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Will the Perseid Meteor Shower Ever Run Dry?

Earth's annual passage through the debris stream left by Comet Swift-Tuttle is not likely to make a dent in the meteor shower's intensity anytime soon
Perseid meteor from 2008



Nick Ares/Flickr

One of the biggest sky shows of the year peaks the evening of August 12, when the Perseid meteor shower will be most active in the night sky. The Perseids, which may deliver as many as 80 meteors per hour, should also be visible on the night of August 13.

The Perseid shower reaches its peak once a year, in mid-August, when Earth's orbit carries the planet through the debris stream left behind by Comet Swift-Tuttle, a 26-kilometer body that sheds ice and dust as it orbits the sun. As Earth approaches the tiny bits of cometary debris, the meteoroids burn up in the atmosphere, generating the bright streaks in the night sky that are often known as shooting stars.

But with each orbit of Earth, the annual meteor shower carves out a portion of Comet Swift-Tuttle's debris stream. So will the Perseids eventually exhaust the reservoir of meteoroids? To find out more about the origin and future of the Perseids, we spoke to Bill Cooke, the Meteoroid Environment Office lead at the NASA Marshall Spaceflight Center in Huntsville, Ala.

[An edited transcript of the interview follows.]

What is the origin of annual meteor showers, such as the Perseids?
Most of the annual meteor showers are caused by debris left behind by comets. The Earth in its orbit will run into these streams from the comets that have deposited debris near us. Basically meteor showers are caused by ice particles left behind by comets, and we run into them.

One wouldn't think of ice particles burning up when they hit the atmosphere, but that is what happens?
Oh, they do. The Perseids, for example, hit the top of the atmosphere at 130,000 miles per hour [210,000 kilometers per hour]. Pretty much anything is going to burn up at that speed.

How big are most of these particles?
Most of the ones you see will be somewhere around a few millimeters across. So not big at all. The very bright ones might be a couple of inches across.

So, the reason that a meteor shower such as this one happens annually is that the debris stream is more or less stationary with respect to the solar system?
Yeah, that's a pretty good way to think of it. The Earth goes around its orbit, and the stuff with the debris that intersects Earth's orbit stays pretty much in the same spot. There's slight movement now and then but it's pretty much in the same place. So that's why the Perseids occur every year, in mid-August.

How recently did the comet that left this debris tail actually pass by?
It was last here in 1993, and it takes about 133 years to go around the sun. It was actually discovered during the Civil War in the early 1860s.

And each time Comet Swift-Tuttle makes a loop, it replenishes this debris stream?
It does. It deposits more debris.

The stuff that we'll encounter tonight is thousands of years old. It's kind of lost its identity. There may be an encounter with some stuff that was deposited in the year 1479.

The deal is, every time the comet comes around the sun its orbit is slightly different. So instead of one stream, there are many streams, and they kind of meld together. So the debris stream left behind in 1479 would be slightly different from the debris stream the comet left behind in 1993. But as time goes along, these debris streams disperse and form the main stream of Perseids that we see.

Some of the younger streams, like this 1479 stream, may be young enough to retain their identity so that we may see a sudden spike at a given time in the number of meteors.

What is unique about that particular stream from 1479?
Nothing, except we are projecting that the Earth will pass close to it. When you look at our computer models, we are expecting to pass close to that stream this year.

Do we pass through the same stream every year?
Well, we pass through the main stream every year. But the younger streams that we pass through, such as the one from 1479, vary from year to year.

Will the comet and its debris stream ever run out of material to generate the Perseids?
This meteor shower has been recorded as far back as 36 A.D. So the Perseids have been seen for close to 2,000 years, and it's at about the same rate today that it was then.

The Perseids are not like the Geminid shower in mid-December, which suddenly turned on in the 1800s and could go away again in the future [as Jupiter pulls on the debris stream]. The Perseids are here to say, at least for the foreseeable future.

What kind of information goes into the projections of how many meteors per hour viewers should see?
With the annual showers, such as the Perseids, we have two ways of doing it. One is simply past experience. The Perseids are a very active shower, and we know from many years of observations that they have a range of anywhere between 70 to 100 per hour.

On occasion, such as back in 1993 and more recently in 2009, the Perseids will outburst, and the rate will jump to two or three times what is normal. In order to predict that, we use a technique that we call stream modeling, where we model the comet in a computer and we throw particles off of it and literally track those simulated meteoroids for thousands of years to find out when they intersect Earth. By using this technique, we can forecast the timing of meteor showers very accurately, and we can also get an idea of how intense the shower will be.

The models aren't perfect, but they give us a good indication of whether it will be a normal year or an outburst year. This one looks to be a normal year.

On average, how do the Perseids rank against other meteor showers?
It's the second best in terms of number of meteors and brightness of meteors. The best of the year is the Geminids, which has meteors in excess of 120 per hour. But the problem with the Geminids is it's mid-December, and no one wants to get out and freeze their rear off to observe it.

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