I heard one of the children cry out, in a voice expressive
of alarm: "Come to the door, father, the world is surely
coming to an end." ... It did appear as if every star had
left its moorings....
First-hand account of the 1833 Leonid Meteor Shower.
Elder Samuel Rogers.
 June 22, 1999: 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 Regulus,
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.
Right: This stunning picture of a
-10 magnitude Leonid fireball was captured by Lorenzo Lovato
on November 17, 1998 from Monteromano, Italy. He used Fuji 800
film with a 16 mm f/2.8 lens for an exposure time of 15 minutes.
Copyright 1998, Lorenzo Lovato, all rights reserved.
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.
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.
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.
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.
Left: A false
color image of comet Tempel-Tuttle obtained on February 19, 1998.
The comet was 0.98 AU from the sun and 1.22 AU from the Earth at the time.
Photo Credit: Lowell Observatory.
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
deflected so much by gravitational encounters with other planets (mainly Jupiter)
that a close encounter with Earth's orbit was no longer
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:
"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.
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!"
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 (shooting stars with magnitudes brighter than -3). 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 [ref].
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: The
International Meteor Organization.
 The
period of maximum activity during the 1998 Leonid shower took
place about 12 hours before the earth crossed Tempel-Tuttle's
orbital plane. The early activity caught many observers by surprise,
but it was business as usual for the unpredictable Leonids. Rainer
Arlt of the 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 Bulletin
13 of the International Leonid Watch: The 1998 Leonid Meteor
Shower, Arlt wrote:
[T]he radar, visual, and photographic records of the 1965
Leonids indicate an activity profile which resembles that of
the 1998 Leonids. Even the low population index seems comparable.
Judging from these phenomenological facts, we may expect 1999
to show a similar shape of activity as in 1966. The actual maximum
meteor numbers are hardly predictable. [ref].
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 Sky
&Telescope he says:
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. [ref]
Rao bases his argument on historical precedent and the Earth-comet
geometry. During the seven most recent Leonid storms when Earth
crossed Tempel-Tuttle's orbit soon after the comet, the average
distance between the comet and Earth was 0.0068 astronomical
unit. The average number of days between the
comet's passage and the Earth's arrival
at the plane of the comet's orbit was 602.8 days. With the 1999 values of 0.0080 a.u.
and 622.5 days, Rao says we ought to be in a prime position to
see significant, if not storm-level, activity.
Rao is also a meteorologist for News 12 Westchester, which
seems a suitable occupation for predicting meteor showers.
In 1999, the
Earth will pass nearly three times
as far from the comet's orbital path as it did in 1966 and
more than six times further than it did during
the great storm of 1833.
If the peak of the
Leonids arrives exactly when the Earth passes
through the comet's orbital plane, Donald Yeomans of JPL
gives 01:48 UT on November 18, 1999 as the most likely time
for the 1999 maximum [ref].
That
would make Europe and North Africa the best places to watch the
show. However, Leonid meteor showers frequently arrive much earlier
or later than predicted, so any place on the globe could be favored.
A blast from the past reveals the future
The spectacular display of fireballs in 1998 was a treat for
observers, but it posed some interesting questions for astronomers.
According to David Asher of the Armagh Observatory and his colleagues,
the intensity and duration of this exceptional event indicated
that the Earth must have passed through an extremely dense, narrow
stream of large dust grains and particles. The timing implied
that these debris particles occupied an orbit somewhat different
from the main stream, and that they left the comet's nucleus
many hundreds of years ago. But in that case, how did the stream
has hold together so tightly for so long?
To solve the problem, Dr David Asher and his co-workers calculated
the motion of large dust grains ejected from comet Tempel-Tuttle
at each of the last 42 occasions when it made its closest approach
to the Sun. They checked to see whether any of the particles
could explain the fireballs seen in 1998, and identified September
1333 as the time when most of the observed particles must have
been released.
 Left: A short video segment showing
a Leonid fireball as seen from the stratosphere. It was recorded
by a digital video camera carried aloft by a NASA weather balloon
on Nov. 17, 1998. More images and video may be
seen at LeonidsLive.com.
The particles were kept in a tight stream by a process known
as "gravitational resonance." A similar phenomenon
gives rise to the fine structure seen in Saturn's rings. In
this case, grains ejected from the comet in 1333 were kept in
step by the gravitational influence of Jupiter. Instead of spreading
around the whole orbit, they were nudged by periodic "kicks" from
Jupiter's gravity into a rather short arc of large particles,
distinct from the 'normal' stream of small particles ahead of
and behind the comet. Their calculations showed that in November
1998 most of the resonant arcs missed the Earth by a wide margin,
but the arc of particles released in 1333 cut right through the
Earth's orbit, and the calculated time for when this happened
matched the observed fireball maximum to the hour.
Armagh and co-workers are not expecting a repeat performance
of bright fireballs in November in 1999. All the resonant strands
in the meteoroid stream will be well past Earth in space. However,
a strong 'normal' display is likely, peaking at about 2 a.m.
on November 18th, due to meteoroids ejected from Comet Tempel-Tuttle
in the years 1866, 1899 and 1932, which have not yet had time
to disperse around the comet's orbit.
Rob McNaught of the Research School of Astronomy and Astrophysics
at the Australian National University and colleagues have
examined the motions of Tempel-Tuttle
debris particles ejected from the comet within the last 200
years. Their calculations predict that the maximum hourly rate of
meteors in 1999 will be 1000 to 1500 per hour -- not exactly
a major storm, but still a remarkable display -- and that the
best years to observe could be in the next millennium when hourly
rates might reach 10,000 - 35,000 in 2001 and 25,000 in 2002.
These conclusions differ from most other
recent studies which predict greatest
activity during the years 1998 - 2000, but the Leonids do not always
adhere to conventional wisdom.
Ready or not, here they come!
All sorts of conjectures were made by all sorts of people
... We may learn of this that, when men are in a high state of
excitement, their testimony must be taken with many grains of
allowance.
First-hand
account of the 1833 Leonid Meteor Shower. Elder Samuel Rogers
There seems to be plenty of room for debate about the
upcoming Leonid meteor showers. The exact timing of the display,
the number of fireballs vs. fainter meteors, and the best observing
sites are all uncertain. Nevertheless, even the most pessimistic
predictions for 1999 presage a memorable show.
One thing seems sure, no matter where you live: The Leonids
are coming and, on Nov 17, 1999 the place to be is outside, looking
up!
|
The
period of maximum activity during the 1998 Leonid shower took
place about 12 hours before the earth crossed Tempel-Tuttle's
orbital plane. The early activity caught many observers by surprise,
but it was business as usual for the unpredictable Leonids. Rainer
Arlt of the 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 Bulletin
13 of the International Leonid Watch: The 1998 Leonid Meteor
Shower, Arlt wrote:
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