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(meteorobs) Excerpt from "CCNet, 5 October 1999"




CCNet, 5 October 1999
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(5) RETURN OF THE LEONIDS: GREATEST SHOW IN THE SKY
    EXPLOREZONE.COM


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[...]

(5) RETURN OF THE LEONIDS: GREATEST SHOW IN THE SKY

>From EXPLOREZONE.COM
http://explorezone.com/columns/space/1999/october_leonids.htm

In 1833, people woke to a sky that seemed ablaze. The hubbub was over
the Leonid meteor shower, set for a possibly grand encore Nov. 17.

By Wil Milan for explorezone.com

The Earth may be about to witness the greatest show ever visible in
our skies, a very rare event which, if it comes off, will be the
headline of every newspaper, cause many people to panic, and leave
everyone in awe. But to understand what is happening, lets first
talk about bees:

Bees on the windshield

Imagine that you are driving at 60 miles per hour down a rural 
highway. Unbeknownst to you, a swarm of bees is crossing the highway
just ahead of you. In the blink of an eye your vehicle plows right
through the middle of the swarm, and in that instant dozens of bees
are splattered over the front of your car.

Had you arrived at that point on the highway only seconds later, you
would have encountered only the trailing portion of the swarm and
struck only a few bees. Had you been even later than that you might
only have hit one or two stragglers and missed the main swarm
altogether.

Bigger and faster "bees"

Striking a few bees with your car is not a big deal, and we wouldnt
give it much thought. But much the same thing happens on much larger
scale, not with your car, but with the entire Earth. The Earth
travels around the Sun at a tremendous speed, over 100,000 kilometers
per hour (about 67,000 miles per hour or about 18 miles per second).

The space through which the Earth travels is largely empty, so there
is not much for the Earth to hit as it speeds along. But it is not
totally empty; there are scattered bits of dust and the occasional
little rock, and when the Earth collides with one the dust grain or
small rock plows into the atmosphere at a combined speed sometimes
reaching hundreds of kilometers per second.

At that speed the friction of the object passing through the
atmosphere is so intense that the tiny object is instantly heated to
many thousands of degrees. The heat is so intense that the tiny
grains of dust are completely burned up in a fraction of a second,
leaving only a momentary bright streak and a bit of vaporized ash
floating in the air. If it were night and had you been looking in the
sky at that moment you would have seen what is sometimes called a
"shooting star," the flash of a meteor burning itself out in the
upper atmosphere. On any given night under dark skies you can see a
dozen or more bright meteors, heavens reward for having your eyes on 
the sky.

Bigger swarms

But sometimes the Earth encounters not just random bits of dust, but
a more concentrated swarm. In those cases many more than a dozen
meteors will be seen, and the rate at which meteors are seen may rise
to 30, 50, even over 100 per hour. These are called meteor showers,
and they are recurring events that take place at the same time each
year. Thus the Lyrids meteor shower occurs in April 21-22 of each
year, the Perseids on August 11-12, the Leonids take place the night
of Nov. 17 each year, the Geminids on December 13-14, and so on.

(The names "Lyrids," "Perseids," etc. refer to the constellation from
which the meteors appear to originate. The Lyrids appear to come
from the constellation Lyra, the Perseids from Perseus, etc. They
dont really come from the constellation, of course; its only an
optical illusion due to the combined directions of travel of the
meteors and the Earth.)

The reason that the dates of meteor showers remain the same from year
to year is that on those dates the Earth reaches points in its orbit
where there are known concentrations of space dust and debris. These
bits of dust and debris are not stationary  nothing in space is
truly stationary -- but they are in orbit about the Sun in a
racetrack pattern, just as the Earth follows its own "racetrack"
around the Sun. But because the two "racetracks" cross each other,
each year when the Earth reaches the point where the orbits of the
Earth and the dust swarm cross each other, the Earth plows through
the thin trail of dust and for a few hours we see more meteors in the
sky.

The source of the swarms

What causes these "racetrack" trails of dust is comets. Comets are
clumps of dust and ice a few miles across and they are very loosely
held together. As they travel in their orbits about the Sun they are
continually scattering dust and debris in their wake, and over time
the path of their orbits become one continuous trail of thin dust and
debris. The orbits of most comets dont cross the Earths orbit, but
when one does then the Earth, in subsequent years, will cross the dust
trail of the comet and a meteor shower will occur at that point each
year.

Thus it is that every meteor shower is believed to be associated with
a comet. In some cases the parent comet of a meteor shower can be
clearly identified: The Perseids appear to be associated with comet
Swift-Tuttle, the Leonids with comet Tempel-Tuttle, and both the
Orionids and Eta Aquarids meteor showers appear to be associated with
Comet Halley (because the Earth crosses Halleys orbit in two
places). In some cases the parent comet is unknown and believed to be
long extinct, but the dust trail remains to mark its former orbit.

A near miss

So what would happen if the Earth did not cross the comets trail far 
back from the comet, but rather very close behind the comet? In other
words, what if we crossed the comets orbit right after the comet had 
just passed? Obviously we would be passing fairly close to the comet,
and there would be more comet dust and debris to encounter.

What happens in those cases is that the meteor shower that would
normally take place becomes much more intense. The meteor rate may
increase from one or two a minute to tens or hundreds of meteors per
minute, and there have been instances when hundreds of meteors per
second have been seen for short periods of time.

Those very rare cases where the rate reaches dozens or hundreds per
minute are known as "meteor storms," and the meteor storm that is
credited with launching the modern study of meteors occurred during
the Leonids meteor shower on the night of Nov. 12-13, 1833. Meteor
storms had been observed before, and just the year before the Leonids
had put on a spectacular show, with one observer in Boston counting
over 8,000 meteors in only 15 minutes.

But what occurred when the Leonids returned in 1833 was far beyond
what anyone had ever seen or even imagined possible. For several
hours over the United States there was a continual blaze of thousands
and thousands of meteors at a time. One estimate was that over
240,000 meteors fell during that period, so many meteors in the sky
at a time that many people were woken from their beds and stared at
the sky in panic, believing the sky to be on fire. Many feared that
it was the end of the world and dreaded what they would see at
daybreak.

At daybreak, of course, everything was back to normal. Hollywood
movies notwithstanding, meteors typically vaporize in the atmosphere,
a few drop harmlessly to the ground, and there is only one known
incident in history when a meteor struck someone (and she only got a
bruise from it). The only living thing ever known to have been killed
by a meteor was a very unlucky dog in Egypt many years ago. You
are more likely to be struck by lightning seven times in a row than to
be hit by a meteor.

The show returns

When the Leonids returned in 1834 it was again a good meteor show,
but nothing like the sky-on-fire spectacle of 1833. The great meteor
storm was back 33 years later in 1866. Astronomers predicted that
the meteor storm would return every 33 years, but it failed to
materialize in 1899 or 1933.

Astronomers began to think that perhaps the great meteor storms would
not be repeated, but right on time in 1966 the great meteor storm was
back, particularly over the western United States. During a peak
period which lasted less than an hour there were hundreds of meteors
in the sky at once, and rates as high as 40 per second were observed.

Orbital observations by then had pinpointed the source of the meteor
storm as Comet Tempel-Tuttle, which has a 33-year orbit. Those
occasions when the meteor storm occurred were linked with times when
the Earth had passed close behind the comet in its orbit. But the
theory is not fully worked out; though there clearly is a link with
the comets position, there is no good explanation why there was no
meteor shower when we passed close to the comet in 1899 and 1933.

33 years later 

Youve probably already done the math: The last Leonids meteor storm
was in 1966, it appears to take place at 33-year intervals, and 33
years later is this year, 1999. And so it is that astronomers
everywhere are eagerly awaiting the night of Nov. 17/18 of this year.
If the pattern holds, on that night we may again witness perhaps the
grandest spectacle in the sky, a great meteor storm that for a few
minutes to a few hours sets the sky on fire with thousands of meteors
and fireballs.

There is already some evidence of a build-up in Leonids. Though
nowhere near the rate of a true meteor storm, last years Leonids
meteor shower was much stronger than usual, with rates several times
the norm. Several locations around the world reported rates of
several hundred meteors per hour. At my location in Arizona several
of us were treated a great show; as daybreak approached the rate
was increasing and we could often seen several meteors in the air at
once. Still not a meteor storm, but a great harbinger of a great event in
1999.

How to see the show

The Leonids meteor show will be visible the night of Nov. 17 through
the early hours of the morning of Nov. 18, and to see the event you
need only one thing: dark skies. Bright city skies make it impossible
to see most meteors, so youll need to head out to the country where
the skies are clear and dark. No equipment of any kind is required,
and in fact telescopes and binoculars only hinder the view. Bring
warm clothes, a comfortable chair, then sit back and enjoy the show,
but do be prepared to stay awake past midnight. There is usually
little activity in the early evening, but after midnight (when the
part of the Earth where you are located is on the "front" of the
Earth as it travels around the Sun) the activity will pick up and
often accelerate as daybreak approaches.

There is no guarantee that the 1999 Leonids will be a spectacular
meteor storm as in 1966 and 1833, but if the pattern holds, this is
the year it is most likely to occur. Even if the great meteor storm
doesnt develop, some kind of meteor shower is guaranteed to occur,
and that alone is worth the watching. Any night under the stars is a
great experience, and enjoying a meteor shower with a few friends and
warm drinks under a clear sky is one of the finest ways to spend a
few hours. A meteor shower is a fireworks show that Man can never
match, and if the great meteor storm does develop, for most people it
will be the most memorable event they have ever seen.

Enjoy the show, and dont forget to bring the hot chocolate.

Copyright 1999, explorezone.com
http://explorezone.com


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