Meteors and Meteorites
A meteor is the visible path of a meteoroid that enters the Earth's (or another body's) atmosphere, commonly called a shooting star or falling star. 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 dexcitation following the individual collisions between vaporized meteor atoms and atmospheric constituents.
A very bright meteor may be called a fireball or bolide. The International Meteor Organization defines fireballs as being meteors of magnitude -3 or brighter.
The meteor section of the British Astronomical Association on the other hand has a much stricter definition, requiring the meteor to be magnitude -5 or brighter.If a meteoroid survives its transit of the atmosphere to come to rest on the surface, the resulting object is called a meteorite.
A meteor striking the Earth or other object may produce an impact crater.
Molten terrestrial material "splashed" from such a crater can cool and solidify into an object known as a tektite.
Meteor dust particles left by falling meteoroids 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 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 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. Curiously, although the waves are electromagnetic, they can be heard: they are powerful enough to make grasses, plants, eyeglass frames, frizzy hair, the middle ear and other materials vibrate.
A meteorite is an extraterrestrial body that survives its impact with the Earth's surface without being destroyed. While in space it is called a meteoroid. When it enters the atmosphere, air resistance causes the body to heat up and emit light, thus forming a fireball, also known as a meteor or shooting star.
More generally, a meteorite on a celestial body is an object that has come from elsewhere in space. The only known examples of meteorites that didn't fall on Earth are Heat Shield Rock, which was found on Mars, and two tiny fragments of asteroids that were found among the samples collected on the Moon by Apollo 12 (1969) and Apollo 15 (1971) astronauts.
Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites are largely composed of metallic iron-nickel; and, stony-iron meteorites contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy.
Most meteoroids disintegrate when entering the Earth's atmosphere, however an estimated 500 meteorites ranging in size from peas to basketballs or larger do reach the surface each year; only 5 or 6 of these are typically recovered and made known to scientists.
Few meteorites are large enough to create impact craters. Instead, they typically arrive at the surface at their terminal velocity (free-fall) and, at most, create a small pit. Even so, falling meteorites have caused damage to property, livestock, and even people in historic times.
Very large meteoroids may strike the ground with a significant fraction of their cosmic velocity, leaving behind a hypervelocity impact crater. The kind of crater will depend on the size, composition, degree of fragmentation, and incoming angle of the impactor. The force of such collisions has the potential to cause widespread destruction.
The most frequent hypervelocity cratering events on the Earth are caused by iron meteoroids, which are most easily able to transit the atmosphere intact. Examples of craters caused by iron meteoroids include Barringer (Meteor Crater), Odessa Meteor Crater, Wabar craters, and Wolfe Creek crater; iron meteorites are found in association with all of these craters.
In contrast, even relatively large stony or icy bodies like small comets or asteroids, up to millions of tons, are disrupted in the atmosphere, and do not make impact craters. Although such disruption events are uncommon, they can cause a considerable concussion to occur; the famed Tunguska Event likely resulted from such an incident.
Many events in Kazantsev's tale were subsequently confused with the actual occurrences at Tunguska. The nuclear-powered UFO hypothesis was adopted by TV drama critics Thomas Atkins and John Baxter in their book The Fire Came By (1976). The 1998 television series The Secret KGB UFO Files (Phenomenon: The Lost Archives), broadcast on Turner Network Television, referred to the Tunguska event as "the Russian Roswell" and claimed that crashed UFO debris had been recovered from the site. In 2004, a group from the Tunguska Space Phenomenon Public State Fund claimed to have found the wreck of an alien spacecraft at the site.
The proponents of the UFO hypothesis have never been able to provide any significant evidence for their claims. It should be noted that the Tunguska site is downrange from the Baikonur Cosmodrome and has been contaminated repeatedly by Russian space debris, most notably by the failed launch of the fifth Vostok test flight on December 22, 1960. The payload landed close to the Tunguska impact site, and a team of engineers was dispatched there to recover the capsule and its two canine passengers (which survived).
Very large stony objects, hundreds of meters in diameter or more, weighing tens-of-millions of tons or more, can reach the surface and cause large craters, but are very rare. However, such events are generally so energetic that the impactor is completely destroyed, leaving no meteorites. (The very first example of a stony meteorite found in association with a large impact crater, the Morokweng Crater in South Africa, was reported in May, 2006.)
About 86% of the meteorites that fall on Earth are chondrites, which are named for the small, round particles they contain. These particles, or chondrules, are composed mostly of silicate minerals that appear to have been melted while they were free-floating objects in space.
Chondrites also contain small amounts of organic matter, including amino acids, and presolar grains. Chondrites are typically about 4.55 billion years old and are thought to represent material from the asteroid belt that never formed into large bodies. Like comets, chondritic asteroids are some of the oldest and most primitive materials in the solar system. Chondrites are often considered to be "the building blocks of the planets."
About 8% of the meteorites that fall on Earth are achondrites, some of which appear to be similar to terrestrial mafic igneous rocks. Most achondrites are also ancient rocks, and are thought to represent crustal material of asteroids. One large family of achondrites may have originated on the asteroid 4 Vesta. Others derive from different asteroids.
Two small groups of achondrites are special, as they are younger and do not appear to come from the asteroid belt. One of these groups comes from the Moon, and includes rocks similar to those brought back to Earth by Apollo and Luna programs. The other group is almost certainly from Mars and are the only materials from other planets ever recovered by man.
About 5% of meteorites that fall are iron meteorites with intergrowths of iron-nickel alloys, such as kamacite and taenite. Most iron meteorites are thought to come from the core of a number of asteroids that were once molten. As on Earth, the denser metal separated from silicate material and sank toward the center of the asteroid, forming a core. After the asteroid solidified, it broke up in a collision with another asteroid.
Stony-iron meteorites constitute the remaining 1%. They are a mixture of iron-nickel metal and silicate minerals. One group, called pallasites, are thought to have originated in the boundary zone above the core regions where iron meteorites originated. The other major group of stony-iron meteorites are called mesosiderites.
Tektites (from Greek tektos, molten), natural glass objects up to a few centimeters in size, were formed--according to most scientists--by the impact of large meteorites on Earth's surface, although a few researchers have favored an origin from the Moon as volcanic ejecta. Tektites are NOT meteorites.
One theory suggests that a large meteorite impact caused the mass extinction of the dinosaurs. It is also theorized that meteorites caused other mass extinction events throughout the history of the Earth.
The only reported fatality from meteorite impacts is an Egyptian dog who was killed in 1911, although this report is disputed. The meteorites that struck this area were identified in the 1980s as Martian in origin.
The first known modern case of a human hit by a space rock occurred on November 30, 1954 in Sylacauga, Alabama.
There a 4 kg stone chondrite meteorite crashed through a roof and hit Ann Hodges in her living room after it bounced off her radio. She was badly bruised. Several persons have since claimed to have been struck by 'meteorites' but no verifiable meteorites have resulted.
Indigenous peoples often prized iron-nickel meteorites as an easy, if limited, source of iron metal. For example, the Inuit used chips of the Cape York meteorite to form cutting edges for tools.
Entry into the atmosphere
When they hit the atmosphere, meteors rub against air particles and create friction, heating the meteors to more than 3000 degrees Fahrenheit. The heat vaporizes most meteors, creating what we call shooting stars (most become visible at around 60 miles up). Some large meteors splatter, causing a brighter flash called a fireball, which can often be heard up to 30 miles away. When meteors hit the ground, they're called meteorites. Some meteors are bits broken off asteroids, others -- mere cosmic dust -- are cast off by comets. (And one more term: A meteoroid is an object in space that may, if it enters our atmosphere, become a meteor.)
Meteor breakup
Whether an object breaks apart depends on its composition, speed, and angle of entry. A faster meteor at an oblique angle suffers greater stress. Meteors made of iron withstand the stress better than those of stone. Even an iron meteor will usually break up as the atmosphere becomes more dense, around 5 to 7 miles up. A meteor sometimes explodes above the surface, causing widespread damage from the blast and ensuing fire. This happened in 1908 over over Siberia.
Impact with Earth
Extraterrestrial objects that hit the ground, their speed roughly half what it was upon entry, blast out craters 12 to 20 times their size. Craters on Earth form much as they would on the moon or any rocky planet. Smaller objects create simple, bowl-shaped craters. Larger impacts cause a rebound that creates a central peak; slipping along the rim forms terraces. The largest impacts form basins in which multiple rebounds form several inner peaks.
"Fall" means the meteorite was witnessed by someone as it fell from the sky. A "find" means the meteorite was not witnessed and the meteorite was found after the fact. About 33% of the meteorites are witnessed falls.
Quadrantids: January
Virginids: March/April
Lyrids: April
Scorpiids: May
Capricomids: July
Perseids August
Piscids: September
Orionids: October
Leonids: November
Geminids: December
History
In ancient times, objects in the night sky conjured superstition and were associated with gods and religion. But the misunderstandings about meteors lasted longer than those of most celestial objects.
Meteorites (the pieces that make it to Earth) were long ago thought to be cast down as gifts from angels. Others thought the gods were displaying their anger. As late as the 17th Century, many believed they fell from thunderstorms (they were nicknamed "thunderstones"). Many scientists were skeptical that stones could fall from the the clouds or the heavens, and often they simply didn't believe the accounts of people who claimed to have seen such things.
In 1807, a fireball exploded over Connecticut, and several meteorites rained down. By then, the first handful of asteroids had been discovered, and a new theory emerged suggesting meteorites were broken bits off asteroids or other planets. (A theory that still holds.)
One of the most significant meteorite events in recent history destroyed hundreds of square miles of forest in Siberia on June 30, 1908. Across hundreds of miles, witnesses of the Tunguska event saw a ball of fire streak through the sky, suggesting the meteor entered the atmosphere at an oblique angle. It exploded, sending out hot winds and loud noises and shaking the ground enough to break windows in nearby villages. Small particles blown into the atmosphere lit the night sky for several days. No meteorite was ever found, and for years many scientists thought the devastation was caused by a comet. Now, the prevailing theory holds that a meteor exploded just above the surface.
The largest meteorite recovered in the United States fell in a wheat field in southern Nebraska in 1948. Witnesses saw a giant fireball in the afternoon that some said was brighter than the sun. The meteorite was found buried 10 feet deep in the ground. It weighed 2,360 pounds.
The most famous meteorite crater in the United States is misnamed Meteor Crater. It's in Arizona, and it's huge. The rim rises 150 feet from the surrounding plain, and the hole is 600 feet deep and nearly a mile wide. It was the first crater proved to be caused by a meteorite impact, which occurred between 20,000 and 50,000 years ago.
For decades astronomers have reaped the values of written records kept for
over 2500 years in China and Europe. Eclipses, planets, and comets are
mentioned and have brought about refinements to our current understanding of
the long-term motion of these bodies‹especially for comets such as Halley
and Swift-Tuttle. Several meteor showers have also been identified which
brought about a better understanding of their evolution as well.
No other culture can provide comparable information as that gathered from
the Chinese and European records, but this need not be a deterrent from
learning about how other cultures felt about these moving bodies in the sky
and one of the richest regions of meteor and comet lore in the world is North
America.
During the last 15 to 20 years, archeoastronomy has uncovered much
concerning the astronomical beliefs of native Americans. Unfortunately, the
methods of keeping records of astronomical events were not as straight forward
as those of the Chinese and Europeans, as there are no books lying around.
Instead, the methods of record keeping included rock and cave drawings, stick
notching, beadwork, pictures on animal skins and jars, and story
telling‹most of which are not dateable.
One of the few dateable events among the various records of native Americans
was the 1833 appearance of the Leonid meteor shower. Historically recognized
as one of the greatest meteor storms on record, it made a lasting impression
among the peoples of North America.
The most obvious accounts of the Leonid storm appear among the various
bands of the Sioux of the North American plains. The Sioux kept records called
"winter counts," which were a chronological, pictographic account of
each year painted on animal skin. In 1984, Von Del Chamberlain (Smithsonian
Institution) listed the astronomical references for 50 Sioux winter counts, of
which 45 plainly referred to an intense meteor shower during 1833/1834. In
addition, he listed 19 winter counts kept by other plains Indian tribes, of
which 14 obviously referred to the Leonid storm.
The Leonids also appear among the Maricopa, who used calendar sticks with
notches to represent the passage of a year, with the owner remembering the
events. The owner of one stick claimed records had been kept that way
"since the stars fell." The first notch on his stick represented
1833.
Story telling was a very important method of record keeping among most
native Americans and several seem to have been influenced by the Leonids of
1833. A member of the Papago, named Kutox, was born around 1847 or 1848. He
claimed that 14 years prior to his birth "the stars rained all over the
sky."
A less obvious Leonid reference may exist in the journal kept by Alexander
M. Stephen, which detailed his visit with the Hopi Indians and mentions a talk
he had with Old Djasjini on December 11, 1892. That Hopi Indian said "How
old am I? Fifty, maybe a hundred years, I can not tell. When I was a boy of so
big (eight or ten years) there was a great comet in the sky and at night all
the above was full of shooting stars‹ah! that was a very long time ago,
maybe a hundred years, maybe more." During the probable lifetime of Old
Djasjini there was never a "great comet" and a sky full of meteors
in the same year, but he might be referring to two separate events such as the
sungrazing comet 1843 I and the great Leonid storm of 1833, both of which
occurred early in his life.
The Pawnee have a story about a person known as Pahokatawa, who was
supposedly killed by an enemy and eaten by animals, but then brought back to
life by the gods. He was said to have come to Earth as a meteor and told the
people that when meteors were seen falling in great numbers it was not a sign
that the world would end. When the Pawnee tribe witnessed the time "the
stars fell upon the earth," which was in 1833, there was a panic, but the
leader of the tribe spoke up and said, "Remember the words of
Pahokatawa" and the people were no longer afraid.
Although the Pawnee learned not to be afraid, there were native Americans
who feared meteors. Why such beliefs came about is almost impossible to guess,
but some of the best examples are as follows:
There were other beliefs which generally did not strike fear into the
hearts of native Americans. Some of these are as follows:
Interestingly, one of the most widely accepted beliefs was that meteors
were the feces of stars. Such lore existed in the stories of the Nunamiut
Eskimos, the Koasati of Louisiana (formerly located in Tennessee), and
numerous southern California tribes. A slight variation of this came from the
Kiliwa (Baja California) who believe meteors were the fiery urine of the
constellation Xsmii [Xsmii has not been defined‹GWK].
Many of the beliefs mentioned above are also attributed to comets, and
most story telling seems to rarely provide conclusive evidence that the object
being discussed is indeed a meteor. Because of this a very interesting story
is being included which originates from the Great Lakes region.
The Ojibwa of the upper Great Lakes region had a story about Genondahwayanung, which meant "Long Tailed Heavenly Climbing Star." During the 1980's, Thor Conway visited the Ojibwa and talked to Fred Pine, an Ojibwa shaman. Pine's story about the creation notes that Genondahwayanung was a star with a long, wide tail which would return and destroy the world someday. He said, "It came down here once, thousands of years ago. Just like a sun. It had radiation and burning heat in its tail." The comet was said to have scorched the earth so that nothing was left, except the native americans, who were warned ahead of time by Chimanitou, a Holy Spirit, and had gone to a bog and rolled themselves up in the mud to protect themselves from the heat. Pine continued, "It was just so hot that everything, even the stones, were cooked. The giant animals were killed off. You can find their bones today in the earth. It is said that the comet came down and spread his tail for miles and miles." Thereafter, all comet and meteors were treated as serious omens which required the interpretation of the Ojibwa shamans.
There are other stories of a great fire coming from the sky and destroying
everything except for certain native american tribes. In some cases the tribes
claimed they were warned, while others claimed they just ran for the nearest
bodies of water.
Another form of record keeping were rock petroglyphs, or pictures carved
into rock. The western United States abounds with these pictures, but any
dating is virtually impossible. Once again it is frequently difficult to
determine whether the object carefully carved into rock is a meteor or a comet.
One rock drawing frequently debated as to its exact depiction was produced
by the Ventureño tribelet of the Chumash at Burro Flats. A pair of disks
with long tails are located on the wall of a cave and have been interpreted by
Travis Hudson and Ernest Underhay (1978) as portraits of a comet "seen
over an interval of a few days or weeks." On the other hand, E. C. Krupp
(1983) has pointed out that "the images have a dynamic appearance that
suggests rapid movement and change. If they are celestial at all, I would
associate them with meteors, and, in particular, with the especially bright
and dramatic type known as fireballs."
The most common petroglyphs depict a circle with a wiggly line emanating
from it. Various archealogists have interpreted these as meteors, comets, and
even snakes.
Another form of record keeping appears in the form of pottery art. Although there are not many examples of this, the Field Museum in Chicago contains Hopi jar (designated number 66760) with a very striking scene depicted. Brought to the museum during the 1890s, the jar depicts mountains, above which are stars and three objects falling towards the ground. Although the scene seems to imply meteors, it is not certain whether it is a shower or a spectacular meteor that broke up as it fell.
According to William Grewe-Mullins at the Field Museum, the notes on this jar indicate it was found near Oraibi, Arizona, and was of recent origin. He ventured to guess that the jar might have been made sometime during the 1850s to 1890s.
It might be possible that this jar depicts the Leonid storm of 1833, although it seems difficult to imagine the Hopi would have still been impressed so much by the storm 2 to 5 decades after the event.
On the other hand, it could be a painting of one of two other storms which were observed in various parts of the world in 1872 and 1884, although none of the winter counts mentioned earlier seem to have noted these.
The Nunamiut Eskimos also found meteorites, but believed they came from thunderstorms.
Native Americans - Among the Menomini of the Great Lakes region is the following legend:
"Oddball" Scottish Rocks Formed By Meteorite National Geographic - April 1, 2008
Huge Meteorite Impact Found In UK -- Britain's Largest Science Daily - March 31, 2008
"Giant Fireball" Impact in September 2007 in Peru Upends Meteorite Theory National Geographic - March 11, 2008
Dark Halos/Craters Discovered on Mercury NASA - March 10, 2008
Mysterious Meteorites Stymie Scientists in Antarctica National Geographic - March 13, 2008
Astronomers Capture Rare Video Of Meteor Falling To Earth; Hunt For Meteorite Science Daily - March 9, 2008
Giant Meteor Fireball Explodes Over Northwest U.S. National Geographic - February 21, 2008
Report clarifies some, but not all of impact New Scientist - September 28, 2007
Meteor Crash in Peru Caused Mysterious Illness National Geographic - September 21, 2007
Meteorite yields life origin clue BBC - December 1, 2006
Meteorite's Organic Matter Older Than the Sun, Study Says National Geographic - December 1, 2006
The ancient materials could offer a glimpse into the
solar system's planet-building past and may even
provide clues to how life on Earth first arose.
Mystery of Arizona's Meteor Crater Solved - Crashed 49,000 year ago BBC - March 10, 2005
Largest ever impact field of meteor craters uncovered under Sahara New Scientist - November 7, 2004
Rare space rock 'a gem' BBC - July 22, 2002
British scientists have confirmed that one of the rarest meteorites ever to fall to Earth is from a time when the Solar System was born.
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