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 October, 1999 Wil's
column appears on the
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space reference skywatching space
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RETURN
OF THE LEONIDS
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 This illustration, among the most famous depictions of the 1833 Leonid meteor shower, was produced some 50 years after the event. |
In 1833, people woke to a sky that seemed ablaze. The hubbub was over the Leonid meteor shower, set for a possibly grand encore Nov. 17.
By Wil Milan for explorezone.com
The Earth may be about to witness the greatest show ever visible in our skies, a very rare event which, if it comes off, will be the headline of every newspaper, cause many people to panic, and leave everyone in awe. But to understand what is happening, let’s first talk about bees:
Bees on the windshield
Imagine that you are driving at 60 miles per hour down a rural highway. Unbeknownst to you, a swarm of bees is crossing the highway just ahead of you. In the blink of an eye your vehicle plows right through the middle of the swarm, and in that instant dozens of bees are splattered over the front of your car.
Had you arrived at that point on the highway only seconds later, you would have encountered only the trailing portion of the swarm and struck only a few bees. Had you been even later than that you might only have hit one or two stragglers and missed the main swarm altogether.
Bigger and faster "bees"
 This Leonids fireball, a splattering meteor, was photographed by James Young at JPL's Table Mountain Observatory in 1966. IMAGE: NASA/ARC/James Young |
Striking a few bees with your car is not a big deal, and we wouldn’t give it much thought. But much the same thing happens on much larger scale, not with your car, but with the entire Earth. The Earth travels around the Sun at a tremendous speed, over 100,000 kilometers per hour (about 67,000 miles per hour or about 18 miles per second).
The space through which the Earth travels is largely empty, so there is not much for the Earth to hit as it speeds along. But it is not totally empty; there are scattered bits of dust and the occasional little rock, and when the Earth collides with one the dust grain or small rock plows into the atmosphere at a combined speed sometimes reaching hundreds of kilometers per second.
At that speed the friction of the object passing through the atmosphere is so intense that the tiny object is instantly heated to many thousands of degrees. The heat is so intense that the tiny grains of dust are completely burned up in a fraction of a second, leaving only a momentary bright streak and a bit of vaporized ash floating in the air. If it were night and had you been looking in the sky at that moment you would have seen what is sometimes called a "shooting star," the flash of a meteor burning itself out in the upper atmosphere. On any given night under dark skies you can see a dozen or more bright meteors, heaven’s reward for having your eyes on the sky.
Bigger swarms
But sometimes the Earth encounters not just random bits of dust, but a more concentrated swarm. In those cases many more than a dozen meteors will be seen, and the rate at which meteors are seen may rise to 30, 50, even over 100 per hour. These are called meteor showers, and they are recurring events that take place at the same time each year. Thus the Lyrids meteor shower occurs in April 21-22 of each year, the Perseids on August 11-12, the Leonids take place the night of Nov. 17 each year, the Geminids on December 13-14, and so on.
(The names "Lyrids," "Perseids," etc. refer to the constellation from which the meteors appear to originate. The Lyrids appear to come from the constellation Lyra, the Perseids from Perseus, etc. They don’t really come from the constellation, of course; it’s only an optical illusion due to the combined directions of travel of the meteors and the Earth.)
The reason that the dates of meteor showers remain the same from year to year is that on those dates the Earth reaches points in its orbit where there are known concentrations of space dust and debris. These bits of dust and debris are not stationary – nothing in space is truly stationary -- but they are in orbit about the Sun in a racetrack pattern, just as the Earth follows its own "racetrack" around the Sun. But because the two "racetracks" cross each other, each year when the Earth reaches the point where the orbits of the Earth and the dust swarm cross each other, the Earth plows through the thin trail of dust and for a few hours we see more meteors in the sky.
The source of the swarms
What causes these "racetrack" trails of dust is comets. Comets are clumps of dust and ice a few miles across and they are very loosely held together. As they travel in their orbits about the Sun they are continually scattering dust and debris in their wake, and over time the path of their orbits become one continuous trail of thin dust and debris. The orbits of most comets don’t cross the Earth’s orbit, but when one does then the Earth, in subsequent years, will cross the dust trail of the comet and a meteor shower will occur at that point each year.
 The comet Tempel-Tuttle, responsible for the Leonid meteor shower, is seen here as it appeared during its closest approach to the Earth. This image was obtained on Jan. 18, 1998, when the comet was 1.18 AU from the Sun and 0.36 AU from the Earth. The glowing head, called a coma, extends at least 30,000 miles from the nucleus, but no tail is evident. The long star trails are due to the rapid motion of the comet as it passes the Earth. IMAGE: Lowell Observatory |
Thus it is that every meteor shower is believed to be associated with a comet. In some cases the parent comet of a meteor shower can be clearly identified: The Perseids appear to be associated with comet Swift-Tuttle, the Leonids with comet Tempel-Tuttle, and both the Orionids and Eta Aquarids meteor showers appear to be associated with Comet Halley (because the Earth crosses Halley’s orbit in two places). In some cases the parent comet is unknown and believed to be long extinct, but the dust trail remains to mark its former orbit.
A near miss
So what would happen if the Earth did not cross the comet’s trail far back from the comet, but rather very close behind the comet? In other words, what if we crossed the comet’s orbit right after the comet had just passed? Obviously we would be passing fairly close to the comet, and there would be more comet dust and debris to encounter.
What happens in those cases is that the meteor shower that would normally take place becomes much more intense. The meteor rate may increase from one or two a minute to tens or hundreds of meteors per minute, and there have been instances when hundreds of meteors per second have been seen for short periods of time.
Those very rare cases where the rate reaches dozens or hundreds per minute are known as "meteor storms," and the meteor storm that is credited with launching the modern study of meteors occurred during the Leonids meteor shower on the night of Nov. 12-13, 1833. Meteor storms had been observed before, and just the year before the Leonids had put on a spectacular show, with one observer in Boston counting over 8,000 meteors in only 15 minutes.
But what occurred when the Leonids returned in 1833 was far beyond what anyone had ever seen or even imagined possible. For several hours over the United States there was a continual blaze of thousands and thousands of meteors at a time. One estimate was that over 240,000 meteors fell during that period, so many meteors in the sky at a time that many people were woken from their beds and stared at the sky in panic, believing the sky to be on fire. Many feared that it was the end of the world and dreaded what they would see at daybreak.
At daybreak, of course, everything was back to normal. Hollywood movies notwithstanding, meteors typically vaporize in the atmosphere, a few drop harmlessly to the ground, and there is only one known incident in history when a meteor struck someone (and she only got a bruise from it). The only living thing ever known to have been killed by a meteor was a very unlucky dog in Egypt many years ago. You are more likely to be struck by lightning seven times in a row than to be hit by a meteor.
The show returns
When the Leonids returned in 1834 it was again a good meteor show, but nothing like the sky-on-fire spectacle of 1833. The great meteor storm was back 33 years later in 1866. Astronomers predicted that the meteor storm would return every 33 years, but it failed to materialize in 1899 or 1933.
Astronomers began to think that perhaps the great meteor storms would not be repeated, but right on time in 1966 the great meteor storm was back, particularly over the western United States. During a peak period which lasted less than an hour there were hundreds of meteors in the sky at once, and rates as high as 40 per second were observed.
Orbital observations by then had pinpointed the source of the meteor storm as Comet Tempel-Tuttle, which has a 33-year orbit. Those occasions when the meteor storm occurred were linked with times when the Earth had passed close behind the comet in its orbit. But the theory is not fully worked out; though there clearly is a link with the comet’s position, there is no good explanation why there was no meteor shower when we passed close to the comet in 1899 and 1933.
33 years later …
You’ve probably already done the math: The last Leonids meteor storm was in 1966, it appears to take place at 33-year intervals, and 33 years later is this year, 1999. And so it is that astronomers everywhere are eagerly awaiting the night of Nov. 17/18 of this year. If the pattern holds, on that night we may again witness perhaps the grandest spectacle in the sky, a great meteor storm that for a few minutes to a few hours sets the sky on fire with thousands of meteors and fireballs.
There is already some evidence of a build-up in Leonids. Though nowhere near the rate of a true meteor storm, last year’s Leonids meteor shower was much stronger than usual, with rates several times the norm. Several locations around the world reported rates of several hundred meteors per hour. At my location in Arizona several of us were treated a great show; as daybreak approached the rate was increasing and we could often seen several meteors in the air at once. Still not a meteor storm, but a great harbinger of a great event in 1999.
How to see the show
The Leonids meteor show will be visible the night of Nov. 17 through the early hours of the morning of Nov. 18, and to see the event you need only one thing: dark skies. Bright city skies make it impossible to see most meteors, so you’ll need to head out to the country where the skies are clear and dark. No equipment of any kind is required, and in fact telescopes and binoculars only hinder the view. Bring warm clothes, a comfortable chair, then sit back and enjoy the show, but do be prepared to stay awake past midnight. There is usually little activity in the early evening, but after midnight (when the part of the Earth where you are located is on the "front" of the Earth as it travels around the Sun) the activity will pick up and often accelerate as daybreak approaches.
There is no guarantee that the 1999 Leonids will be a spectacular meteor storm as in 1966 and 1833, but if the pattern holds, this is the year it is most likely to occur. Even if the great meteor storm doesn’t develop, some kind of meteor shower is guaranteed to occur, and that alone is worth the watching. Any night under the stars is a great experience, and enjoying a meteor shower with a few friends and warm drinks under a clear sky is one of the finest ways to spend a few hours. A meteor shower is a fireworks show that Man can never match, and if the great meteor storm does develop, for most people it will be the most memorable event they have ever seen.
Enjoy the show, and don’t forget to bring the hot chocolate.
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Explore Comets
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Wil Milan (wmilan@airdigital.com) is an astrophotographer based in Arizona. He will be out under the stars on the night of Nov. 17, hot chocolate in hand. Some of his work can be seen at www.astrophotographer.com.
Will
Milan's Universe appears on the first Monday of each month.
