Heavenly half-storm nears

This year has been a bit of a bust for meteor showers. For the normally good Perseids, back in August, a third-quarter moon hindered a mediocre shower. But what could prove to be the best meteor shower of the year comes Nov. 17 with the Leonid meteor shower.

Meteor showers occur when Earth passes through the debris baked off a comet. Comets are dirty iceballs 10 miles across or so. When a comet comes into the inner solar system, the sun’s tremendous heat vaporizes the surface layer of the comet, releasing any trapped dust and rock chunks.

When that debris trail slams into Earth’s atmosphere, it creates a meteor shower. Earth reaches the same spot in its orbit on the same day each year, so meteor showers are annual events.

Some readers may remember the Leonid meteor storms in 1999 and 2001. A meteor storm is defined as 1,000 or more meteors per hour.

"On Nov. 17, we expect the Leonids to produce upward of 500 meteors per hour,” says Bill Cooke of the NASA Marshall Space Flight Center. "That’s a very strong display.”

That would make the 2009 Leonids a half-storm, like what occurred in 1998. But there is a difference between this year’s Leonid shower and 1998’s. That half-storm occurred when Earth passed through a stream of particles released from the parent comet Tempel-Tuttle in 1333, and it was rich in nugget-size pieces, which create dramatic fireballs.

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