Meteor Shower

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Meteor Shower (illustrated)

A meteor shower occurs when the Earth crosses the path of a dust, or "debris trail" left behind by a comet. Most meteor showers appear to be coming from a fixed point in the sky, this is referred to as the radiant.

Comets are, as some refer to them, "dirty snowballs" which consist of ice, rock, gas and dust. It is thought that comets are leftover "debris" from the formation of the outer planets. Most comets orbit our sun and as it approaches the sun the ice evaporates and the dust particles are thrown away from the comet creating the comet's "debris trail". Because some comets have an orbit that is close to the Earth's orbit, Earth passes through this trail at certain times of the year.

1 What is a meteor?
2 How does a meteor shower get it's name?
3 Differences: Meteoroids/Meteors/Meteorites
4 Earthgrazers, Fireballs and Bolides
5 Viewing
6 Meteor Shower vs. Meteor Storm
7 Dangers
8 External Links
9 Related Discussion

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What is a meteor?

Particles from the "debris trail" that produce a meteor shower are typically no larger than a grain of sand to pebble size. As these particles enter and interact with Earth's atmosphere they heat up thus becoming the visible meteors. The heat that is produced by a meteoroid entering our atmosphere is a result of the ram pressure, not friction as commonly thought. Ram pressure refers to the pressure exerted on a moving body through a fluid medium. The ram pressure (caused by the rapid compression of the air in front of the meteoroid) heats up the air which then heats the meteor and results in the streak of light that we see.

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How does a meteor shower get it's name?

Leonid Radiant (Courtesy of NASA)

Meteor showers are named after the constellations from which they appear to radiant from. For example, the Perseid meteor shower (mid-August) appears to originate from within the constellation Perseus while the Leonids (November) appear to originate from the constellation Leo.

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Differences: Meteoroids/Meteors/Meteorites

Once a meteoroid enters our atmosphere it becomes a meteor. That interaction with our atmosphere produces the bright streak of visible light. If that meteor is large enough to survive our atmosphere it is then referred to as a meteorite.

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Earthgrazers, Fireballs and Bolides

2003 Perseid earthgrazer (Courtesy of stardate.org).

Earthgrazers are meteors that skim the Earth's atmosphere horizontally and put on quite a beautiful show with bright colors and long lasting tails. However rare to view, your chances of seeing one greatly increase just as the radiant of a meteor shower is near the horizon.

A fireball is generally considered to be a meteor with a magnitude of -4 or greater.

A bolide is synonymous with a fireball, however, some astronomers use the term to describe a very bright fireball that explodes in the atmosphere.

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Viewing

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What is peak viewing?

Peak refers to a meteor shower's period of highest activity. Meteorologists as well as local astronomers can usually suggest peak hours and nights for best viewing. Typically, the best hours for viewing meteor showers range from midnight to dawn.

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Hourly Meteor Rate

Meteor showers can produce anywhere from 10 - 15 meteors per hour during the low-rate meteor showers and as much as 50 - 100 meteors per hour during the high-rate meteor showers such as the Perseids and Leonids.

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Tips

There are several things that you can do to increase your chances of maximum viewing and the good news is that you don't need any complicated equipment.

Be prepared to stay up late. Peak times and dates for meteor showers vary.
There are two things that will greatly decrease your viewing capability. First is light pollution. Get as far away from light pollution as possible, you will need a full view of the sky. Second, the moon. A bright, full moon will pretty much obliterate your chances of seeing meteors. Try to get out before the moon rises.
Dress warm, take a jacket and extra clothing, even for those summer showers.
Take reclining chairs and/or blankets. You want to have a full view of the sky so it is best to be either reclined in a chair or just laying back on a blanket.
Give your eyes time to adjust to the dark.
While most meteor showers have a radiant (the point in the sky which they appear to originate from), you will see meteors all around. Continuously scan the sky with your eyes.
Just lay back and be patient, the more light pollution and/or moon glow the longer you may have to wait between meteors.

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Most Notable Meteor Showers

Name---Activity Dates/Peak Dates---Associated Comet

*Quadrantids---January 1-5/January 4, morning---2003 EH1
*Lyrids---April 16-26/Night of April 21/22---Thatcher
*Eta Aquarids---late April-early May/May 5, morning/evening---Halley's Comet
*Perseids---August 3-14/August 12, morning---Swift-Tuttle
*Orionids---October 2-November 7/October 21, morning---Halley's Comet
*Leonids---November 14-21/November 17, morning---Temple-Tuttle
*Geminids---December 7-17/December 13, evening---3200 Phaethon

Note: Activity/Peak dates may vary. See the following link for current meteor shower information.

SPA Meteor Shower List

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Meteor Shower vs. Meteor Storm

Even at it's most active, a meteor shower typically produces on average 50 - 100 meteors per hour. A meteor storm can produce thousands. A meteor storm occurs when Earth passes through a dense pocket of debris left behind by the host comet. One of the most noted meteor storms was the Leonids in 2002 where as many as 10 meteors per second were reported.

This illustration (right) of the 1833 Leonid meteor storm by Swiss artist Karl Jauslin is based on a first hand account by eyewitness and minister, Joseph Waggoner en route from Florida to New Orleans.

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Dangers

Meteor showers don't pose much of a threat to us, however, they do pose a risk to Earth orbiting objects such as spacecraft and satellites. Even an impact from the smallest meteoroid particle can severely damaged or disable orbiting spacecraft and satellites. The November Leonids poses one of the greatest risks because of the dense meteoroid stream associated with it and the fact they are among the fastest meteoroids.

HST meteoroid impact (Courtesy of NASA).

HST meteor shower orientation (Courtesy of NASA).

In 1994 a meteoroid impact (360 microns deep/900 microns diameter) was discovered on the Hubble Space Telescope believed to have occurred during the 1993 leonid meteor shower. HST mirrors are turned away and solar panels rotated to minimize the risk of a meteoroid impact.

The greatest potential hazard to orbiting spacecraft and satellites is the plasma cloud that is produced when a meteoroid particle impacts a spacecraft or satellite. Upon impact, the meteoroid particle disintegrates creating an electrically charged plasma cloud that can severely short circuit systems. As a precaution, satellites at risk can be powered down or ramped off.

In 1993, the ESA (European Space Agency) lost the Olympus communications satellite when it was hit by a meteoroid during the August Perseid meteor shower creating an electrically charged plasma cloud which disabled a gyro and sent the satellite tumbling. Once power was restored it had used all of it's fuel and was permanently disabled.

The International Space Station is protected by hundreds of protective shields including meteoroid/debris blankets made of the same material used in bullet proof vests.

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