The Leonids Return

© 2002 by Robert C. Moler

The Sky chart above is looking nearly southward at 5:30 a.m. EST on Tuesday November 19th. It is about the time the Leonid meteor shower will reach its peak. The famous meteor storms of this shower have, over the fast 5 years, thrilled people all over the world. Now it's North America's turn.

The meteor storm will rise quickly and reach its peak around 5:30 a.m. The bad news is that the moon will be nearly full. Still, up to a thousand meteors may be visible an hour. This meteor storm is the second of two expected that night. The first, which is projected to reach peak at 11 p.m. will occur just before Leo rises. The radiant point will rise about 11:30. Perhaps some long meteor streaks then would be visible from our location as the meteoroids just skim the atmosphere overhead from us.

According to astronomers David Asher of Armagh Observatory, Northern Ireland, and Rob McNaught of the Australian National University. This is next to the last hurrah for this go around of Comet Tempel-Tuttle. The comet last passed close to the earth and sun in 1998. The last projected not quite meteor storm will be in on November 19, 2006 with a projected 100 meteors an hour that will be best seen in Europe and Africa.

What is the reason for these meteor storms? Meteor showers and meteor storms are caused by debris liberated by comets when they come close to the sun. Comets are frozen bodies of gasses with imbedded dust and bits of rock.

Comets originally come from the very outer part of the solar system, a nearly spherical volume called the Oort comet cloud, named after Dutch astronomer Jan Oort who proposed it. It is thought that this cloud extends far beyond the planet Pluto.

Astronomers surmise that every so often a star passes close enough to the sun to pass through the Oort cloud. This scatters the comets near it in all directions. Some are ejected from the solar system entirely, but some are sent towards the sun. If a comet comes close to a planet, especially Jupiter, it can be captured in a smaller orbit of the sun. Such is the case with Comet Tempel-Tuttle, which now has a period of only 33 years.

Every time the comet passes close to the sun it is heated causing the frozen gasses to sublimate (turn directly from a solid to a gas), liberating the embedded bits of dust and rock. The heating action occurs on the sunlit side of the comet. So the larger particles, the ones we will see as meteors will be ejected towards the sun. This has the effect of increasing their velocity slightly with respect to the sun compared to the comet. These particles, then will eventually travel farther from the sun than the comet, and have a longer period. Thus they return to the vicinity of the sun after the comet does. This is why the big meteor storms occur after the comet passes through.

The material of each pass of the comet slowly disperses along the orbit. After 6 or 7 orbits after being released by the comet, the material is pretty much strewn all over the orbit. Before that the bits of material are still compact enough to provide a meteor storm. The material the causes the meteor storm at 11 p.m. on the 18th was liberated in 1767. The 5:30 a.m. storm on the 19th, is from the 1866 pass of the comet by the sun.

One of the reasons why the Leonids show this activity is that the comet reaches perihelion, its closest to the sun, when it’s closest to the earth. The particles released by the comet will tend to return to the location they were liberated from.

Let’s hope for clear skies, and a great show.

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Updated 10/28/02