November 2000

Meteors!

Imagine a comet.

The classic descriptive phrase is a "dirty snowball," but that's not perfect. First, there aren't many snowballs ten kilometers across. Second, snow is frozen water. Comets also have frozen carbon dioxide, methane, ammonia, and a few other chemicals. Third, snow is too dense. Comets are delicate, gossamer objects. Cometary material makes snow look like lead. Fourth, "dirty" isn't a strong enough word. These things are saturated with carbon and silicates, making them as black as tar.

Have you seen that nasty snow by the side of the road after the plow has been by? It's crammed full of gravel and bits of dirt and stained with who-knows-what. Cometary material is a little like that.

Most of the time, comets spend their time in the supercold outer reaches of the solar system. Out there, they are frozen solid, inert and unchanging. But when one enters the inner solar system, sunlight starts to work it over, heating it and vaporizing the ice on the surface. The ice, now steam (and charged particles), expands and is pushed away from the comet by the solar wind, making the familiar tail. A good tail can be ten million kilometers long, at least.

Remember all those bits of rock that were lodged in the ice? When the ice vaporizes, they're free! The comet doesn't have the gravity to hang on to most of them, so they keep drifting, entering orbits around the Sun that are similar to the comet's orbit.

We're talking about tiny pebbles, smaller than marbles and bigger than grains of sand. Trillions of them. The solar system is full of them, but you can find high concentrations of them along the orbital paths that comets follow.

Not surprisingly, the Earth nails these things all the time. As they zip through the atmosphere, they compress and heat a tiny region of air in front of them. This hot air glows and disintegrates the pebble. You see it as a streak of light across the sky.

"Shooting star!" you might say. But if you know better, you'd say, " Meteor!" At a dark location, you might see a couple per hour, on any night.

But what about those high concentrations of pebbles that follow the comets around? If a comet's orbit intersects the Earth's orbit, and the comet has passed by recently, then there could be a lot of junk in the way when the Earth moves through there. The result: we can get quite a show!

And that's what we're in for this month, with luck. Comet Tempel-Tuttle orbits the Sun every 33 years, and passed through the Earth's orbit in 1998. Now, the comet is on its way back to the outer solar system, but its stream of debris is still here. The Earth will pass through it on November 17. Rather than a few dozen meteors per hour, we might see a few thousand per hour for a short time.

Details: The debris stream is narrow, and the Earth can pass through it in an hour. If you're in daylight during that hour, you'll miss it. Also, The odds of hitting a thick clump of the stream (and seeing a meteor storm) is not as good this year as it was in 1998 or 1999, but it won't get better until the early 2030's, when the comet comes around again. So be ready!

You may have heard the name of this meteor shower: the Leonids. That's a good name, because these meteors all seem to come from a particular spot in the sky, in the constellation Leo. Why is that?

Because in late November, the Earth is travelling toward Leo in its orbit around the Sun. The meteoric debris is in the way, and we plow through it. Because of perspective, the parallel paths of the meteors seem to start at a "vanishing point" in front of us, that is, in Leo.

This year, the Moon will appear in Leo during the shower. I wonder if the meteors will look like they are being launched from the Moon.

Good news! The Kitt Peak Visitor Center is having a special Nightly Observing Program on November 17. Twenty people (and no more) will be able to admire the Leonid meteors as they zip through the dark skies of Kitt Peak. The program on that night will be extended until 1 am, prime meteor-watching time! Call the Visitor Center for details at

(520) 318-8726.

Steve White
Nightly Observing Program
Kitt Peak Visitor Center

PS: For more details about the Leonids, click here. -S

PPS: As I write this, it is August 27th, my last day. So this is my last article! Again, thanks for reading, and thanks to everyone at Kitt Peak! -S

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Telescope Operators/Guides for Nightly Program:
Adam Block (ablock@noao.edu) Lead Observer
Steve White (swhite@noao.edu)
Flynn Haase (fhaase@noao.edu)
Richard Barchfield (richardb@noao.edu)

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Updated: 8/27/2000