A Leonid meteor.  Photograph courtesy of Carter Roberts.

Children of Comets

Meteors might be called the "children" of comets. You may have heard comets described as "big dirty snowballs." The "dirty" part refers to little bits of stone, not unlike sand, embedded in the comet's icy nucleus. Comets whose orbits carry them through the inner solar system pass close enough to the Sun for some of their ices to be vaporized and outgassed into space, in the process releasing the embedded dust and forming long trails of gas and dust. When the Earth's orbit carries us through such a trail, the Earth sweeps through the dust at a speed of 18.5 miles per second (the speed at which Earth moves around the Sun). As the dust grains enter our atmosphere at a very high speed (the speed of the Earth plus the speed of the dust particles, up to 45 miles per second), friction with air molecules heats them, usually causing them to vaporize in a flash of light. When this happens, a meteor is born, and we see its fiery streak through our night skies. 

When Can I See Meteors?

Meteors can be seen on just about any night. The number of meteors that you might observe on a given night increases when the Earth moves into an existing cometary dust trail. We call this event a "meteor shower." We name meteor showers that occur annually based on the constellation that they appear to be coming from. So, the annual meteor shower that appears to radiate from the constellation Leo is called the "Leonid Meteor Shower," and the Perseid Meteor Shower comes from Perseus, and so on.

Comet tails blown by upper atmospheric winds.   Photo courtesy of Carter Roberts.

How Many Meteors Will I See? 

The number of meteors that one might see during a shower depends on the specific shower. Comet dust trails are not all created equal; some are littered with a higher density of dust particles than others, either due to the amount of debris shed by the parent comet, or by the spreading out and thinning of the dust lane over time, or both. Older dust lanes can be "resupplied" when the parent comet returns on a regular orbit from time to time. In recent times, the Leonid Meteor Shower's parent comet, Comet Temple-Tuttle, passed through the inner solar system, stimulating predictions of particularly intense showers in 1998 and 1999. The Leonid shower was more intense than usual, particularly in 1999, producing hundreds or even thousands of meteors per hour (depending on the viewer's location). A popular annual shower is the Perseid shower (occurring on August 12), which produces between 50 and 60 meteors per hour.

Annual Meteor Showers Table