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 small 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. 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. Larger than that, the object is an asteroid; smaller than that, it is interplanetary dust.

Meteor

For other uses, see Meteor (disambiguation).

A meteor is the visible event that occurs when a meteoroid 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.

Fireball

A fireball is a very bright meteor. There is no official definition of how bright a meteor must be to be called a fireball. A frequently used definition is that a fireball is a meteor bright enough to cast a visible shadow on a moonless night with a dark sky. The International Meteor Organization defines a fireball as a meteor that would have a magnitude of -3 or brighter if seen at zenith. The meteor section of the British Astronomical Association requires a magnitude brighter than Venus or magnitude -5 or brighter (Venus is magnitude -4 at its brightest). However, both of the above organizations are amateur astronomy clubs so their definitions carry no official weight.

Bolide

A bolide is an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball). The word comes from the Greek βολις, (bolis) which can mean a missile or to flash. Like fireball, there is no official definition of bolide. An alternate definition holds that a meteor is a bolide if sound can be heard coming from its passage. (Example: 3 December 2005 near Albany in south-west Western Australia). The term bolide is sometime used synonymously with meteorite (see below). In this sense a bolide is a exceptionally bright fireball that may or may not explode and may or may not hit the ground, but probably does one or the other. When a bolide explodes it may do so in the air or upon impact with the ground. In either case an impact crater may be formed.

Meteorite

A meteorite is meteoroid that survives its entry into the atmosphere and strikes the ground. A meteorite striking the Earth or other object may produce an impact crater. Bolide and meteorite are sometimes use synonymously.

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.

Tektite

Molten terrestrial material "splashed" from such 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 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.

Large meteoroids 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, frizzy hair, the middle ear and other 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. Others 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 orbit around the Sun. Not all of the orbits are the same. Some meteoroids 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 meteoroids 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 event as the Earth orbits in the direction of east a dawn. 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.

Spacecraft Damage

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
• Meteor Captured above Colorado Video

See also

• Baetylus
• Impact crater
• Meteor shower
• Meteoroid
• Meteorite
• Tektite
• Bolide
• Green Fireballs
• Impact event