Over the next few days Earth will sweep through the debris trail of an ancient comet named Swift-Tuttle. Thousands of bits of dust and rock will crash into our planet's atmosphere, lighting up the night skies with dozens of flashing meteors per hour.

This spectacular event is known as the Perseid meteor shower, and it usually provides the best meteor show of the year.

The trail of a comet

There are hundreds of comets known, and most of them have orbits that never come near Earth. But a few comets have orbits that do cross Earth's orbit, which means that once a year our planet crashes through the debris trail those comets have left behind. When that happens we experience a meteor shower, a night or two when flurries of meteors are seen, apparently emanating from a specific section of the night sky.

The Perseid meteor shower in August of each year is associated with Comet Swift-Tuttle, a well-known comet that has a 134-year orbit and was last near the sun in 1992. The debris it has left on its orbital path is what makes up the meteors of the Perseid shower.

Pelting the atmosphere

A meteor shower can be likened to a car driving at high speed through a swarm of bees, splattering bees on the windshield as it speeds through the swarm.

The "bees" in this case are the particles left by the comet in its path. Earth travels in its orbit at very high speed (about 66,000 miles, or 106,000 kilometers per hour), and when it plows through the swarm of thousands of tiny rocks, pebbles and dust, they smash into Earth's atmosphere. The combined speed of Earth and the particles is so high -- many times faster than a rifle bullet -- that the debris are immediately vaporized in a flash of light. That brief streaking flash is what we see as a meteor in the night sky. (A common term for these is "shooting stars," but they really have nothing to do with stars.)

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No danger to humans -- well, maybe not much

Meteor showers such as the Perseids pose little danger to those of us on Earth's surface. The tiny size of these particles -- most are smaller than a quarter inch (6 millimeters) -- ensures that most of them never reach the ground, but are instead completely burned up in the atmosphere. Larger meteors -- about the size of a fist or larger -- can survive the trip through the atmosphere and fall to the ground, but those are relatively rare and overall pose no great risk.

The evidence: In all of human history there is no record of anyone ever being killed by a meteor. But there have been people struck by them, as well as many near misses.

A real danger -- but not now

There is a possible danger associated with the Perseids, but not from the meteor shower. In the year 2126, the Perseids' parent comet, Swift-Tuttle, is predicted to return. On this pass of its orbit, it is expected to come very close to Earth. Because comet orbits cannot be predicted exactly, there is a small but significant danger that 126 years from now, Comet Swift-Tuttle will hit the Earth directly. That would be major worldwide catastrophe, but it's not something we need to worry about anytime soon.

Mostly entertainment

Remote risks aside, for most people around the world the Perseids and other meteor showers are not a source of fear but of enjoyment. This year, on August 12 and 13, hundreds of thousands of people will turn out to see the peak of the Perseid shower, spending a night under the stars looking for these tiny celestial travelers. Though a near-full moon will make it harder to see them this year, it should still be a show worth seeing.