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THE LEONID METEOR STORM: Is This the Year?

Last year we got spectacular fireballs. Now the Leonid shower may be about to produce a blizzard of fainter meteors on the morning of November 18th. Skywatchers in Europe and Africa have the best chance, but North America is still in the running.

The time is rolling around again. As November approaches, amateur astronomers are eagerly awaiting this year's passage of the Earth through the Leonid meteor stream. After putting on a dazzling show last year (for which much of North America was clouded out!) the Leonid shower seems poised to produce a rich display of fainter meteors early on the morning of November 18th for some portion of the world. Europe and Africa are the areas most likely to be having their early-morning Leonid-watching hours at the time when Earth plunges through the stream of meteoroids. But North Americans have a chance to see the show too, especially those in the East.

Ever since November 17, 1966, when an awesome outburst of shooting stars filled the skies of western North America for close to an hour, meteor observers have been impatiently waiting for another such spectacle. The Leonids return in cycles of approximately 33 years. The shower began building strength in the 1990s on schedule - and then last year it delivered a surprise. Instead of the brief, rich shower of average- brightness meteors that was anticipated, nearly the whole world got a sparser, longer-lasting display of brilliant fireballs. The bombardment started a full day ahead of predictions and lasted roughly 18 hours (February issue, page 123).

Now, with the 1999 Leonids nearly upon us, some astronomers predict that this could truly be the year of the storm.

Forecasting unusual meteor showers is tantalizingly tricky, much like predicting the weather was 50 years ago. Enough is known about the Leonids and other important showers to get it partly right much of the time, with occasional dead-on hits. But there is still room for a complete miss.

The Leonid meteoroids are small bits of debris shed by the periodic comet 55P/Tempel-Tuttle. Like a dirt-filled dump truck on a bumpy road, the comet leaves a moving "river of rubble" along its path. Every particle in the stream orbits the Sun in close to a 33-year period, like the comet itself. Each November the Earth passes through this stream; the result is a meteor shower.

The geometry of the encounter ensures that the meteors always appear to originate from the direction of the constellation Leo. Because they travel in parallel, the illusion of perspective makes them appear to radiate from one spot in the sky, the shower's "radiant" point (just inside the cutting edge of Leo's Sickle asterism, at right ascension 10h 12m, declination +22u). The meteors themselves flash into view all over the sky, not just near Leo. But their directions of flight, if traced back far enough, would all intersect this point.

In the case of old, reliable, annual showers such as the Perseids, Orionids, and Geminids, the particles in the stream have had time to spread out fairly uniformly all around the parent comet's orbit. But the Leonids are different. They are still mostly confined to the vicinity of Comet Tempel-Tuttle, running just a few years ahead of and behind it in a thin, ribbonlike streamer. Most years an observer counts no more than 10 or 15 Leonids per hour under ideal conditions. Spectacular storms, in which an observer at a dark site can see more than 1,000 per hour, have occurred only around the years when Comet Tempel-Tuttle returns to the inner solar system and passes Earth's neighborhood. The comet last came by in early 1998.

A Sheaf of Ribbons

A dense, concentrated meteoroid stream like the Leonids is likely to have a complex structure, which is maddeningly hard to map because we can't see it. We can only sample the few places along it through which Earth plunges once a year. From there we can try to extrapolate based on what we know (or guess) about how a comet sheds debris when warmed by the Sun, how the debris responds to the Sun's radiation pressure over the years, and the effects of gravitational perturbations by the planets on each particle's orbit.

One important finding is that a comet yields not just one trail of meteoroids but many, one for each of its passes through the warmth of the Sun. These can form a complex sheaf of parallel ribbons very close together. New ones are sharp and dense, old ones are more fuzzy and scattered, and the very oldest blend into one wide, relatively sparse swarm. The particles responsible for the "storm component" of the Leonid stream are in the thin, dense ribbons that have not yet had time to disperse much.

A ribbon must be at least several astronomical units long if we encounter it for several years running. But based on the brief time it takes Earth to pass through it, the ribbon can only be about a ten-thousandth as thick. Its width remains more or less unknown. All the ribbons lie within or very close to the plane of the comet's orbit.

Many astronomers had thought that 1998 would bring a Leonid storm, because the Earth crossed the plane of Comet Tempel-Tuttle's orbit a scant 257 days after the comet itself passed closest to this intersection point (the "descending node"). However, no true storm took place.

1998 Fireballs: Relics of the 14th Century

To many people, last November's brilliant fireball display seemed like a dramatic confirmation of meteor-shower predictions in this magazine and elsewhere. But the nature of the display, and its arrival many hours early, took meteor astronomers by surprise. Only later was the situation figured out. Last spring Mark E. Bailey and David J. Asher (Armagh Observatory) and Vacheslav Emelyanenko (South Ural University, Chelyabinsk, Russia) demonstrated that the fireball outburst occurred when the Earth passed through a dense stream of particles that was shed by Comet Tempel-Tuttle in the year 1333, fully 20 revolutions ago. These large particles (typically pea to marble size) became locked in a 5:14 resonance orbit with Jupiter. That is, for every 14 revolutions of Jupiter around the Sun, the largest particles - like the comet itself - complete 5 orbits. (Similar orbital resonances give rise to the structures in Saturn's rings.) As a result, rather than spreading out over the centuries, these resonant meteoroids were shepherded together into a localized concentration within the larger Leonid stream. Small particles, more affected by solar radiation pressure, were largely removed.

This analysis tells us that the 1998 fireball shower is history; it won't repeat. The Jupiter-shepherded concentration of large particles is now well past Earth. In fact last year's fireballs, grand as they were, were a sideshow that ended up telling us essentially nothing about what we are heading into this November.

The Signs Point to 1999

Four years ago I predicted that 1997 through 2000 would provide the best opportunities for a 1966-type Leonid storm. So far it hasn't happened.

In order to help sort out the chances for 1999, we should examine the circumstances of past historic Leonid displays. Donald K. Yeomans (Jet Propulsion Laboratory) and British meteor historian John W. Mason have each compiled lists of strong Leonid showers over the centuries, often from very fragmentary evidence that is subject to interpretation. Eleven Leonid storms made it onto both lists. All of these occurred many months after Comet Tempel-Tuttle passed its closest to Earth's orbit. A summary of these 11 events (1966 and earlier) is provided in the table below; it is probably closer to the truth than lists I have shown here before.

The first column gives the date of the observed Leonid maximum. (The date has shifted from mid-October to mid-November during the past millennium for three reasons: the change from the Julian calendar to the present Gregorian calendar in 1582, the precession of Earth's axis, and gravitational perturbations by the planets.) In the second column, Earth at Node tells how many days the shower occurred after the parent comet passed by. C-E is the distance between the comet's orbit and Earth at that point, measured in astronomical units; the minus sign in every case means the comet passed inside the Earth's orbit (closer to the Sun). Last are contemporary descriptions and/or the estimated peak rate as would be seen by a single observer.

Notice that not since 1533 has a storm in the list occurred with the Earth trailing the parent comet by as little as 257 days. So it's no wonder that 1998 did not produce a true storm.

If we average the 11 values for Earth at Node as well as the nine C-E values, we get 623 days and -0.0068 a.u., respectively. As can be seen, these compare very favorably with 1999. So we can state a clear conclusion: if you hope to observe a classical Leonid meteor storm, 1999 seems like the most likely year!

How about 2000? A Leonid storm is not out of the question, but the odds appear nowhere near as good. By then the Earth will be following Comet Tempel-Tuttle to the node by 987 days. On just two other occasions in the table have Leonid storms occurred with the Earth trailing farther behind the comet.

As an added handicap next year, the waning gibbous Moon will shine brightly in Cancer just one constellation away from the Leonid radiant. This year the Moon sets around 1 a.m., leaving the sky dark for the best meteor-watching hours before dawn.

When to Watch

Exactly when might the 1999 Leonids reach their peak? That's the question on every skywatcher's mind, and we probably have a pretty good answer.

Part of the answer comes from a close analysis of last year's display. According to Rainer Arlt of the International Meteor Organization, there was more to the shower than the attention-grabbing fireballs. Also present was a lesser, very narrow peak of activity that arrived on schedule about 19 hours after the main peak. This brief "storm component" was caused by a relatively fresh, narrow ribbon of debris. The fireball display apparently reached a maximum zenithal hourly rate* of 340 around 1:40 Universal Time on November 17th. That was a full 18 hours before Earth crossed the comet's orbital plane. The more modest "storm component" peaked at 180 per hour around 20:30 UT on November 17th, just 45 minutes after Earth crossed the comet's plane.

Fresh, dense ejections of meteoroids, the kind that cause storms, are most likely to lie especially close to the plane of a comet's orbit. They haven't had much time to drift away from it. A recent study by David Asher finds that in the great Leonid storms of 1833 and 1966, Earth likely passed through meteoroid streams shed during the comet's 1800 and 1899 returns, respectively. The stream from 1800 had made a single revolution around the Sun when it caused the historic Leonid storm of 1833. The stream released in 1899 made two revolutions before producing the now epic 1966 Leonids.

The bottom line of Asher's study? To get a storm in 1999, we will again have to encounter the stream shed in 1899. Asher's calculations suggest that this will indeed happen around 2:08 UT on November 18th. If he is right, Europe and Africa will have the ringside seats. In North America the Leonid radiant will not yet have risen, so we'll miss the peak completely, according to this prediction. Only the late fringe of the display will fall in our window of early-morning observing time after the radiant rises.

A different way to estimate the time of the peak, however, is simply to extrapolate the trend of the last few years. Since 1996 the shower's maximum has been coming a little less than two hours later each year.

To make this calculation, we can't go by the clock and calendar directly; our civil timekeeping system jitters around with respect to Earth's position in its orbit by up to 18 hours from one year to the next. Instead, we need to specify the time by the Earth's position directly. Meteor astronomers do this by noting the ecliptic longitude of the Sun as seen from Earth (referred to the equinox of 2000.0). In this system the storm component of the Leonids has shifted from solar longitude 235.17u in 1996, to 235.22u in 1997, to 235.31u in 1998. Extrapolating this forward gives us 235.38u in 1999 for the storm-component peak. That works out to 4:15 UT November 18th, or 2 hours 30 minutes after Earth crosses the comet's orbital plane. This timing would be ideal for Western Europe and West Africa.

It would also raise an exciting possibility for people near the East Coast of North America. The Leonid radiant will be just rising at that time, so Easterners would see "Earth-grazing" meteors skimming almost horizontally through the upper atmosphere. Such meteors are unusually long and dramatic, streaking far across the sky. Easterners got a taste of exactly such meteors on the evening of August 11, 1993, when the Perseid shower underwent a predicted outburst while the radiant was still near the horizon.

Norman McLeod, assistant visual program director of the American Meteor Society, notes that if a really major storm does arrive three or more hours after the orbit-plane crossing, "the sky ought to have a continuous show of Earth-grazing Leonids in view from the eastern states. Considering that each Earth-grazer typically lasts three to six seconds, there ought to be more than one visible at any instant for a while."

Adds Robert Lunsford, secretary-general of the International Meteor Organization: "If I were staying in North America, I would certainly recommend that you observe from a location as far east as possible."

How Strong a Shower?

While the timing of a possible storm can be predicted with some confidence, how much meteor activity we can expect is open to considerably more debate. Peter Brown, a meteor scientist at the University of Western Ontario, recently published a study of historical Leonid observations in the planetary-sciences journal Icarus. He concludes that a strong, 1966-class storm is unlikely this year. However, single-observer hourly rates "on the order of 1,000 may be reached."

Peter Jenniskens, a meteor specialist at NASA's Ames Research Center, says, "I am optimistic that we may get rates as high as 7,000 per hour or so."

David Asher's analysis of meteoroid streams that are three revolutions old suggests that 1999 compares somewhat favorably to the one-revolution storm of 1833 and the two-revolution storm of 1966.

Asher and Robert McNaught (Australian National University) have carefully examined the motions of particles ejected from Comet Tempel-Tuttle within the last 200 years. They predict a maximum hourly rate of 1,000 to 1,500 for 1999 and 2000 - not exactly major storms, but still the meteor showers of a lifetime for most observers. They also predict that the best years are yet to come, with single-observer hourly rates possibly reaching 10,000 to 35,000 in 2001 and 25,000 in 2002. This stands in contrast to most other recent studies, which predict greatest activity from 1998 to 2000. But the Leonids do not always follow conventional wisdom.

On the other hand, Zidian Wu and Iwan P. Williams in Great Britain are standing by their prediction that in 1999, "only a few Leonids will be seen."

My own investigation suggests that the swarm of particles that produced the great Leonid storm of 1966 will pass close by Earth, just 0.0026 a.u. (400,000 kilometers) inside our orbit, shortly after 2:00 UT November 18th. This seems to suggest (at least to me) possible meteor rates on the order of tens of thousands per hour! However, caution is strongly advised before trumpeting such a prediction. We unfortunately know very little about whether the planetary perturbations wrought upon the Leonid stream since 1966 will have an advantageous or adverse effect on its coherence.

In last March's issue I commented that skywatchers worldwide are entered in a 1999 "Leonid lottery." If the shower hits just 4 to 8 hours earlier than anticipated, the show would shift to eastern Asia and Australia. Conversely, if it peaks 4 to 8 hours late, the Americas will be looking into the eye of the storm. It is always risky business to make definite predictions concerning unusual meteor showers, especially one with a history of being unpredictable. As NASA's Donald Yeomans noted several years ago, "That's the way it usually is with the Leonids. . . . You can say 'probably,' but if you say 'definitely' they'll get you every time."

Good luck and clear skies to you all!

Joe Rao is a meteorologist for News 12 Westchester and an instructor/lecturer at New York's Hayden Planetarium. He thanks David J. Asher, John E. Bortle, Edward M. Brooks, Peter Jenniskens, and Donald K. Yeomans for their helpful comments and suggestions.
Date   Earth at Node   C-E (a.u.)   Peak Hourly Rate / Remarks
902   Oct. 13   597 days after comet   -0.0113   "Small starlike
fire."
1002 Oct. 14   634 days after   -0.0129   "Stars flew early in the
morning."
1202 Oct. 18   613 days after   -0.0059   "Stars rushed across the
heaven."
1238 Oct. 18   1,456 days after   -0.0031   "Countless large and
small meteors."
1533 Oct. 25   230 days after   -0.0065   "Countless meteors till
dawn."
1601 Nov.  5   465 days after   -0.0102   "Stars became like rain."
1833 Nov. 13   308 days after   -0.0013   ~100,000/hr; "Stars
descend like snow." (U.S.)
1866 Nov. 14   299 days after   -0.0065   ~5,000/hr; maximum over
Europe around 1:10 UT.
1867 Nov. 13   664 days after         u   ~3,600/hr; maximum over
N. America; bright Moon.
1868 Nov. 13   1,030 days after         u   ~1,500/hr
1966 Nov. 17   561 days after   -0.0033   <150,000/hr; "A rain of
shooting stars." (U.S.)
1997 Nov. 17.5   108 days after   -0.0080   >100
1998 Nov. 17.1   257 days after   u     u   340
1999 Nov. 18.1   622 days after   u     u   Go look!
2000 Nov. 17.3   989 days after   u     u   Go look!


RELATED ARTICLE : Planning Your Leonid Watch

Alan M. MacRobert

Prepare to miss a lot of sleep on the night of November 17-18. You might try to nap until 11 or midnight. That's about when the radiant of the Leonid shower rises above your east-northeastern horizon (for skywatchers at midnorthern latitudes). Before then there will be nothing to see no matter how intensely the Leonid meteors may be storming against the other side of the world.

As soon as the radiant clears the horizon, you have a chance to see long, Earth-grazing meteors crossing the sky, as described in the text - if a major storm happens to be in progress at the time. The bright waxing gibbous Moon will still be up, washing out the view to some extent. The Moon sets around 1 a.m. (depending somewhat on your location), leaving a dark sky for the rest of the night.

The later you wait, the higher the radiant rises. The meteors will plunge more directly downward into the atmosphere, and, all else being equal, you would see them in greater numbers. But all else won't be equal. If a storm occurs it will probably last for an hour or less. And if you're in North America, each hour that ticks by carries you farther past the predicted time. So early after radiant-rise may be the best bet.

How to Watch

All you really need to do is find a dark observing site with a wide-open view of the sky. Bring a reclining lawn chair, bundle up warmly, and lie back and watch the stars. Be patient. Give your eyes at least 15 minutes to adapt to the dark. No optical aid is necessary; your eyes alone provide the widest-field view. It doesn't much matter which direction you look. Straight up may be best, since that's where the least light pollution is likely to be. If you have a choice, face more or less east.

Rather than simply watching for the show, however, it's much more fun and rewarding to do a scientific meteor count by standardized methods that will yield meaningful results - ones that can be compared with other observers' results from around the world. You'll need to determine the faintest star you can see with the naked eye and keep good records (best done by voice into a tape recorder) of the meteors you see. Record the starting and stopping times of your observing session, the times of any breaks you take, and also make a note of the time about every half hour. The fraction of your view obstructed by clouds or other obstacles also needs to be logged. Full instructions, including how to report your results to the International Meteor Organization for inclusion in their worldwide data base, are in the August 1997 Sky & Telescope, page 90, and at http://www.skypub.com/sights/meteors/meteorwatch.html.

A new and promising meteor-recording method is low-light video. Sensitive, inexpensive video cameras can now match or beat the eye's performance in the dark. During the Geminid shower last December Tony Phillips in Aspendell, California, recorded a 20u-wide field centered on Orion for three hours using an Astrovid 2000 CCD video camera with a 12- millimeter f/1.2 lens. He simultaneously counted meteors by eye, then compared the results of the two methods the next day. The camera captured twice as many meteors.

"I have 20/20 vision, and the limiting magnitude was +6 for both me and the video recording system," Phillips has said. "It was a fair competition, but the camera recorded many more meteors than I did. The ones that I missed tended to be faint, short, and fast moving. When I played back the tape they were there, as clear as day. If lots of amateurs begin using recording devices like this, we may discover all sorts of new things about meteor showers."
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Author:Rao, Joe
Publication:Sky & Telescope
Geographic Code:1USA
Date:Nov 1, 1999
Words:3693
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