What are meteoroid streams and meteor showers?

Meteor shower displays are associated with the Earth’s passage through meteor streams. These streams consist of the debris left over from the passage of comets (see diagram to the left for the Leonids meteor stream). As comets pass through the inner solar system, the radiation from the Sun causes them heat up, evaporating the dusty-icy materials of the comet.   These particles are left in the wake of the comets passage creating a stream of small debris that is strewn along the comets orbital path.  If the orbit of the Earth intersects the orbital path of a comet, then at regular predictable times throughout the year the Earth will pass through the stream of debris creating a meteor shower.

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How do I observe a meteor shower?

Enjoying a meteor shower is easy. You need only a site far from the blinding lights of the city.  Take with you a lawn chair or sleeping bag, perhaps a blanket or hot chocolate to keep warm, and gaze upward towards the sky.  Absolutely no equipment is needed to enjoy the cosmic show.  For safety reasons, never observe alone in a secluded place. Meteors come in all brightnesses, sizes, and colors, and there is no way to predict what might appear on any given night.  Most will be dim, but some can appear quite spectacular.  If our Earth encounters a much larger meteoroid than is common–say, the size of a marble–it might appear much brighter than any of the stars or planets in the sky. We would see its demise as a brilliant smoking or flaming fireball.  More than 50,000 fireballs occur in our atmosphere each year, though most are not seen since they occur in daylight, over un-populated areas or open ocean.  If a fireball produces a sonic boom, it is then called a bolide.  Only about one out of every ten fireballs behaves in this way.

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When do the most intense outbursts occur?

Your best chance of seeing meteors comes after midnight and before dawn. The reason for this peculiarity lies in the motion of our Earth. In the hours following sunset, our hemisphere faces the direction opposite our orbital motion–that is, we are on the trailing side of the Earth and are looking out the Earth’s “rear window.” Only those meteoroids that catch up to us at a speed of at least 18 miles per second (29 kilometers per second) can fall into our atmosphere. With clear weather we might see two to six sporadic meteors each hour during early evening. After midnight, however, we lie on the leading side, facing the direction of the Earth’s orbital motion. Now, when we gaze into the sky, we peer out our planet’s “front window” and can see all those particles being swept into the atmosphere “head-on.” The nearer to dawn it becomes, the more sporadic meteors we can expect to see–perhaps as many as 14 per hour-as well as shower meteors.

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What is a meteor storm?

Typically, a meteor shower only produces a couple dozen meteors per hour.  But, occasionally a meteor shower occurs soon after the comet (whose debris is the “stuff” of the meteor shower) has made a recent visit.  As a result, dense pockets of debris are left in the comet’s wake.  If the Earth encounters one of these dense pockets of debris, a meteor storm occurs.  One of the more famous meteor showers is the Leonids (peaking around mid-November each year).  The meteor shower is caused by the comet Tempel-Tuttle with a 33 year orbit.  In November of 1966, the last time Comet Tempel-Tuttle swung by our planet and left in its wake a fresh supply of dust, skywatchers were treated to a cosmic extravaganza as meteors fell at the staggering rate of 41 per second!  In a typical year, the Leonids produces a meteor rate of about 60 to 100 meteors per hour falling from the direction of the constellation Leo in the eastern sky.    However, every 33 years or so, watchers are treated to the show of a lifetime.  It’s certainly happened before, perhaps it will happen again.

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Why are some longitudes favored?

The meteors in any given shower come from a particular direction in space. You need to be on the hemisphere facing that direction to see the meteors. It also has to be night-time, except for incredibly bright fireballs.

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Can damage to satellites occur?

Very high speed impacts of tiny dust grains on satellites can cause plasma to be generated, which can lead to electrical failure. There is evidence that the Olympus communications satellite was disabled owing to the impact of a meteoroid from the Perseid stream in 1993.

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For more information:

Visit the International Meteor Organization (IMO) at http://www.imo.net/

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