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Leonids In The Crystal Ball

Leonids In The Crystal Ball
Marshall Space Flight Center
http://science.nasa.gov/newhome/headlines/ast27oct99_1.htm

Most experts agree that 1999 is a likely year for a Leonids meteor storm.

October 27, 1999: Imagine tuning in to the local TV weather report and
hearing this from the weatherman:

"Good evening space weather lovers! Last night Earth was hit by a
high-pressure solar wind stream. It's expected to persist for 3 or 4 more
days producing a 50% chance of mid-latitude aurora. But the big news today
is the 1999 Leonid meteor shower. Experts are predicting a big storm on
November 18th with up to 100,000 shooting stars per hour. Of course, we
could be off by a couple of years. The storm might hit in 2001 instead. Or
maybe not at all! Hey, if predicting these things were easy we wouldn't need
experts!"

One day, space weather forecasts like this could be commonplace. As our
society comes to rely on satellites, cell phones, and other space-age
gadgets, forecasting solar storms and meteor showers can be just as
important as knowing the chances of rain tomorrow. Three weeks from now we
may be treated to a very visible reminder of space weather when the Leonid
meteor shower strikes on November 18, 1999.

What's the probability of significant meteoroid precipitation? That's what
stargazers and satellite operators everywhere would love to know.

Most experts would agree that predicting the Leonids can be tricky. To
understand why it's helpful to know the difference between a "meteor shower"
and a "meteor storm." Simply put, meteor showers are small and meteor storms
are big. Meteor showers produce a few to a few hundred shooting stars per
hour. Meteor storms produce a few thousand to a few hundred thousand meteors
per hour. A meteor storm, like a total solar eclipse, ranks as one of
Nature's rarest and most beautiful wonders.

A Leonid meteor shower happens 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.

Every 33 years something special happens. Comet Tempel-Tuttle swings through
the inner solar system and brings a dense cloud of debris with it. For 3 or
4 years after its passage the Leonids can be very active. In 1966 for
instance, over 100,000 meteors per hour were seen from parts of North
America. Curiously, there isn't a full-fledged storm every time
Tempel-Tuttle passes by. Sometimes there's simply a stronger-than-average
shower. Sometimes nothing happens at all!

Will there be a storm in 1999? (Probably, yes.)

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.

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. 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.

If the 1999 Leonids are anything like the 1966 storm, stargazers are
definitely in for a treat. The 1966 event 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.

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 AU 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 farther 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 West 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.

If the peak of the Leonids occurs over Europe or the Atlantic Ocean, then
observers in the USA could be in for an unusual treat. The Leonid radiant
would just be rising over North America at the time. In the eastern US sky
watchers would see a large number of earth-grazing meteors skimming
horizontally through the upper atmosphere. "Earth grazers" are typically
long and dramatic, streaking far across the sky.

To look or not to look, that is the question

     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.
               From a first-hand account of the 1833 Leonid Meteor
     Shower. by Elder Samuel Rogers

Most experts agree that 1999 is the most likely year for a Leonids meteor
storm during the current 33 year cycle. However, if 1999 turns out to be a
disappointment, don't despair! There are other studies that suggest 2000,
2001 or even 2002 could be better years. The Leonids are simply hard to
forecast.

If 1999 is the year, when should you look? Most experts predict that the
Leonids peak will occur between 0100 and 0400 Universal Time on November
18th. However, it is important to remember that such predictions are always
uncertain. The 1998 Leonid fireball display occurred nearly 16 hours before
the predicted maximum! No matter where on Earth you live, the morning of
November 18 will probably be the best time to look for Leonids in 1999. This
is true even if morning where you live occurs much earlier or later than
0100-0400 UT.

Conventional wisdom says that meteor observing is always best between
midnight and dawn local time on the date of the shower (November 18 in this
case). For a shower or storm like the Leonids that might be relatively
brief, it is best to start watching no later than midnight. In fact, when
the author of this story went outside last year at midnight to view the 1998
Leonids, the shower was already well underway! With this in mind you may
decide it's a good idea to begin observing even earlier, say, 10 p.m. on
November 17.

In the coming weeks Science@NASA will post more stories about the Leonids
with observing tips for meteor watching with the naked eye, video cameras
and other types of recording devices. One thing seems sure, no matter where
you live: The Leonids are coming and, on Nov 18, 1999 the place to be is
outside, looking up!

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