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Meteors

Our word meteor comes from the Greek word meteoron which means "a thing in the sky."

A meteor, sometimes called a "shooting star," can be the brightest object in the night sky, yet meteoroids are the smallest bodies in the solar system that can be observed by eye. Wandering through space, perhaps as debris left behind by a comet, meteoroids enter the earth's atmosphere, are heated by friction, and for a few seconds streak across the sky as a meteor with a glowing trail. 

A brilliant meteor, called a fireball, may weigh many kilograms, but even a meteor weighing less than a gram can produce a beautiful trail. Some of these visitors from space are large enough to survive (at least partially) their trip through the atmosphere and impact the ground as meteorites. Fireballs are sometimes followed by trails of light that persist for up to 30 minutes; some, called bolides, explode with a loud thunderous sound.

How can a particle the size of a grain of sand produce such a spectacular sight? The answer is the speed at which the meteoroid enters the earth's atmosphere. Many meteoroids travel at 60-70 kilometers per second. As a comparison, the shuttle moves around the earth at about 8 kilometers per second.

During its trip through the atmosphere, meteoroids collide with air molecules, knocking away materials and stripping electrons from the meteor. When the stripped atoms recapture electrons, light is emitted. The color of the light depends on the temperature and the material being "excited."

On almost any night a few meteors an hour will be seen from any one place. However, periodically there are meteor showers, with hundreds of meteors emanating from the same apparent spot in the sky.  These showers typically last from a few hours to several days. These showers are usually associated with comet paths, and are caused by debris expelled by the comet.

Each day as many as 4 billion meteors, most miniscule in size, fall to earth. Their masses total several tons, seemingly a large amount, but negligible compared to the earth's total mass of 6,600,000,000,000,000,000,000 tons.

When a meteorite actually hits, the impact can be tremendous.  Many meteorites actually explode on impact or just above the surface leaving nothing visible but a crater.  During impact, debris is thrown from the crater.   This displaced earth is called the ejecta.  It usually contains rocks of different composition melded together called breccia.

The shape and size of the crater depends on the size and velocity of impact.   Small diameter meteorites (less than 4 kilometers or 6.4 miles)  usually leave a round bowl crater, while larger meteorites cause craters with raised centers called the central peak. This peak is caused by the surface's attempt to rebound from the impact. Huge impacts can leave multiple rings in the earth's surface in the same way a rock creates ripples in a pond.

Source: National Resources Canada

 

The Meteor Crater in the state of Arizona was the first crater to be identified as an impact crater. Between 20,000 to 50,000 years ago, a small asteroid about 80 feet in diameter impacted the Earth and formed the crater.The crater is the best preserved crater on Earth and measures 1.2 km in diameter.

 

In 1908 a 200-foot-wide comet fragment slammed into the atmosphere and exploded over the Tunguska region in Siberia, Russia, with nearly 1,000 times the energy of the atomic bomb dropped on Hiroshima.

 

 

Major Meteor Showers (2012)



Quadrantids
Comet of Origin: 2003 EH1
Radiant: constellation Bootes
Active: Dec. 28, 2011-Jan. 12, 2012
Peak Activity: Jan. 4, 2012
Peak Activity Meteor Count: 120 meteors per hour
Meteor Velocity: 25.5 miles (41 kilometers) per second
Notes: A waxing gibbous moon will set at about 3 a.m. local time, allowing for several dark-sky hours of observing before dawn. This shower has a very sharp peak, usually only lasting a few hours, and is often obscured by winter weather. The alternate name for the Quadrantids is the Bootids. Constellation Quadrant Murales is now defunct, and the meteors appear to radiate from the modern constellation Bootes.

Lyrids
Comet of Origin: C/1861 G1 Thatcher
Radiant: constellation Lyra
Active: April 16-25, 2012
Peak Activity: April 21-22, 2012
Peak Activity Meteor Count: 10-20 meteors per hour
Meteor Velocity: 30 miles (49 kilometers) per second
Notes: A new moon on April 21 guarantees a dark sky in the late night and early morning hours, making this year ideal for observing from 10 p.m. to dawn. Lyrid meteors often produce luminous dust trains observable for several seconds.

Eta Aquarids
Comet of Origin: 1P Halley
Radiant: constellation Aquarius
Active: April 19-May 28, 2012
Peak Activity: May 5-6, 2012
Peak Activity Meteor Count: 10 meteors per hour
Meteor Velocity: 44 miles (66 kilometers) per second
Note: While the shower peaks an hour or two before dawn, the year's closest and largest full moon will be out all night, resulting in a moonlit sky that will wash out all but the brightest meteors. Meteor watchers in the Southern Hemisphere stand the best chance of seeing any meteors.

Delta Aquarids
Comet of Origin: unknown, 96P Machholz suspected
Radiant: constellation Aquarius
Active: July 12-Aug. 23, 2012
Peak Activity: July 28-29, 2012
Peak Activity Meteor Count: Approximately 20 meteors per hour
Meteor Velocity: 25 miles (41 kilometers) per second
Notes: It's not a good year for the Delta Aquarids -- light from the August full moon make them nearly impossible to see.

Perseids
Comet of Origin: 109P/Swift-Tuttle
Radiant: constellation Perseus
Active: July 17-Aug. 24, 2012
Peak Activity: Aug. 12, 2012
Peak Activity Meteor Count: Approximately 100 meteors per hour
Meteor Velocity: 37 miles (59 kilometers) per second
Notes: Moonlight won't be as big a problem as last year, as its waning crescent won't rise until after midnight, and the shower peaks from about 10-11 p.m. local on the night of Aug. 12.

Orionids
Comet of Origin: 1P/Halley
Radiant: Just to the north of constellation Orion's bright star Betelgeuse
Active: Oct. 2-Nov. 7, 2012
Peak Activity: Oct. 21, 2012
Peak Activity Meteor Count: Approximately 25 meteors per hour
Meteor Velocity: 41 miles (66 kilometers) per second
Note: With the second-fastest entry velocity of the annual meteor showers, meteors from the Orionids produce yellow and green colors and have been known to produce an odd fireball from time to time.

Leonids
Comet of Origin: 55P/Tempel-Tuttle
Radiant: constellation Leo
Active: Nov. 6-30, 2012
Peak Activity: Night of Nov. 17, 2012
Peak Activity Meteor Count: Approximately 15 per hour
Meteor Velocity: 44 miles (71 kilometers) per second
Note: The Leonids have not only produced some of the best meteor showers in history, but they have sometimes achieved the status of meteor storm. During a Leonid meteor storm, many thousands of meteors per hour can shoot across the sky. Scientists believe these storms recur in cycles of about 33 years, though the reason is unknown. The last documented Leonid meteor storm occurred in 2002.

Geminids
Comet of Origin: 3200 Phaethon
Radiant: constellation Gemini
Active: Dec. 4-17, 2012
Peak Activity: Dec. 13-14, 2012
Peak Activity Meteor Count: Approximately 120 meteors per hour
Meteor Velocity: 22 miles (35 kilometers) per second
Note: The Geminids are typically one of the best, and most reliable, of the annual meteor showers. This year's peak falls perfectly with a new moon, guaranteeing a dark sky for the show in the nights on either side of the peak date. This shower is considered one of the best opportunities for younger viewers because the show gets going around 9 or 10 p.m.

 

 

 

 

Credit: NASA, International Meteor Organization