[meteorite-list] Meteorites 101

From: Barrett <BarrettWF_at_meteoritecentral.com>
Date: Sat, 15 Jan 2011 18:29:41 -0700
Message-ID: <001401cbb51c$d9fe3b20$8dfab160$_at_net>

The definitive source WIKIPEDIA!! Says;

MeteoroidThe current official definition of a meteoroid from the International Astronomical Union is "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom".[1][2] Beech and Steel, writing in Quarterly Journal of the Royal Astronomical Society, proposed a new definition where a meteoroid is between 100 ?m and 10 m across.[3] The NEO definition includes larger objects, up to 50 m in diameter, in this category. Very small meteoroids are known as micrometeoroids (see also interplanetary dust).

The composition of meteoroids can be determined as they pass through Earth's atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals also give information, especially useful for daytime meteors which are otherwise very difficult to observe. From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see Meteor showers) often associated with a parent comet, others apparently sporadic. Debris from meteoroid streams may eventually be scattered into other orbits. The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice,[4] to nickel-iron rich dense rocks.

Meteoroids travel around the Sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second) through space in the vicinity of Earth's orbit. The Earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the Earth's atmosphere head-on (which would only occur if the meteors were in a retrograde orbit), the combined speed may reach about 44 miles per second (71 kilometers per second).

Meteor"Meteor" and "Meteors" redirect here. For other uses, see Meteor (disambiguation).
See also Hydrometeor.
 
Comet 17P/Holmes and GeminidA meteor is the visible path of a meteoroid that has entered the Earth's atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km.[5] Millions of meteors occur in the Earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between about 40 and 75 miles (65 and 120 kilometers) above the Earth. They disintegrate at altitudes of 30 to 60 miles (50 to 95 kilometers). Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with the Earth as the Earth orbits in the direction of roughly west at noon.[clarification needed] Most meteors are, however, observed at night as low light conditions allow fainter meteors to be observed.

For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres)[clarification needed] the visibility is due to the atmospheric ram pressure (not friction) that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second. A relatively small percentage of meteoroids hit the Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs (for example The Great Daylight 1972 Fireball).

Meteors may occur in showers, which arise when the Earth passes through a trail of debris left by a comet, or as "random" or "sporadic" meteors, not associated with a specific single cause. A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the incoming meteors or meteorites have been calculated. All of them came from orbits from the vicinity of the asteroid belt.[6]

FireballA fireball is a brighter-than-usual meteor. The International Astronomical Union defines a fireball as "a meteor brighter than any of the planets" (magnitude -4 or greater).[7] The International Meteor Organization (an amateur organization that studies meteors) has a more rigid definition. It defines a fireball as a meteor that would have a magnitude of -3 or brighter if seen at zenith. This definition corrects for the greater distance between an observer and a meteor near the horizon. For example, a meteor of magnitude -1 at 5 degrees above the horizon would be classified as a fireball because if the observer had been directly below the meteor it would have appeared as magnitude -6.[8]

Bolide
An especially bright meteor, a bolide (in astronomy)"Bolide" redirects here. For the Swedish guided missile BOLIDE, see RBS 70.
In astronomy
The word bolide comes from the Greek ????? (bolis) which can mean a missile or to flash. The IAU has no official definition of "bolide", and generally considers the term synonymous with "fireball". The bolide term is generally used for fireballs reaching magnitude -14 or brighter.[9] Astronomers tend to use the term to mean an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball).

In geology
Geologists use the term "bolide" more often than astronomers do: in geology it indicates a very large impactor. For example, the USGS uses the term to mean a generic large crater-forming projectile "to imply that we do not know the precise nature of the impacting body ... whether it is a rocky or metallic asteroid, or an icy comet, for example".[10]

SuperbolideIf the magnitude of a bolide reaches -17 or brighter it is known as a superbolide.[9][11]

MeteoriteMain article: meteorite
A meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and impact with the ground without being destroyed.[12] Meteorites are sometimes, but not always, found in association with hypervelocity impact craters; during energetic collisions, the entire impactor may be vaporized, leaving no meteorites.

TektiteMain article: tektite
 
Two tektitesMolten terrestrial material "splashed" from a meteorite impact crater can cool and solidify into an object known as a tektite. These are often mistaken for meteorites.

Meteoric dustMost meteoroids burn up when they enter the atmosphere. The left-over debris is called meteoric dust or just meteor dust. Meteor dust particles can persist in the atmosphere for up to several months. These particles might affect climate, both by scattering electromagnetic radiation and by catalyzing chemical reactions in the upper atmosphere.[13]

Does this help any? Seems about correct.
-Barrett

-----Original Message-----
From: meteorite-list-bounces at meteoritecentral.com [mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of GeoZay at aol.com
Sent: Saturday, January 15, 2011 5:59 PM
To: meteorite-list at meteoritecentral.com
Subject: Re: [meteorite-list] Meteorites 101


>>Bolide is a term that it's good to avoid. It doesn't mean anything... or
rather, it means too many different things. "Fireball" unambiguously means
a
meteor of a specific apparent brightness. "Bolide" is simply confusing.<<



I usually think of a fireball as a meteor with a magnitude brighter than
-3. I also sometimes think of a Bolide as being a fireball that has produced
a sonic boom as well.
GeoZay

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Received on Sat 15 Jan 2011 08:29:41 PM PST