Ionization trail



A meteoroid is a small sand- to boulder-sized particle of debris in the Solar system. The visible path of a meteoroid that enters Earth's (or another body's) atmosphere is a meteor, commonly called a "shooting star" or "falling star". On reaching the ground, a meteor is then called a meteorite. Many meteors are part of a meteor shower. The root word meteor comes from the Greek meteōros, meaning high in the air.


Larger than a meteoroid, 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. The Royal Astronomical Society has proposed a new definition where a meteoroid is between 100 µm and 10 m across. The NEO definition includes larger objects, up to 50 m in diameter, in this category.

The composition of meteoroids can be determined as they pass through Earth's atmosphere from their trajectory and the light spectra of the resulting meteor. Their effects on radio signals also yield 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. 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, to nickel-iron rich dense rocks. A relatively small percentage of meteoroids hit the Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs.


A meteor is the visible event that occurs when a meteoroid or asteroid enters Earth's atmosphere and becomes brightly visible. This typically occurs in the mesosphere, and most visible meteors range in altitude from 75km to 100km.



A fireball is brighter than a usual 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.


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 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". Astronomers tend to use the term to mean an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball).





Meteoric dust


Ionization trails





Many investigators believe the sounds to be imaginary — essentially sound effects added by the mind to go along with a light show. However, the persistence and consistency of the reports have caused others to wonder. Sound recordings made under controlled conditions in Mongolia in 1998 by a team led by Slaven Garaj, a physicist at the Swiss Federal Institute of Technology at Lausanne, support the contention that the sounds are real.



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.


Meteoroids and asteroids orbit around the Sun, in greatly differing orbits. Some of these objects orbit together in streams; these are probably comet remnants that would form a meteor shower. Other meteoroids are not associated with any stream clustering (although there must also be meteoroids clustered in orbits which do not intercept Earth's or any other planet). The fastest objects travel at roughly 42 kilometers per second (26 miles per second) through space in the vicinity of Earth's orbit. Together with the Earth's orbital motion of 29 km/s (18 miles per second), collision speeds can reach 71 km/s (44 miles per second) during head-on collisions. This would only occur if the meteor were in a retrograde orbit. 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. Most meteors are however, observed at night as low light conditions allow fainter meteors to be observed. Meteors are usually seen when they are 60 to 120 km (40 to 75 miles) above the ground.


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


One meteorite recovered at Peekskill, N.Y., for which the event and object gained its name, (at 41.28 deg. N, 81.92 deg. W) had a mass of 12.4 kg (27 lb) 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 spacecrafts. The Hubble Space Telescope has about 572 tiny craters and chipped areas.


See also


External links

Search another word or see Ionization trailon Dictionary | Thesaurus |Spanish
Copyright © 2015, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature