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The Leonid meteor shower

Higher than usual, but not serious: That's how NASA and the United States Air Force forecast the risk to satellites from November's Leonid meteor shower.

And that's how it turned out. According to the Air Force Space Command, military satellites came through unscathed, while two civilian satellites were briefly disabled. Not bad, given that some 650 satellites orbit Earth. (Both satellites were back in service in a matter of days.)

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The satellites' users probably didn't notice a thing, because satellite owners quickly switched to backup "birds," says Lt. Col. Don Jewell of Space Command headquarters in Colorado Springs, Colo.

The two satellites failed when they experienced the aftereffects of a small lightning bolt that went from the satellite to space.

Such sparks, called electrostatic discharges, occur when a satellite is hit by a tiny meteoroid. Meteoroids travel so fast (about 160,000 m.p.h.) that they vaporize on impact. The vapor is plasma: gas so hot that electrons are stripped from their atoms.

As a satellite orbits, it picks up an electrical charge from particles streaming off the sun. Since plasma conducts electricity, plasma from a meteoroid collision acts like a wire and instantly drains the charge into space. The spark generates radio waves, which penetrate the satellite and are turned back into electrical current by circuits inside, overloading them.

Last November's shower was less intense than many scientists expected. After reviewing historical records, some say that this year's Leonids will produce a bigger storm. The display will be at its best over northern Europe.

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