A great deal of the material that makes up meteorites comes from short-period comets. As comets travel close to the Sun, they lose material, creating a trail of debris behind them. These trails, called meteoroid streams, can take many hundreds of years to form, but gradually build up to contain a large amount of loose dust and rock fragments. If Earth’s orbit carries it through one of these streams, then hundreds of meteoroids will enter the atmosphere in a very short time, creating a meteor shower.
Meteor showers occur when the earth’s orbit and that of a comet intersect. What you are seeing is the bits of dust that the comet left behind colliding with the atmosphere at high speed. The friction with the earth’s atmosphere heats up the particles of dust to thousands of degrees until they either vaporize or strike the surface of the earth. In some cases they particles a deflected back into space like a stone skipping on water. When the earth is not in a recent orbital path of a comet there are still loose particles all around the solar system so there is a base rate of about 5 visible meteors an hour in ideal viewing conditions. You can see one strike the atmosphere if its dark enough and you happen to be looking in the right direction. The orbital paths of the earth and many comets intersect once every year and the meteor rate can be much higher when the earth passes through these debris fields. The process of small particles loose in space being swept up by larger bodies like the earth has been going on for billions of years and is what created the sun, planets and comets to begin with. The earth captures 40,000 metric tons of space dust a year currently which is much less than the rate it was 4 billion years ago when the planets were first forming. This makes sense logically as the dust clears the collision rate falls. The geologic record on earth and the moon also support this hypothesis.
Meteor showers associated with particular comet orbits occur at about the same time each year, because it is at those points in the earth’s orbit that the collisions occur. However, because some parts of the comet’s path are richer in debris than others, the strength of a meteor shower may vary from one year to the next. Typically a meteor shower will be strongest when the earth crosses the comet’s path shortly after the parent comet has passed.
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