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Leonids in the Crystal Ball

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

A Leonid fireball, copyright 1998, Lorenzo Lovato, all rights reservedOctober 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!"

Right: Lorenzo Lovato captured this stunning picture of a -10 magnitude Leonid fireball 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.

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.

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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.

Parents and Educators: Please visit Thursday's Classroom for lesson plans and activities related to this story.

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 [ref].

Click for a larger images. 
Data credit: International Meteor OrganizationThe 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].

Above: 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.

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]

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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!

Web Links

Leonids Live! -site of the live webcast of the 1998 Leonids

Awaiting the Storm - by Joe Rao in Sky&Telescope

Bulletin 13 of the International Leonid Watch: The 1998 Leonid Meteor Shower - by Rainer Arlt of the International Meteor Organization

The November Leonids: Will They Roar? - Leonids predictions from Donald Yeomans of JPL

Asher, DJ, Bailey, ME and Emel'yanenko, VV "Resonant meteoroids from Comet Tempel-Tuttle in 1333: the cause of the unexpected Leonid outburst in 1998," Mon. Not. R. Astron. Soc. 304, L53-L56 (1999)

Related Stories:

Hunting for Halley's Comet -- May 7, 1999. A high flying weather balloon ascends to the stratosphere in hopes of capturing an Eta Aquarid meteoroid

Meteors Down Under -- May 3, 1999. Information about the eta Aquarids meteor shower and Halley's comet.

Tuning in to April Meteors -- Apr. 27, 1999.Amateur astronomers capture radio echoes from fiery meteors in April 99

April's Lyrid Meteor Shower -- Apr. 21, 1999. The oldest known meteor shower peaks this year on April 22

A Wild Ride to the Stratosphere -- Apr. 14, 1999. A weather balloon hits the stratosphere in search of meteoroids

Meteor Balloon set for Launch -- Apr. 8, 1999. This weekend scientists will launch a weather balloon designed to capture meteoroids in the stratosphere.

Leonid Sample Return Update -- Apr. 1, 1999. Scientists will describe initial results from a program to catch meteoroids in flight at the NASA/Ames Leonids Workshop April 12-15, 1999.

The Ghost of Fireballs Past -- Dec. 22, 1998. RADAR echoes from Leonid and Geminid meteors.

Bunches & Bunches of Geminids -- Dec. 15, 1998. The Geminids continued to intensify in 1998

The 1998 Leonids: A bust or a blast? -- Nov. 27, 1998. New images of Leonid fireballs and their smoky remnants.

Leonids Sample Return payload recovered! -- Nov. 23, 1998. Scientists are scanning the "comet catcher" for signs of Leonid meteoroids.

Early birds catch the Leonids -- Nov. 19, 1998. The peak of the Leonid meteor shower happened more than 14 hours earlier than experts had predicted.

A high-altitude look at the Leonids -- Nov. 18, 1998. NASA science balloon catches video of 8 fireballs.

The Leonid Sample Return Mission -- Nov. 16, 1998. NASA scientists hope to capture a Leonid meteoroid and return it to Earth.

Great Expectations: the 1998 Leonid meteor shower -- Nov. 10, 1998. The basics of what the Leonids are and what might happen on November 17.


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