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Leonids 2002: the grand finale.

This year may offer our last chance this century to witness a Leonid meteor storm.

There will be a full moon this year, and that will kill most of the meteors; maybe we shall see only a thousand per hour!

--ROY K. MARSHALL

THESE WORDS, BY A WELL-KNOWN Sky & Telescope columnist of yesteryear, were used to describe the viewing prospects for the anticipated storm of Giacobinid meteors in October 1946. Yet Marshall's wisecrack, made more than a half century ago, could just as well sum up the situation we face with the Leonid meteor shower of November 2002.

As most Sky & Telescope readers know, the Leonids are the dross of Comet 55P/Tempel-Tuttle. A meteor stream is like an enormous highway of moving particles, strung out along the entire elliptical path of the comet, and we become aware of only those particles that actually encounter our planet (a comparatively tiny target--a mere dot in space). The geometry of the encounter makes them appear around November 18th each year from the direction of the Sickle of Leo, hence the name "Leonids." We normally see no more than about 10 meteors per hour.

But that's just an average Leonid shower. Every third of a century, for several Novembers in a row, there is the chance that the Leonids will put on a truly prolific showing. There can be hundreds or even thousands of shooting stars per hour. In fact, we are in this window of potential Leonid storms right now. Each time Comet Tempel-Tuttle sweeps through the inner solar system and passes nearest to the Sun, it spews new sets of particles into space that quickly elongate into long, narrow, extradense ribbons of debris. Such ribbons of meteoroids likely have a complex structure and almost certainly contain denser sheets or clusters, each perhaps spawned by an individual outburst on the comet nucleus. A typical dust trail must be at least several astronomical units (Earth-Sun distances) long to have been encountered for several years running, but it is probably only a few Earth diameters thick.

Leonids appear ultrafast compared to the meteors of other showers, for this stream's particles enter our atmosphere at 71 kilometers per second (near the theoretical speed limit for particles belonging to our solar system). Because of their tremendous speeds, Leonids can be extremely bright and are often tinged with hues of blue or green. Roughly half leave luminous vapor trains--some hanging in the air five minutes or more.

The curtain on the current Leonid saga rose in November 1998, just eight months after Comet Tempel-Tuttle passed perihelion. On the night of November 16-17 that year, practically the whole world witnessed a remarkable and unexpected 18-hour bombardment of brilliant fireballs, at times numbering up to a few hundred per hour. In 1999 fewer fireballs were seen, but they were replaced by a true storm of more typical Leonids with rates reaching one per second over Europe, Africa, and the Middle East. A very good display (but no storm) appeared in 2000. Then in 2001, not one but two Leonid storms materialized (see the diagram at left).

And now, in 2002, we prepare for what indeed will be the grand finale. It's one final opportunity to see Leonid displays capable of producing rates in excess of 1,000 per hour--one last chance, for probably a very long time, to see a Leonid storm.

Getting the Big Rocks

What made last year's Leonid storms so successful, especially in the eyes of the general public, was not just their rate of appearance but the unusual brightness of the meteors observed. Indeed, even from large cities perpetually plagued by severe light pollution, many bright meteors could be readily seen streaking across the sky.

Obviously, the larger the particle, the brighter the streak it creates. From a study compiled a decade ago by investigators Peter G. Brown and James Jones (University of Western Ontario, Canada), Leonid particles shed by Comet Tempel-Tuttle can be separated into three basic categories. The first contains particles with a mass of approximately 1 gram, and these would correspond roughly to fireballs with a peak brightness of magnitude -5. Those with a mass of 0.01 gram would correspond to visual meteors of magnitude 0, while the smallest particles of 0.001 gram are associated with meteors of around magnitude +5.

When particles are initially ejected into space by the comet, there is a likely preponderance of those in the 0.01- to 0.001-gram range. As such, during its first few revolutions around the Sun, a "fresh" dust trail is likely to produce mostly meteors that are moderately bright to rather faint. But as the trail ages further, the large meteoroids of 0.01 to 1 gram begin to predominate, partly due to orbital perturbations but mainly because they are less affected by the solar-radiation pressure that, with the passage of time, sweeps away their smaller brethren.

The unusually bright displays in 2001 may have stemmed from the age of the particular dust trails that Earth encountered. The trail that produced the storm over North America originated during the comet's perihelion passage in 1767, or seven revolutions ago. The second storm, witnessed in eastern Asia, arose from a combination of dust trails ejected by the comet in 1699 (nine revolutions ago) and 1866 (four revolutions ago). Interestingly, the four- and seven-revolution trails will again pass very near Earth in 2002.

In contrast, during the 1999 Leonid storm that occurred over Europe and Africa, most observers were struck by an obvious lack of bright meteors. The dust trail responsible for them was ejected by the comet in 1899, just three revolutions ago, and can be considered relatively young.

Our Leonid Guide for 2002

By far the best forecasts of Leonid outbursts during the past three years, in terms of the intensity and timing of peak activity, have been those issued by Esko Lyytinen of the astronomical society URSA in Finland and Thomas Van Flandern of Meta Research, Washington, D.C. Last year, for example, Lyytinen's Leonid model came within just 1 minute of anticipating the maximum of the nine-revolution trail, 4 minutes for the four-revolution trail, and 11 minutes for the seven-revolution trail. The 2002 Leonid predictions have been improved with the assistance of Markku Nissinen, also of URSA.

Equally well known for their past Leonid forecasts are David Asher of Armagh Observatory in Northern Ireland and Robert McNaught of the Australian National University and Siding Spring Observatory. Several years ago they developed their now-famous "dust trail" model of how meteor streams evolve in space. This model received worldwide acclaim after it predicted the peak of the 1999 Leonid storm to within only 6 minutes.

Based on past track records, we feel that the Lyytinen and Asher models will serve as our best guides to how strong the Leonids will become in 2002 and when these peaks in activity will occur.

It turns out that the older four- and seven-revolution dust trails, which performed so well in 2001, are to be encountered one last time in 2002. This set of circumstances holds promise for producing not only large numbers of meteors, but also many bright ones. As we have already suggested, the drawback for all prospective Leonid observers this year--and a huge one--will be the Moon. It will be only hours from reaching full phase, hampering observations under even the clearest skies. For every magnitude step of naked-eye stars hidden by bright moonlight or light pollution, the number of meteors is cut by roughly 60 percent. Thus, if your limiting magnitude is 4.0, you will see only one-tenth the meteors you would see if it were 6.5. But, as we'll explain below, moonlight may not be as grim a hindrance as it first seems.

Storm No. 1. The seven-revolution trail will be encountered first. There is excellent agreement between the Asher and Lyytinen models as to the time when peak activity should occur. They put the peaks at 4:00 and 4:03 Universal Time, respectively, on November 19th.

The Lyytinen model forecasts a zenithal hourly rate (ZHR) of 3,500 for this particular trail, as shown in the bottom diagram on page 96. (The ZHR is the number of meteors that a single observer would see if the radiant were at the zenith in a sky dark enough for 6.5-magnitude stars to be seen. Actual conditions are rarely this good.) The Asher model, however, provides two possible values for this trail. The logic that led to this reasoning is that part of the trail encountered by Earth last November may have been previously disrupted, becoming more and more "stretched out" in space. If what was observed over North America last year reflected such a disruption, then the ZHR for this year should be 1,000. "However," adds McNaught, "if the trail parameters and observations made in 2001 truly represent this dust trail, then the encounter in 2002 should be scaled upward by a factor of 3 (hence to 3,000). Overall, I'd say 1,000 to 3,000 for this trail in 2002!"

Both models also indicate that the seven-revolution dust trail responsible for this first storm will be the richer in bright meteors.

As can be seen in the left-hand diagram above, western Africa as well as western and central sections of Europe are the favored viewing locations. The region that offers the most promise appears to be a strip of northwest Libya (near Tripoli) as well as the adjacent coast of the Mediterranean Sea, where the Leonid radiant will climb to at least 70[degrees] altitude while the bright Moon hangs low over the western horizon. Morning twilight will begin very shortly thereafter.

But wait! Take a closer look at the visibility map. Notice that near the upper-left edge, just protruding into view, are the Maritime Provinces of Canada and the northeastern sections of New England. Here the Leonid radiant will be rising into view above the east-northeast horizon just as the Leonids generated by the 1767 trail reach their peak. This special circumstance could lead to the appearance of Earth-grazing meteors, due to meteoroids that skim through our atmosphere along a path nearly parallel to Earth's surface.

Observers in Newfoundland and Labrador will have the radiant roughly 15[degrees] high at the peak. For Nova Scotia and Prince Edward Island it will be 10[degrees] up, on Cape Cod 5[degrees], while in Greater New York the radiant will be just emerging above the horizon. If this first peak really occurs near 4:00 or 4:03 UT on November 19th, it is not out of the question that a brief bevy of Earth-grazing Leonids may be visible from parts of New England and eastern Canada. Keep in mind that for New England this peak is due on the previous calendar day, November 18th locally, at about 11:00 p.m. Eastern Standard Time. (For the Canadian Maritimes the corresponding time is midnight, and for Newfoundland it is 12:30 a.m. on the 19th.) Unfortunately, the Moon will be near its highest point in the southern sky around this time as well.

Storm No. 2. The 1866 (four-revolution) trail is expected to arrive about 612 hours after the 1767 trail. During that interval, Earth will have rotated to bring much of North America into the prospective viewing zone. As to intensity, the Lyytinen model forecasts a ZHR of 2,600, while the Asher model predicts 3,500. There is again excellent agreement between the two teams as to the time when peak activity should occur; Asher and McNaught cite 10:36 UT, while Lyytinen and Van Flandern give 10:40 UT.

The right-hand Earth view shows that virtually all of North America will be favored. Only near and along the Atlantic Seaboard will advancing morning twilight be an issue. The farther east one goes, the brighter the dawn sky will be. Even so, all viewers are well advised to keep a Leonid watch right up through sunrise, since at past Leonid outbursts brilliant fireballs have been perceived in bright twilight, even broad daylight.

During the 2000 Leonids a last-quarter Moon stood less than 10[degrees] southwest of the Leonid radiant, greatly hindering viewers. In 2002 the Moon will be full and thus about 10 times brighter, but it will also be 10 times farther away from the Leonid radiant (about 100[degrees] away, in eastern Aries). Moreover, in eastern North America where this second peak is forecast for near the break of dawn, the Leonid radiant will stand high in the south-southeast as the Moon sits low toward the western horizon. In the Florida Panhandle, observers near Tallahassee can expect maximum Leonid activity as the radiant appears at its highest, just over 70[degrees] up, as morning twilight sets in.

Van Flandern has developed a special plan to defeat the Moon's adverse effect. "The basic idea," he says, "is to place the observer just east of a high mountain that will block moonlight from view. The major point is not so much that the Moon be invisible (any nearby tree or building could do that much) but to also block most of the scattered moonlight in the surrounding air mass above the observer." Van Flandern plans to lead an expedition to a specially selected observing site west of Asheville, North Carolina, where the nearby high Alleghenies will provide a longer, deeper shadow in moonlight than anywhere else in the zone of optimum meteor visibility. "My best guess would be that our site may be the only one with a good chance to reach observed meteor rates above 1,000 per hour during the storm." Details of this Leonid expedition can be found at http:// eclipseedge.org.

Just a few years ago, I might have added a disclaimer here. I would have emphasized how risky the business of meteor forecasting is, especially for a shower like the Leonids with such a checkered history. But the spectacular verifications of the dust-trail models in the past few years have pretty much taken the guesswork out. To this end, David Asher reminds us, "We now know the dust trails from 1767 and 1866 will be there when the Earth reaches the relevant times this November, and we'll get big meteor displays. There could be surprises in the exact ZHR profile, but we know the trails exist and where they are."

No More Stormy Times Ahead

After 2002, however, there will be no more Leonid storms for a very long time. Donald K. Yeomans of NASA's Jet Propulsion Laboratory as well as Brian G. Marsden and Gareth V. Williams at the Minor Planet Center in Cambridge, Massachusetts, have calculated the path of 55P/ Tempel-Tuttle through future perturbations. They all find that as the comet approaches the Sun toward a May 2031 perihelion, it will pass within 1.5 a.u. of Jupiter in August 2029. This encounter will pull the comet closer to the Sun and increase the distance between Earth's orbit and the comet's to 0.0162 a.u.--their largest separation since 1733. Such a gulf between the two orbits may preclude any substantial meteor activity for the year 2031, and for several years thereafter, when the next cycle of Leonid storms would normally be expected.

In examining this next Leonid cycle, McNaught has found three outlying dust trails that the Earth will approach in the years 2033 and 2034. "Unfortunately," he notes, "they are probably too distant for any reasonable chance of high activity."

There will be little improvement at the comet's subsequent return in 2065, for the separation between the orbits of the comet and the Earth will have diminished only slightly to 0.0146 a.u.

In 2098 the separation of the orbits shrinks to 0.0062 a.u. And in 2131, for the first time since 1633, the comet crosses our orbital plane slightly outside the Earth's orbit at a distance of 0.0089 a.u. Not until one, or both, of these remote years can our great-grandchildren expect to witness a storm of Leonid meteors.

JOE RAO serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes a Sunday astronomy column ("Sky Watch") for the New York Times, and he is also an on-camera meteorologist for News 12 in Westchester, New York.
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Author:Rao, Joe
Publication:Sky & Telescope
Date:Nov 1, 2002
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