyby, can have large craters on its surface that are
remarkably free of the debris usually associated with an impact.
Scientists had previously assumed that the impact debris was
accelerated beyond the escape velocity of the asteroid and thus never

The results, if they hold up, would have an effect on both planetary
formation and planetary defense. Erik Asphaug, a planetary scientist at
the University of California Santa Cruz, notes in an accompanying
article that low-density objects, like primitive asteroids in the early
solar system, would be able to accrete material "like a celestial
Pac-Man." That would make planetary accretion easier to understand, he

On the other hand, a porous asteroid like Mathilde on a collision
course with the Earth would be difficult to deflect, since it would be
able to effectively abosrb the energy of bombs designed to
deflect it off course. The object would simply become slightly more
dense, rather than be deflected off course as intended.

Asphaug isn't terribly concerned about this, however. "We would have to
live ten million years or more, on average, to experience the type of
asteroid collision that could wipe out mankind," he writes. "I am not
betting on that horse."

Copyright 1999, SpaceViews

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