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- Go outside after sunset this month and
look high in the sky above the southwest horizon. The bright star shining
about 20 deg. above, and to the south of Venus is <http://www.skypub.com/sights/sights.shtmlRegulus,
the brightest star in the constellation Leo. The dim, sickle-shaped collection
of stars that make up Leo may not seem impressive now, but in November
1999 they could serve as the backdrop for a once-in-a-lifetime sky show
-- a full-fledged Leonid meteor storm.
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- The Leonid meteor shower takes place
every year around November 17 when Earth passes close to the orbit of comet
Tempel-Tuttle. Usually not much happens. The Earth plows through a diffuse
cloud of old comet dust that shares Tempel-Tuttle's orbit, and the debris
burns up harmlessly in Earth's atmosphere. A typical Leonid meteor shower
consists of a meager 10 to 20 shooting stars per hour.
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- If this were always the case, the Leonids
would be known to a small number of meteor enthusiasts only. Instead they
are famous. At roughly 33 year intervals the Leonid meteor shower can blossom
into what astronomers call a meteor "storm," when hundreds of
thousands of shooting stars per hour rain down from the sky.
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- Leonid storms occur at intervals separated
by multiples of 33 years, the period of comet Tempel-Tuttle's orbit around
the sun. Whenever the comet swings through the inner solar system it brings
a dense cloud of debris with it, so that for 3 or 4 years after its passage
the Leonids can be very active. Curiously, there isn't a full-fledged storm
every time Tempel-Tuttle passes by.
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- Sometimes there's simply a stronger-than-average
shower, never quite rising to the level of a storm, and sometimes nothing
much at all happens to mark the comet's passage. This capricious behavior
makes predicting Leonid meteor storms a bit tricky.
-
- The last great Leonid meteor storm was
in 1966. It was, predictably, somewhat unexpected. The comet had passed
by Earth's orbit in 1965, so astronomers were aware that something might
happen. But, judging by the paucity of the 1899 and 1932 showers, it was
widely thought that the orbit of the debris stream had been perturbed so
that a close encounter with Earth's orbit was not possible. The best predictions
suggested a strong shower over western Europe with 100 or so meteors per
hour.
-
- Instead, there was an stunning display
of shooting stars over Western North America. This recollection by James
Young at JPL's Table Mountain Observatory in California gives a sense of
what the storm was like:
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- "This very noteworthy [1966] meteor
shower was nearly missed altogether... There were 2-5 meteors seen every
second as we scrambled to set up the only two cameras we had, as no
real preparations had been made for any observations or photography.
The shower was expected to occur over the European continent.
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- The shower peaked around 4 a.m., with
some 50 meteors falling per second. We all felt like we needed to put on
'hard hats'! The sky was absolutely full of meteors...a sight never imagined
... and never seen since! To further understand the sheer intensity
of this event, we blinked our eyes open for the same time we normally
blink them closed, and saw the entire sky full of streaks ... everywhere!"
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- The 1966 return of the Leonids was one
of the greatest displays in history, with a maximum rate of 2400 meteors
per minute or 144,000 per hour.
-
- Tempel-Tuttle visited the inner solar
system most recently in late 1997 and early 1998. The subsequent Leonids
display, in Nov. 1998, was marvelous as observers all over the world were
treated to a dazzling display of fireballs. Nevertheless, the 1998 Leonids
were a shower, not a storm. The maximum rate of meteors last year was about
250 per hour. Scientists have learned that if Earth crosses the orbit of
Tempel-Tuttle too soon after the comets passage, then there is no storm,
just a strong shower. Apparently that's what happened in 1998. In recent
history no Leonid storm has ever occurred less than 300 days after Tempel-Tuttle
passed by Earth's orbit. In 1998, Earth followed the comet to the orbit-crossing
point by only 257 days [<http://www.skypub.com/sights/meteors/leonids/9904awaitstorm.htmlref].
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- Below: 1998 Leonids activity based on
visual records from 217 observers who saw more than 47,000 Leonids in 858
observing hours. The vertical axis is the "zenithal hourly rate"
of visual meteors, or the hourly rate of meteors an observer would witness
under ideal conditions with the meteors appearing directly overhead. The
horizontal axis is the solar longitude of Earth, and may also be regarded
as time increasing from left to right. The "Fireball peak" corresponds
to the impressive fireball display of Nov. 17, 1998. The smaller "Storm
peak" occurred approximately 12 hours later just as Earth was crossing
the orbital plane of Tempel-Tuttle. Credit: <http://www.imo.net/articles/shower/leo98.htmlThe
International Meteor Organization.
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- <http://science.nasa.gov/newhome/headlines/images/meteors/leo98plot.gifThe
period of maximum activity during the 1998 Leonid shower took place about
12 hours before the earth crossed Tempel-Tuttle's orbit. The early activity
caught many observers by surprise, but it was business as usual for the
unpredictable Leonids. Rainer Arlt of the <http://www.imo.net/International
Meteor Organization noted that while the maximum activity came early, there
was a secondary maximum when the Earth passed the comet's orbit (see left).
This pattern is similar to that observed in 1965, the year that preceded
the great Leonids storm of 1966. In his report <http://www.imo.net/articles/shower/leo98.htmlBulletin
13 of the International Leonid Watch: The 1998 Leonid Meteor Shower, Arlt
wrote:
-
- Joe Rao, a Leonids expert who lectures
at New York's Hayden Planetarium, also advocates 1999 as possibly the best
year for a storm during this 33 year cycle. Writing for <http://www.skypub.com/sights/meteors/leonids/9904awaitstorm.htmlSky
&Telescope he says:
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- Based on what happened last November,
I will venture a prediction. If a meteor storm is to take place at all,
1999 would appear to be the most likely year for it to happen. But even
if this year's Leonids are richer in number, observers should not expect
the same high proportion of fireballs that were seen in 1998. Instead,
a more even mix of bright and faint meteors is likely. [<http://www.skypub.com/sights/meteors/leonids/9904awaitstorm.htmlref]
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