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 called a meteor, or commonly a "shooting star" or "falling star". If a meteoroid reaches the ground, it 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.
If the object is larger than a meteoroid, it is called 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.
For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres) 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 is simply the visible event rather than an object itself.
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).
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. 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.
Molten terrestrial material "splashed" from a crater can cool and solidify into an object known as a tektite. These are often mistaken for meteorites.
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.
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 burst communications.
Numerous people have over the years reported sounds being heard while bright meteors flared overhead. This would seem impossible, given the relatively slow speed of sound. Any sound generated by a meteor in the upper atmosphere, such as a sonic boom, should not be heard until many seconds after the meteor disappeared. However, in certain instances, for example during the Leonid meteor shower of 2001, several people reported sounds described as "crackling", "swishing", or "hissing" occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth's auroras.
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.
How these sounds could be generated, assuming they are in fact real, remains something of a mystery. It has been hypothesized that the turbulent ionized wake of a meteor interacts with the magnetic field of the Earth, generating pulses of radio waves. As the trail dissipates, megawatts of electromagnetic energy could be released, with a peak in the power spectrum at audio frequencies. Physical vibrations induced by the electromagnetic impulses would then be heard if they are powerful enough to make grasses, plants, eyeglass frames, and other conductive materials vibrate. This proposed mechanism, although proven to be plausible by laboratory work, remains unsupported by corresponding measurements in the field.
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.
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. None of them came from orbits from the vicinity of the asteroid belt.
Eyewitness accounts indicate that the fireball entry of the Peekskill meteorite started over Montana at 23:48 UT (±1 min). The fireball, which traveled in a northeasterly direction had a pronounced purpleish colour, and attained an estimated peak visual magnitude of -136. During a luminous flight time that exceeded 407 seconds the fireball covered a ground path of some 739.8 to 872.9 km.
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 H2 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.
Although shooting stars have been known since ancient times, they weren't know to be an astronomical phenomenon until early in the 19th century. Prior to that, they were seen in the west as an atmospheric phenomenon, like lightning, and weren't connected with strange stories of rocks falling from the sky. Thomas Jefferson wrote "I would more easily believe that (a) Yankee professor would lie than that stones would fall from heaven." He was referring to Yale chemistry professor Benjamin Silliman' investigation of an 1807 meteorite that fell in Connecticut. Silliman believed the meteor had a cosmic origin, but meteors didn't attract much attention from astronomers until the spectacular meteor storm of November 1833. People all across the Eastern US saw thousands of meteors, radiating from a single point in the sky. Astute observers noticed that the radiant, as the point is now called, moved with the stars, staying in the constellation Leo.  The astronomer Denison Olmsted made an extensive study of this storm, and concluded it had a cosmic origin. After reviewing historical records, Heinrich Wilhelm Matthias Olbers predicted its return in 1867, which drew the attention of other astronomers. Hubert A. Newton's more thorough historical work led to a refined prediction of 1866, which proved to be correct. With Giovanni Schiaparelli's success in connecting the Leonids (as they are now called) with comet Tempel-Tuttle, the cosmic origin of meteors was now firmly established. Still, they remain an atmospheric phenomenon, and retain their name "meteor" from the Greek word for "atmospheric."
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- North American Meteor Network
- International Meteor Organization
- American Meteor Society (AMS)
- Impact crater
- Impact event
- Meteor shower
- Tollmann's hypothetical bolide
- Green fireballs
- Stratospheric micrometeorites
- ↑ http://www.imo.net/glossary Glossary International Meteor Association
- ↑ Beech, M.; Steel, D. I. (September 1995). "On the Definition of the Term Meteoroid". Quarterly Journal of the Royal Astronomical Society 36 (3): 281–284, http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995QJRAS..36..281B&db_key=AST&data_type=HTML&format=&high=44b52c369007834. Retrieved on 31 August 2006. )
- ↑ Povenmire, H. PHYSICAL DYNAMICS OF THE UPSILON PEGASID FIREBALL – EUROPEAN NETWORK 190882A. Florida Institute of Technology
- ↑ Philip J. Erickson. "Millstone Hill UHF Meteor Observations: Preliminary Results".
- ↑ MeteorObs Explanations and Definitions (states IAU definition of a fireball)
- ↑ International Meteor Organization - Fireball Observations
- ↑ usgs.gov - What is a Bolide?
- ↑ The Oxford Illustrated Dictionary. 1976. Second Edition. Oxford University Press. page 533
- ↑ Psst! Sounds like a meteor: in the debate about whether or not meteors make noise, skeptics have had the upper hand until now - Now Hear This | Natural History | Find Articles at BNET.com
- ↑ Listening to Leonids
- ↑ Hearing Sensations in Electric Fields
- ↑ Human auditory system response to Modulated electromagnetic energy.
- ↑ Human Perception of Illumination with Pulsed Ultrahigh-Frequency Electromagnetic Energy
- ↑ NASA Home > World Book @ NASA, Meteors
- ↑ Diagram 2: the orbit of the Peekskill meteorite along with the orbits derived for several other meteorite falls
- ↑ The Peekskill Meteorite October 9, 1992 Videos
- ↑ "Meteoritical Bull", by Wlotzka, R. published in "Meteoritics", # 5, 28, (5), 92, 1994.
- ↑ 18.0 18.1 amsmeteors.org The Early Years of Meteor Observations in the USA
- ↑ 19.0 19.1 meteorshowersonline.com The Leonids and the Birth of Meteor Astronomy
- ↑ Hitchcock, Prof. Edward (January 1834). "On the Meteors of Nov. 13, 1833". The American Journal of Science and Arts XXV, http://books.google.com/books?id=HjcPAAAAYAAJ&pg=PA407#PPA405,M1.
- ↑ astroprofspage.com October's Orionid Meteors
- ↑ SPACE.com - How Hubble Has Survived a Decade of Impacts
- Astronomers spot meteor streaking across central Ontario sky - CBC News March 7, 2008
- Meteoroids Page at NASA's Solar System Exploration
- 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 shower predictions
- Society for Popular Astronomy - Meteor Section
- Video - Meteor brilliantly lights up sky - and breaks into pieces - Canada - Times Online - November 24, 2008
- How meteor showers were linked to comets
- Science and History of the Perseid Meteor Shower