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The Leonids: King of the Meteor Showers
By Joe Rao
Sky & Telescope
LATE EVERY AUTUMN around November 17th or 18th, in the still, silent hours before dawn, dedicated meteor watchers have long kept a vigil. In the cold early morning darkness the sky glimmers with a preview of early spring constellations. Leading them up in the east is Leo. Its familiar Sickle asterism, a backward question mark, seems especially full of meaning these nights, for on the cutting edge of the Sickle's curved hook is the radiant of the greatest of all meteor showers.
Most of the time nothing happens. Only occasionally does the watcher, lying in blankets against the cold, glimpse a meteor somewhere in the sky whose flight direction betrays its invisible origin in the Sickle. Perhaps 8 or 10 Leonids per hour make it onto the watcher's clipboard. As the first hint of dawn appears he checks the time, makes one final note, and gets to his feet, pleased with the night's gleanings of data but saying, as always, "Not this year."
Meanwhile, invisible in the wastes of the outer solar system, a cloud of meteoroids has been preparing itself for a dash around the Sun. This swarm, perhaps the richest known, has been narrowing, lengthening, and falling with ever increasing speed toward the spot the Earth occupies every November 17-18. In 1966, when it last swept by, the Earth plowed through it head-on. For an hour the upper atmosphere on the forward-facing side of our planet blazed with meteors storming like a fiery rain from the Sickle of Leo.
Now the swarm is heading toward us once more. The next few years will tell whether it will hit us again and fire the sky to streaking luminescence, or whether it will skim by just to one side of Earth, unseen.
Already the signs are good. The Leonid shower picked up strength in 1994, after two decades of hardly qualifying as more than a "minor" shower. In 1994 Leonid rates briefly matched those of the Perseids and Geminids, normally the year's two leading showers. What will happen this November 17-18 and in the next few years is anyone's guess. But we just might be in for the show of a lifetime.
On the night of November 12, 1833, the Western Hemisphere unexpectedly came under attack. A firestorm of shooting stars, silent but overwhelming, filled the sky. Here is a part of Victorian astronomy writer Agnes Clerke's classic description of that incredible scene:
"On the night of November 12-13, 1833, a tempest of falling stars broke over the earth.... the sky was scored in every direction with shining tracks and illuminated with majestic fireballs. At Boston, the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm. Their numbers ... were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed, on the basis of that much-diminished rate, that 240,000 must have been visible during the nine hours they continued to fall."
Apparently the night was clear and starry from Halifax to the Gulf of Mexico and, judging by Plains Indian records of the shower, farther west as well. By midnight some people may have noted an unusual number of meteors streaking from the east. But it was the early morning hours of the 13th that made the greatest impression. A reliable observer, A. C. Twyning at West Point, New York, estimated that at the height of the storm at least 10,000 bright meteors were visible per hour. Another observer, believing that the meteors were stars, thought there would be no stars left in the sky the next night. Some of the meteors were said to be as bright as streaking full Moons.
"Imagine a constant succession of fireballs, resembling rockets, radiating in all directions from a point in the heavens," wrote Yale professor Denison Olmsted, an eyewitness to the great Leonid meteor storm of November 1833. The light and commotion apparently woke most Americans, and the display left a lasting impression on the country; this portrayal, the most famous, was drawn more than 50 years later for a religious primer illustrating biblical prophecies fulfilled (S&T: September 1987, page 252).
The meteor storm made a deep and terrifying impression on the American people. According to newspaper reports almost everyone saw it, awakened either by the commotion in the streets or by the moving glare of fireballs shining into bedroom windows. In 1878 the historian R. M. Devens listed it as one of the 100 most memorable events in U.S. history. "During the three hours of its continuance," he wrote, "the day of judgment was believed to be only waiting for sunrise, and, long after the shower had ceased, the morbid and superstitious still were impressed with the idea that the final day was at least only a week ahead. Impromptu meetings for prayer were held in many places, and many other scenes of religious devotion, or terror, or abandonment of worldly affairs, transpired, under the influence of fear occasioned by so sudden and awful a display."
Woodcuts like these appeared in many newspapers and magazines after the Leonid storm of 1833. This print depicts the shower as seen at Niagara Falls, New York. Notice that the vast majority of the meteors appear to radiate from a single spot on the sky. Mechanics' Magazine said this illustration was made by an editor named Pickering "who witnessed the scene." Many other versions were redrawn from it.
Indeed, the 1833 shower has been credited with contributing to the intense religious revivals that swept the United States in the 1830s, which permanently influenced the national character and spread new sects and denominations that are well established on the American scene today.
The meteor storm burst upon a world largely ignorant of the possibility of meteor showers. Yet records examined afterward showed that the storm could have been anticipated, if not actually predicted. The fault lay as much with the astronomers of the era as with anyone. Up until only some years earlier, they had refused to believe that meteors -- those little streaks of light so commonly seen in the upper atmosphere -- could have any astronomical connection at all. But the shower of 1833 dispelled all doubts. Many observers clearly reported that the meteors seemed to radiate from a spot in Leo and that, as the constellation moved slowly westward during the night, the radiant point moved with it.
Within weeks a Yale mathematician, Denison Olmsted, demonstrated that this radiant point was simply an effect of perspective. The millions of meteors that fell that night had in fact been moving along parallel paths. They appeared to diverge from a point in Leo for the same reason that parallel railroad tracks, or any other parallel lines on the ground, appear to diverge from a point on the horizon.
Following this realization, the meteors were given the Latin family name for their apparent place of origin: the Leonids. This remarkable finding, that meteors are visitors from astronomical realms, was as striking in its own way as the shower itself. It sparked intense study into this new field of astronomy.
Today we know of many annual showers. More than a dozen produce enough meteors every year for observers to consider them worth a special watch, and hundreds of weaker showers have been identified at least tentatively.
All are caused by streams of particles traveling around the Sun in more or less well-defined orbits that happen to cross the orbit of Earth. As the Earth circles around the Sun it passes through each stream at about the same date every year. The side of the Earth facing into the stream gets peppered with meteoroids, which vaporize in the upper atmosphere at altitudes of about 40 to 80 miles. The meteoroids themselves are tiny, generally ranging from the size of large sand grains to small pebbles.
The source of a meteor stream was first identified in 1866 by the Italian astronomer Giovanni Schiaparelli (of Martian canali fame). In that year he established that the orbit of another famous shower, the August Perseids, closely matches the orbit of Periodic Comet Swift-Tuttle (now designated 109P/Swift-Tuttle). In the same work Schiaparelli published his calculations for the orbit of the Leonid stream. Other experts in celestial mechanics, notably Urbain Le Verrier and Theodor von Oppolzer, independently spotted a striking resemblance of this Leonid orbit to that of the newly discovered Periodic Comet Tempel-Tuttle (Comet 1866 I, now called 55P/Tempel-Tuttle). Other matches between comet and meteor-stream orbits were subsequently found. Today, even though not all prominent meteor showers have been matched to known comets, the relationship is clear: meteor streams are the debris of crumbling comets.
The Leonids Roar Back
While the shower of November 12, 1833, sparked the beginning of serious meteor astronomy, it was neither the beginning nor the end of Leonid history. After the event reports were unearthed in which observers from the Urals, Arabia, Mauritius, and Europe, as well as ship captains in the North Atlantic, described large numbers of meteors appearing one year earlier, on November 12, 1832.
Other accounts were brought to light of a shower of thousands of bright meteors on November 12, 1799, described by the Prussian scientist and explorer Alexander von Humboldt from his camp in Cumanã, Venezuela. As he described it, there was "no part of the sky so large as twice the Moon's diameter not filled each instant by meteors." An observer in Florida that same night noted that the meteors were "at any one instant as numerous as the stars," while at Iserstadt, Germany, "bright streaks and flashes" were seen even though day had already broken.
Interestingly, Humboldt's inquiry among the South American natives elicited the information that in 1766 a similar "rain of stars" had also been seen.
After 1833 many astronomers researched the history of November meteors in ancient European, Arab, and Chinese documents. In 1837 the German physician and astronomer Heinrich Olbers suggested that better-than-average displays occurred in cycles of 33 or 34 years. The great storms of meteors had come in November at these intervals, and, he suggested, they could be expected to continue as long as the meteor swarm remained intact.
In 1863 Yale professor Hubert Anson Newton succeeded in tracing accounts of the Leonids for almost a thousand years. Particularly impressive displays were found to have taken place in 1533, 1366, 1202, 1037, 967, 934, and 902. Even these few dates suffice to indicate a periodicity of about 33 years. Indeed, it was later surmised that a dense cloud of matter revolves around the Sun in a period that, in 1866, was established as 33.25 years.
Based on the history of the Leonids, as well as on their association with P/Tempel-Tuttle, astronomers predicted that another major shower would occur in 1866 or 1867. And indeed it did, though it was apparently not as spectacular as in 1833. The rates for a single observer were reported to be 5,000 per hour from Europe on November 13-14, 1866, and about 1,000 per hour (despite bright moonlight) from North America on November 13, 1867.
This behavior is typical of the Leonid shower -- that is, one part of the world may see a tremendous downpouring while elsewhere the event is relatively minor. Apparently a true Leonid storm lasts only a few hours at most. So when it hits, only observers in a few time zones will be lucky enough to be in predawn darkness with Leo high in their sky.
The return of the shower was anticipated again in 1899. In the intervening years the Leonids had produced only modest numbers of meteors, about 30 to 50 per hour visible to a single observer at maximum. But 1899 was another year in the cycle, and fairly wide publicity was given to the possibility of a reenactment of the events of 1867 and especially 1833. Unfortunately nothing much happened, and the faith of the public in the infallibility of astronomical calculations was rather badly shaken. The American meteor expert Charles P. Olivier later wrote: "This was the worst blow ever suffered by astronomy in the eyes of the public." The failure of the shower to manifest itself undoubtedly led to a serious diminution of interest in meteor astronomy.
In fairness to astronomers, however, some did issue cautions in advance. Calculations by John C. Adams and George Stoney in England demonstrated that the swarm of particles passed sufficiently near to Saturn in 1870, and to Jupiter in 1898, that it might be deflected into another orbit. Indeed, by 1899 the orbit of the swarm had been given a severe shift, so that it passed 0.0117 astronomical unit closer to the Sun than the Earth at the supposed meeting point.
After 1899 interest in the Leonids never revived. This was very unfortunate, since almost inexplicably the Leonids roared to life one year later in 1900. On November 15-16 rates of more than 1,000 per hour reportedly caused "a panic" in a small community near Hudson Bay, Canada. Then in 1901, at Pomona College in Claremont, California, Leonids were observed to fall at almost 2,000 per hour for a brief interval, while at Tucson, Arizona, and Tuape, Mexico, the meteors were described as "too thick to count." But since most astronomers and the public felt burned by the flop of 1899, very few witnessed the spectacular displays of the next two years.
If the turn-of-the-century years were disappointing, the 1930s were worse. Even though hourly rates reached 190 in 1931 and 240 in 1932, nothing the least bit like the meteor storms of the past was reported anywhere. In addition, for the second time in a row, the Leonids' parent comet P/Tempel-Tuttle was not recovered, despite a diligent search when it should have been passing through the inner solar system on its 33-year orbit. The consensus was that, like the ill-fated Biela's Comet, P/Tempel-Tuttle had somehow broken apart and had vanished forever, leaving its fragments behind. The fragments of Biela's Comet had produced incredible meteor displays in 1872 and 1885, but these had since diminished greatly. Now it looked as though the once-great Leonids too had finally petered out.
Observers generally ignored the Leonids during the 1940s and 50s, and this state of neglect probably caused many to miss the enhanced Leonid activity in 1961, when observed rates climbed to more than 50 per hour. Many of these were brilliant meteors with long-enduring trains.
From 1962 through 1964 hourly rates of 15 to 30 were recorded. Then in 1965 P/Tempel-Tuttle, lost for nearly a century, was rediscovered (as Comet 1965 IV). Calculations would later reveal that it passed closer to the Earth's orbit (0.0032 a.u., hardly more than the Moon's distance) than on any occasion since 1833. And indeed the 1965 Leonids produced rates of up to 120 meteors per hour as seen from such widely separated locations as Hawaii and Australia. From the Smithsonian tracking stations at Maui and Woomera came reports of Leonids as bright as magnitude -5, at times several in the sky together, leaving luminous trains lasting several seconds. These reports were reminiscent of the brightness, if not the numbers, of the objects seen in 1799 and 1833.
Then one year later, on November 17, 1966, a tremendous storm of tens of thousands of Leonids fell for a short interval timed for skywatchers in the central and western United States. This display probably rivaled the historic showers of 1799 and 1833. Within just two hours, observed rates increased sharply from about 40 per hour to, by various frenzied estimates, 10 per second, 40 per second, or 200 per second!
The Leonid rate per minute throughout the brief but intense 1966 storm, as counted by Dennis Milon and others at Kitt Peak in southern Arizona. Other estimates of the maximum were drastically higher and lower than the 2,400 per minute (40 per second) estimated by this group around 11:55 UT November 17th.
"We saw a rain of meteors turn into a hail of meteors and finally a storm of meteors too numerous to count," wrote Charles Capen in the San Gabriel Mountains of Southern California. Watching from Kitt Peak in southern Arizona, Dennis Milon stated, "The meteors were so intense that we were guessing how many could be seen in a one-second sweep of the observer's head.... A rate of about 150,000 per hour was seen for about 20 minutes." At New Mexico State University Observatory, Thomas B. Kirby and Thomas P. Pope estimated a rate of 200,000 to 1 million meteors per hour. Bradford A. Smith estimated 500,000 to 1 million per hour including the faintest visible in a very dark sky. But even the widely quoted rate of 150,000 per hour may be too high; in a 1994 reanalysis Peter Jenniskens (Dutch Meteor Society and NASA/Ames Research Center) settled on a relatively modest 15,000 per hour (Astronomy & Astrophysics, March (I) 1995).
Whatever the rate, it was spectacular enough. Many people commented that on looking toward the Leonid radiant they got the stupendous impression that the world was hurtling through space -- as in fact it is. It was an extremely rare opportunity to perceive the Earth's motion directly. "I had the feeling that I should be hearing something," wrote Dana K. Bailey in northern Colorado. Said Capen, "Instinctively we sought to shield our upturned faces from imagined celestial debris."
In the aftermath of this incredible display, Canadian meteor expert Peter M. Millman used radar data to determine the thickness of the ribbonlike Leonid stream. It was a mere 35,000 kilometers (22,000 miles) thick, not much more than the Earth itself. This is now generally regarded as the thickness of the debris filament that contained the greatest concentration of meteoroids. The Earth swept through it in just one hour.
One reason the Leonids can be so good. The meteor stream is concentrated near the orbit of Comet 55P/Tempel-Tuttle, which is inclined only 17° to Earth's orbital plane. The comet and meteoroids travel in the direction opposite Earth, so we hit the meteors head-on at 71 kilometers per second -- a lucky arrangement that increases their brilliance and the numbers visible. In addition, the low angle at which we pass through the meteors' orbital plane increases the shower's duration. Inset: The dense ribbon of meteoroids that caused the 1966 Leonid shower was only 35,000 kilometers thick, according to radar studies by Peter Millman. Earth is drawn to scale.
The Leonids provided one more surprise as the 1960s came to a close. On November 17, 1969, Leonids were seen at a rate of four per minute for less than an hour by observers in the northeastern United States. Elsewhere the shower was reported weak.
Now, with P/Tempel-Tuttle due to return to perihelion around February 27, 1998, the cycle is coming around again.
Waiting and Watching
Meteor forecasting is still an inexact science. But like weather forecasting, it is better than it used to be.
Like other comets, P/Tempel-Tuttle is a cosmic litterbug, spreading a long "river of rubble" in front of and behind itself along its orbit. Each particle in this stream orbits the Sun independently in a roughly 33-year period. Many Leonid meteoroids have become widely scattered along and away from the comet's orbit (a narrow ellipse that reaches all the way out to the orbit of Uranus). These stray particles are the ones that produce the ordinary, weak annual Leonid shower, which lasts for several days. But the narrow, densest part of the swarm apparently remains within a few astronomical units of the comet itself, following it around. This narrow ribbon must be several a.u. long, long enough to intersect the orbit of the Earth for several years running.
The locations where meteoroids have been thickest around Comet 55P/Tempel-Tuttle, as revealed by Earth getting an unusually rich Leonid shower on passing through the comet's orbital plane (the plane of the screen). The meteors are closely confined to this plane. The vertical axis is the distance in astronomical units outside (+) or inside (-) the comet's orbit with respect to the Sun. The horizontal axis shows the number of days that particles preceded the parent comet (-) or lagged behind it (+) in orbit. The horizontal scale is compressed about 2,000 times compared to the vertical scale. Large dots indicate where a major meteor storm occurred; small dots mark a strong shower. Open circles show where Earth passes through the comet's plane each year from 1995 to 2001. Adapted from "Comet Tempel-Tuttle and the Leonid Meteors" by Donald K. Yeomans, Icarus, Vol. 47, page 492 (1981).
The weak annual shower typically amounts to no more than 10 meteors per hour even at its November 17-18 peak. Since, like their parent comet, the meteoroids orbit the Sun backward (in a retrograde orbit), they collide with the Earth nearly head-on. They rip into our atmosphere at 71 km (44 miles) per second, faster than particles from any other major shower, producing bright, swift streaks of white, green, and blue. Many leave long-enduring trains.
It's in the years just before and after P/Tempel-Tuttle passes by that we stand a chance of getting hit by the full onslaught.
In 1981 Donald K. Yeomans of NASA's Jet Propulsion Laboratory put forward what many consider to be the definitive study of P/Tempel-Tuttle's orbit and its implications for future Leonid activity. By studying historical data from 902 to 1969, Yeomans was able to map the distribution of meteoric material surrounding the comet. We seem to go almost through the middle of the richest zone.
A lot of factors shape a meteoroid stream over time. If nothing mattered but the particles' ejection velocities from the comet, quadrant IV would be heavily populated. In fact that quadrant is nearly empty. This leads Yeomans to conclude that solar radiation pressure and gravitational perturbations by the planets cause the Leonid particles to evolve rapidly into a position behind the comet and outside its orbit: into quadrant II.
Strong meteor displays, those with hourly rates of more than 100, appear to be possible for about six or seven years before and after P/Tempel-Tuttle's perihelion -- if the comet passes less than 0.015 a.u. inside or 0.010 a.u. outside the Earth's orbit. Indeed, there are signs that activity is picking up. In 1994, despite a full Moon, both visual and radio observations of the Leonids suggested strong activity from 6:00 to 7:00 Universal Time on November 18th -- perhaps with zenithal hourly rates (ZHRs) of 80 to 100. Shelby Ennis is a ham radio operator (W8WN) who has monitored meteor showers by radio for more than three decades. "While not a great shower, [the Leonids] were very good," he noted, "better than I can remember from way back. They were more like the normal peak of the Perseids or Geminids."
The likelihood of a stupendous display appears best when the Earth encounters the particles not far behind the comet in quadrant II, as will happen in 1998 and 1999. It should also be noted that the great meteor storm of 1799 occurred when the Earth brushed past P/Tempel-Tuttle's orbit a little before the comet came by, in quadrant I. Similar circumstances will again prevail in 1997. On the other hand, the great storms of the last two centuries have happened quite close to the comet's orbit, and we're not so near to it this time.
As we have already seen, while the Leonids have the potential to storm every 33 or 34 years, they don't always do so. Recall that when storm conditions appeared favorable in 1899 and 1932 the hoped-for Leonid blizzards failed to materialize. This leaves the spectacular displays that some have predicted for the close of the century rather uncertain. "That's the way it is with meteor showers," says Yeomans. "You can say 'probably,' but if you say 'definitely' they'll get you every time."
In addition, observers should remember that only a small part of the Earth has a favorable orientation at any given hour -- putting Leo well above the horizon in the few hours before local dawn. Trying to predict the exact hour when the Leonid peak will hit in a given year gives astronomers additional headaches.
A big problem is that there may be not just one dense ribbon of meteoroids along the orbit of P/Tempel-Tuttle but several, each having evolved from the debris that crumbled off the comet's nucleus at a different perihelion passage. These ribbons themselves likely consist of irregular clusters, each perhaps spewed by individual outbursts on the comet nucleus. The 1966 and 1969 Leonid displays, for example, may have been caused by particles ejected from P/Tempel-Tuttle as far back as 1767 -- six revolutions ago.
But we can at least make a stab at a time prediction. If we knew in advance the orbit plane of a Leonid ribbon, the calculation would be simple, for the encounter happens when the Earth goes through this plane. Assuming that the meteoroids remain in exactly the same plane as P/Tempel-Tuttle, Yeomans recently recomputing some exact times.
Joe Rao is a meteorologist for News 12 Westchester and an instructor/lecturer at New York's Hayden Planetarium.
© 1998 Sky Publishing Corp. All rights reserved.
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