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Let's Define This Special Meteor
A fireball is a meteor that is unusually bright. A bolide meteor is an exceptionally brilliant fireball, which sometimes explode causing fragmentation of the meteor. A meteor fireball will usually leave a trail across the sky as it travels through the Earth's atmosphere. Bolides can produce single flashes or sometimes multiple flashes before disappearing.
Meteors travel around the Sun in varying orbits and different velocities. When a meteor enters the Earth's atmosphere, the resultant speed of the bolide is a combination of the speed of the Earth and that of the bolide. Its resultant kinetic energy will be dissipated as it passes through the atmosphere.
Bolides reach magnitudes of -14 or sometimes even -17 making them superbolides. The chemical composition of a bolide can vary. They can be either composed of rocks, be metallic in nature or contain ice or a combination of all the above. Exploding bolides are called detonating fireballs.
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What Causes the Bright Flash?
When a meteor enters the upper atmosphere of the Earth, ram pressure causes the meteor to heat up. The atoms within the meteor begin to boil off due to the extreme heat generated. As the atoms boil off, they begin to interact with the atmospheric particles producing ionized particles. The ionized particles begin to form a column of plasma around the meteor, which causes it to glow. A bolide flashes brightly due to greater amounts of ionized particles leaving a larger column of plasma.
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Bolides that Explode
Exploding bolides are also called detonating fireballs. The reason why some bolides explode has to do with the composition of the object. If the internal composition is water-ice then the bolide will likely explode. This is because as the meteor enters the Earth's atmosphere and begins to descend, ram pressure generates a shock wave produced by the compression of gases in the Earth's atmosphere. This ram pressure causes the meteor to heat up which causes the water-ice inside the meteor to boil and when it reaches a critical stage it explodes violently, fragmenting the object into much smaller pieces. The smaller pieces will burn up through the atmosphere, seldom causing any injury to humans or damage to property.
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Determining the Composition of a Bolide
Scientists use the trajectory of the bolide and the light spectrum of the bolide to determine its composition. The light spectrum of the bolide, using spectroscopy, can yield useful information about the composition of the bolide. Scientists use emission lines and absorption lines to determine the chemical composition of a bolide.
Trajectory measurements of bolides have yielded objects that have been found to be grouped in streams around a parent comet. Many have also been determined to be sporadically circling the comet.
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Image Credits:WIkimediaCommons/ThomasGrau (http://commons.wikimedia.org/wiki/File:Boliden-Phasen.jpg)