Space is generally thought of as being a vacuum, but in reality it is a filthy place which is in serious need of a good sweeping out. When a piece of this cosmic dirt comes in contact with the Earth's atmosphere, you see the flash of a meteor in the night sky.

On any given night, there will be an average of seven meteors streaking across the heavens. In a year, 500 meteorites reach the surface of the Earth each year, but only ten or so are recovered as the majority fall and sink into the oceans.

Some Definitions

A meteor is the object which flashes across the sky; a meteorite is the remains of the object which flashed across the sky when it reaches Earth; and a meteoroid is the object floating is space before it comes in contact with the Earth and the Earth's atmosphere.

There are three main types of meteorites:

• Stony, which are virtually identical to some types of terrestrial stones.

• Irons, which are a combination of iron and nickel and are easily identifiable as they are heavy, resist weathering and often look like slag from a furnace.

• Stony-Irons, which are a combination of the above two types.

Of all the meteorites to reach Earth, the largest is in southern Africa, remaining where it fell. It measures nine by eight feet and weights about 132,000 pounds. There are other signs of impact craters on the surface of the Earth, most notably Arizona's Meteor Crater which dug a crater over 4000 feet across and almost 600 feet deep, pushing the walls 150 feet above the surface of the surrounding topography. But don't worry, the odds of a person being injured in a meteor strike in a given year are 10 trillion to one.

Meteor Showers

It is estimated that 100 million meteors enter the Earth's atmosphere each year and burn up, falling to the ground as dust. The total weight of this dust is about 4 million tons per year, or enough to cover the Earth with a layer one inch thick in 5000 years. Many of these meteors go unobserved as they streak across the daytime sky, or are too dim to see even at night.

However, when the Earth's orbit crosses through a large cloud of debris, the odds of seeing bright meteors greatly increases as the number of meteors can swell to hundreds per hour. This event is known as a meteor shower.

Most meteor showers are related to the trail of debris left behind as comets streak toward the Sun through the inner solar system. For example, the famous Perseids meteor shower in August is related to Comet Swift-Tuttle, which was last in the inner Solar System in 1992.

In meteor showers, there is a higher concentration of large particles which are able to light up the sky. For a meteor to be as bright as Sirius, the brightest star in the northern sky, it would have to weigh less than one-hundredth of an ounce. A meteor the size of a walnut would appear as bright as the full moon. The great Tunguska fireball of 1908 was brighter than the Sun.

In a meteor shower, the meteors all appear to be coming from one part of the sky, called the radiant. Meteor showers are named for the part of the sky in which the radiant lies - so the Perseids appear to come from the constellation of Perseus, and the Leonids from the constellation of Leo.

Calendar of Meteor Showers

Most of the major meteor showers occur at or around the same time every year. The following is a handy list of some of the big ones, along with the approximate date listed for peak activity. It should be remembered that the meteors per hour figure is an approximation based on past showers' activity. Actual results may vary.

How to Observe a Meteor Shower

The best way to observe a meteor shower is to look slightly away from the meteor shower's radiant. This is because if you look right at the radiant, meteors will appear to come right at you as a point of light, rather than streaking across the sky.

Do not use a telescope or pair of binoculars. During a meteor shower, you want to be able to see the entire sky. To help you see as much of the sky as possible, bring along a lawn chair or blanket and sit down, looking up at the sky. Dress warmly because even in the summer months it can be quite chilly as the dew begins to settle.

The best time to observe a shower is after midnight local time because the Earth's rotation will take you into the heart of the cosmic debris. Having a good dark site is important as well. Even moonlight can overpower the dim light of some meteors.

Some people like to observe the same shower every year and compare the rates over time. Others set up cameras and take time-lapse photographs of the sky to record the number of meteors that pass overhead. And some people just like to hang out with friends and watch a heavenly fireworks display.

Few gatherings of science-minded people can match the excitement of a meteor shower. You'll hear adults and children squeal with delight when a particularly bright fireball passes overhead, crackling as it burns up in the atmosphere and leaving a trail of smoke behind. If you get the opportunity to see one, don't miss it!