2006 Leonids
Published on Nov 15, 2006 at 5:52 pm.
7 Comments.
Filed under meteors.
The Leonid Meteors are famous among meteor observers. For much of my life in astronomy, I have heard about the 1966 Leonid storm. The entire sky was lit up with meteors, perhaps a dozen per second. Even that paled compared to the 1833 Leonid storm, which is what really started astronomers looking at meteors. Until that time, most had thought of them as atmospheric rather than astronomical in nature.
But, while the Leonids have had such major outbursts, most Leonid showers are pretty poor. A typical Leonid meteor shower consists of a meteor every 10 minutes or so. This year may be more active than average, according to some predictions, but nowhere near meteor storm level. At best, it will be a pretty good meteor shower.Â
To understand why this may be true, you need to know where the meteors come from. A meteor is basically the flash of light caused by a particle shooting through the atmosphere. The particles themselves are called meteoroids. Most of the meteoroids are tiny — not much larger than sand grains.  When they hit the atmosphere, moving at dozens of miles per second, friction between the meteoroids slows and heats them. They burn up. But, in the process they streak through the air stripping electrons from the atoms of the air. As these atoms recombine, they release energy in the form of light. This light is what you see. You don’t really normally see the meteorids themselves — they are too small, and too far away (they whole event is normally 40 to 50 miles or so up).Â
On any given night, some random meteors will hit Earth’s atmosphere. However, now and then, Earth passes through a swarm of meteoroids, and so you get multiple meteors coming from the same direction. This is a meteor shower. Normally, a meteor shower is only a few meteors per hour from a particular direction. Some meteor showers are far more active, and you get dozens of meteors per hour, and a few meteor showers are very good, yielding 100 or more meteors per hour. Only rare times does Earth pass through a thick enough swarm that you get thousands or more meteors per hour. If the activity reaches or exceeds 3600 meteors per hour (one per second), then you call it a meteor storm.
Since the meteoroids are moving in a swarm, they will appear to come from a particular part of the sky when they strike Earth’s atmosphere. For an observer on the ground, this means that they will seem to radiate from a particular constellation. We generally name the meteor shower by the constellation that it seems associated with. For example, the Leonid Meteors of mid-November appear to radiate away from Leo. December’s Geminid Meteors appear to radiate from Gemini, October’s Orionid Meteors radiate from Orion, and August’s Perseid Meteors appear to radiate away from the constellation Perseus.
But, where do the swarms come from?  In 1866, Giovanni Schiaparelli showed that the orbit of the Perseid meteoroids matched up with that of Comet Swift-Tuttle. Later, he showed that the Leonid meteoroids seemed to have nearly the same orbit as the newly discovered Comet Tempel-Tuttle. But, how do comets produce meteoroids? It turns out that comets are composed of dust, rock, ice, and frozen gasses. When they come close to the Sun, the ices and frozen gasses sublimate, and blow out into space, carrying dust with them.  Many of these dust grains then follow orbits similar to those of the comet.  Over time, the dust grains spread out along the orbit, and every time that the Earth passes through the cloud of dust grains, you get a meteor shower. But, for young comets, the clouds of dust grains haven’t gotten a chance to spread out, so they remain clumped together. Comet Tempel-Tuttle is such a comet. Each time it swings by the Sun, about once every 33 years, it sheds material, and so those meteoroids continue to move along in orbits near that of the comet.Â
So, the comets shed material with each passage by the Sun. The material shed includes gas, dust, and small rocks. The gas doesn’t concern us as far as meteor showers go. The dust and rocks, though, yield meteors. The dust yields small meteors, and the rocks yield bigger, brighter meteors called bolides or sometimes fireballs. Since Comet Tempel-Tuttle seems to be young (as far as such things go), its meteoroid streams are very tightly clumped. So, most of the time, they miss Earth, and we only get stray meteoroids, and hence poor meteor showers. But, when Earth passes directly through one of these clumps, it can be really impressive. But, the clumps are small, and so the Leonids are only impressive for a short time — perhaps a few minutes or hours. If you are in the wrong place, you miss out.
Until recently, forecasting meteor shower activity has been just a wild guess. However, fantastic strides have been made in recent years. David Asher, of the Armagh Observatory, seems to be very good at this. He is predicting that in 2006, Earth will be passing through a region of space containing several clumps of dust and rocks shed during 1833. 
We should be passing through the thickest patch early on November 19. Specifically, his predictions indicate that the peak activity should be about 04:45UT (That is about 10:45pm on November 18 here in Texas). Unfortunately at that time, Leo won’t be above the horizon for most of North America. Most of Europe misses the event, but the western half of Europe would get a good view just before dawn.  The best place to observe the Leonids if his predictions are the case would be Bermuda. 
Here is a map showing where you’d have to be on Earth to see the peak (click on it to get a full sized view).  According to Asher’s predictions, the Leonids should be rather active, with nearly 120 meteors per hour expected. Note that this figure is assuming DARK skies (no light pollution), clear (no haze), and a seasoned observer. Usually, I tell people to expect to see about half of the predicted rate. But, even so, that would be very impressive! Also, just because you are not in the area of prime visibility, don’t count yourself totally out. It turns out that even with the major strides that have been made in predicting meteors, there is still a lot of guesswork. These meteoroid streams might not be exactly where they are predicted to be. Also, gravitational interactions with the planets (mainly Jupiter) move them around a bit. So, the peak may come several hours earlier or later. Given Asher’s track record, though, I’d expect his predictions to be pretty accurate.
So, what should you expect to see? The Leonids come in FAST. They are among the fastest meteors hitting Earth, moving at over 70 kilometers per second. Most should be small, faint, and fast, if predictions are correct. But, depending upon how Jupiter has shoved some of the bigger particles around, there may be a few bolides mixed in. In 1998, the sky was alight with bolides from the Leonids, but I don’t really expect a repeat this year. Most activity, whatever you get, should be after local midnight (That’s midnight local mean time, not clock time. For central Texas, that is about 12:30am, Central Standard Time).  A common mistake is that people go out in the evening and look, but they don’t see any meteors.  Unfortunately, the best activity is almost always from local midnight until dawn. The Leonids are normally not so spectacular, but there is a chance that you might see a better than average show. Even if you don’t see a Leonid storm, or even a major Leonid shower, you should see a few anytime the morning of November 19.
If you want to read all about the history of the Leonid Meteor Shower, a fantastic easy to read book is Mark Littmann’s The Heavens on Fire. Littmann tells of the great 1833 meteor storm that got the whole ball rolling on meteor shower studies and how the connection to comets came about. The book was published in 1998, just before the long anticipated resurgence in Leonid activity, and slightly before much of the most spectacular work on understanding and predicting meteor showers, but it is a wonderful reference for the layman or educator on the history of meteor shower studies.
So, if you’ve got clear skies this weekend, go look for meteors!
-Astroprof
(PS:Â If you are in the North Texas area, feel free to join us at Lake Mineral Wells State park Saturday night at our star party.)
(Images courtesy of Ferris Hall, Lowell Observatory, David Asher (Armagh Observatory), NASA)
A Ler-- Rastos de Luz on November 16, 2006 at 10:43 am: 1
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Scott on November 16, 2006 at 11:07 am: 2
It is a bit of a mistake to say that the incoming meteoroids generate heat by friction with the air. It is really compressional heating of the air, and the hot air radiatively heats the meteoroid. The meteoroid is moving so fast (much faster than the speed of sound) that the air in front of it cannot flow out of the way. The air in front of the meteor gets compressed and lots of compressed air builds up in front of the meteoroid. Compression, of course, raises the temperature of the air.
Astroprof on November 16, 2006 at 12:06 pm: 3
That is correct. Thanks for clarifying that, Scott. I guess that I was getting lazy and pressed for time, and the focus of the post was on the Leonids rather than the mechanics of entry, so I just gave the standard description that you see a lot: friction with the atmosphere (which is how many intro textbooks describe it). But, as you say, is not really a proper description, and saying something that I know isn’t quite right is below the standards that I want to set for this page.
It isn’t really friction heating things up (unless you are very broad in your definition of friction). It is also the pressure in front of the meteoroid that slows it down. This is the same effect as that experienced by spacecraft upon reentry into the atmosphere.
Nam on November 17, 2006 at 7:22 pm: 4
Is the party still happening? Do you expect to be able to see much? I will be in Dallas on 11/18/2006. Thanks.
Astroprof on November 17, 2006 at 7:50 pm: 5
Yes. It it is the annual North Texas Skywatch star party. We are holding it at Lake Mineral Wells State Park, a little west of Fort Worth. There’ll be speakers around dusk, and then telescopes set up until everyone gets tired, or the Sun rises, whichever comes first.
Nam on November 17, 2006 at 8:03 pm: 6
Do you expect to be able to see the Leonids at all?
Astroprof on November 17, 2006 at 8:44 pm: 7
I am hopeful, but doubtful that we’ll see much here in Texas. Of course, meteor predictions are notoriously inaccurate, so we might get lucky.