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Leonids on the Horizon

What's in store for the 1999 Leonid meteor shower? Experts make their predictions.

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.

A Leonid fireball, copyright 1998, Lorenzo Lovato, all rights reservedJune 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.

Comet Tempel-TuttleLeft: 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.

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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.

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].

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 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.

A Leonid fireball recorded by a digital camera on board the 1998 Leonids science balloonLeft: 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!

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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