Meteoroid

"Meteor" redirects here. For other uses, see Meteor (disambiguation).

Photo of a part of the sky during a meteor shower over an extended exposure time. The meteors may have actually occurred several seconds to several minutes apart.

A meteoroid is a large sand to boulder-sized particle of debris in the Solar system. The visible path of a meteoroid that enters the Earth's (or another body's) atmosphere is a meteor, commonly called a shooting star or falling star. Many meteors are part of a meteor shower.

Definitions

The root word meteor comes from the Greek meteōros, meaning high in the air.

Meteoroid

A meteoroid is a small sand to boulder-sized particle of debris in the Solar system. Larger than that, the object is an asteroid; smaller than that, it is interplanetary dust. The 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 or molecule." The Royal Astronomical Society has proposed a new definition where a meteroid is between 100 µm and 10 m across.

Meteor

A meteor is the visible event that occurs when a meteoroid or asteroid enters the earth's atmosphere and becomes brightly visible. For bodies with a size scale larger than the atmospheric mean free path (10 cm to several meters) the visibility is due to the heat produced by the ram pressure (not friction, as is commonly assumed) of atmospheric entry. Since the majority of meteors are from small sand-grain size meteoroid bodies, most visible signatures are caused by electron relaxation following the individual collisions between vaporized meteor atoms and atmospheric constituents. The meteor's just what we see.

Fireball

A fireball is a very bright meteor. The International Astronomical Union defines a fireball as "a meteor brighter than any of the planets" (magnitude -4 or greater). 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.

Bolide

The word bolide comes from the Greek βολις, (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 term is more-often used among geologists than astronomers where it means a very large meterorite. 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". Astronomers tend to use the term to mean an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball).

Meteorite

A meteorite is a meteoroid or asteroid that survives its entry into the atmosphere and strikes the ground. A meteorite striking the Earth or other object may produce an impact crater. Geologists use the term bolide to mean a very large meteorite (see above).

In the 20th century, two people were reportedly struck by meteorites. Both survived with minor injuries. In the 1990s an empty car was struck and heavily damaged by a meteorite. The car sold as a collector's item for several tens of thousands of dollars. In all there are some dozen cases of meteorite falls documented to have hit people, property, or animals.

Tektite

Molten terrestrial material "splashed" from a crater can cool and solidify into an object known as a tektite. These are often mistaken for meteorites.

Meteoric dust

Most meteoroids are destroyed 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.

Ionization trails

During the entry of a meteoroid or asteroid into the upper atmosphere, an ionization trail is created, where the molecules in the upper atmosphere are ionized by the passage of the meteor. Such ionization trails can last up to 45 minutes at a time. Small, sand-grain sized meteoroids are entering the atmosphere constantly, essentially every few seconds in a given region, and thus ionization trails can be found in the upper atmosphere more or less continuously. When radio waves are bounced off these trails, it is called meteor scatter communication.

Meteor scatter has been used for experimental secure military battlefield communications systems. The basic idea of such a system is that such an ion trail will act as a mirror for radio waves, which can be bounced off the trail. Security arises from the fact that as a mirror, only receivers in the correct position will hear the transmitter, much as with a real mirror, what is seen in reflection depends upon one's position with respect to the mirror. Because of the sporadic nature of meteor entry, such systems are limited to low data rates, typically 459600 baud.

Amateur radio operators sometimes use meteor scatter communication on VHF bands. Snowpack information from the Sierra Nevada mountains in California is transmitted from remote sites via meteor scatter. Meteor radars can measure atmospheric density and winds by measuring the decay rate and Doppler shift of a meteor trail.

Meteors can leave behind very large ionization trails, which then interact with the Earth's magnetic field. As the trail dissipates, megawatts of electromagnetic energy can be released, with a peak in the power spectrum at audio frequencies. Physical vibrations induced by the electromagnetic impulses can be heard: they are powerful enough to make grasses, plants, eyeglass frames, and other conductive materials vibrate. See for example, Listening to Leonids (NASA, 2001), Hearing Sensations in Electric Fields (1964), Human auditory system response to Modulated electromagnetic energy (J. Appl. Physiol. 17(4):689-692. 1962), Human Perception of Illumination with Pulsed Ultrahigh-Frequency Electromagnetic Energy (Science 27 July 1973 Vol. 181. no. 4097, pp. 356 - 358) for details and references on the electrophonic auditory phenomenon.

Formation

Many meteoroids are formed by impacts between asteroids though many are also left in trails behind comets that form meteor showers and many members of those trails are eventually scattered into other orbits forming random meteors too. Other sources of meteors are known to have come from impacts on the Moon, or Mars as some meteorites from them have been identified. See Lunar meteorites and Mars meteorites.

Orbit

Meteoroids and asteroids orbit around the Sun. Not all of the orbits are the same. Some of these objects orbit together (stream component); these are probably comet remnants that would form a meteor shower. Other meteors are not associated with any clustering of meteoroids to form a shower (though there must also be meteoroids clustered in orbits which do not intercept the Earth's or any other planet.). The fastest objects travel at roughly 26 miles per second (42 km per second) through space in the vicinity of the Earth's orbit. Together with the Earth's orbital motion of 18 miles per second (29 km per second) speeds can reach 44 miles per second (71 kilometers per second) in head-on collisions. This means the meteor would also be in a retrograde orbit and a daylight (or near daylight) event as the Earth orbits in the direction of roughly east a noon. Most meteors are observed at night as light conditions allow fainter observations but meteors are still seen from 40 to 75 miles away high in the sky above observers.

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.

Perhaps the best-known meteor/meteorite fall is the Peekskill Meteorite which was filmed on October 9, 1992 by at least 16 independent videographers

Eyewitness accounts indicate that the fireball entry of the Peekskill meteorite started over West Virginia at 23:48 UT (+/- 1 min.). The fireball, which traveled in an northeasterly direction had a pronounced greenish colour, and attained an estimated peak visual magnitude of -13 (comparable to the Full Moon). During a luminous flight time that exceeded 40 seconds the fireball covered a ground path of some 700 to 800 km.

One meteorite recovered at Peekskill, N.Y., for which the event and object gained it's name, (at 41.28 deg. N, 81.92 deg. W) had a mass of 12.4 kg and was subsequently identified as an H6 monomict breccia meteorite. The video record suggests that the Peekskill meteorite probably had several companions over a wide area especially in the harsh terrain in the vicinity of Peekskill.

Spacecraft damage

Even very small meteoroids can damage spacecraft. The Hubble Space Telescope for example, has about 100 tiny craters and chipped areas.

External links

• International Meteor Organization fireball page
• British Astronomical Society fireball page
• A Goddard Space Flight Center Science Question of the Week where the answer mentions that a fireball will cast a shadow.
• Meteor showers - view tips

See also

• Baetylus
• Impact crater
• Meteor shower
• Meteorite
• Tektite
• Bolide
• Green fireballs
• Impact event