Meteors, Meteoroids, and Meteorites Meteors, Meteoroids, and Meteorites Meteorite Impacts And Showers A meteoroid is a solid body in interplanetary space before it reaches the Earth's atmosphere. A meteor denotes the fiery streak or "shooting star" which appears when a tiny meteoroid strikes the Earth's atmosphere and burns up. Most meteors result from meteoroids no more than a few centimeters in diameter. A larger meteoroid which survives the fiery passage through the Earth's atmosphere as a meteor and strikes the Earth's surface is called a meteorite. Meteoric material less than a tenth of a millimeter in diameter is called cosmic dust. Meteors A Perseid meteor on August 12, 1986. Meteors appear when centimeter-sized meteoroids travelling at least 11 km/sec, but more usually 30 to 55 km/sec, strike the Earth's atmosphere. The maximum speed is 72 km/sec. The meteoroid's kinetic energy of motion converts into heat, vaporizing the meteoroid at heights above 60 km. The hot vapor trail is what we see as a meteor.

The vaporized material may reach temperatures of 1,000 to 2,000 Kelvins. The period during which the meteor flashes is called incandescent flight. The period after the light phenomena cease is called dark flight. Meteorites the size of a golf ball (two or three centimeters) or larger vaporize in exceptionally brilliant flashes called fireballs or bolides. These may also produce a variety of sounds . Theodor Abrahamsen's photo at the right captures a Perseid meteor on August 12, 1986. Meteor Showers You can see five or six meteors each hour from any given vantage point on Earth when atmospheric conditions allow. Up to 25 million meteors arrive each day, dropping about 100 tons of material. Most meteors are composed of debris left behind by comets as they orbit the Sun. A meteor shower occurs when the Earth intersects a comet's path and moves through the stream of debris and dust emitted by the comet. The meteors in a shower appear to originate from one area of the sky called the radiant. The meteor shower is usually named after the constellation in which the radiant lies.

Meteor showers occurs at the same time each year. Common meteor showers result in ten to fifty meteors per hour. Typically the best time to observe is in the early morning. The following table lists a few of the more prominent meteor showers. Some Prominent Meteor Showers Meteor Storms Leonid Storm of November, 1833 Occasionally the Earth passes through an unusually heavy concentration of cometary debris resulting in a meteor storm. Hundreds or even thousands of meteors may flash each hour. One of the historically most prominent meteor storms, the Leonid storm, occurs at about thirty-three year intervals. The Leonid shower normally produces about ten meteors per hour. When they storm, the Leonids can produce the equivalent of over one hundred thousand meteors per hour for a short period. The woodcut at the right by artist Adolf Vollmy, based upon an original painting by the Swiss artist Karl Jauslin, portrays the great Leonid meteor storm of November 12-13, 1833.

Victorian era astronomy writer Agnes Clerke described that storm as follows: "On the night of November 12-13, 1833, a tempest of falling stars broke over the earth.... the sky was scored in every direction with shining tracks and illuminated with majestic fireballs. At Boston, the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm. Their numbers ... were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed, on the basis of that much-diminished rate, that 240,000 must have been visible during the nine hours they continued to fall." Periodic comet Temple-Tuttle is the parent (originating object) of the debris for the Leonid meteor shower. Leonids are among the fastest known meteors, striking the Earth's atmosphere at speeds of 71 km/sec on average. Curiously, no meteorite fall has ever been associated with a meteor shower. So far all meteorites observed to fall during a meteor shower followed different orbits than the material forming the meteor shower.

Most meteoroids large and solid enough to reach the Earth's surface as meteorites are asteroid fragments. A few meteorites appear to be debris lifted off the Moon and Mars by large impact events. A handful may be fragments of comets. Meteorites Meteoroids the size of a fist or larger may survive the trip through the atmosphere to land on the Earth's surface. They are then known as meteorites. A meteorite located after a witnessed descent is called a fall. A meteorite from an unwitnessed descent is called a find. Meteorites are usually named for a post office or another geographic landmark close to the place where the meteorite was found. The name of the meteorite can refer to either a specimen of the meteorite itself or to the locality in which it was found. Meteorites include some of the oldest and most primitive solar system material. Radiometric dates suggest some meteorites are as much as 4.54 billion years old. Some even include cosmic material formed before the solar system itself was born. Because many meteorites have changed so little in the intervening eons, they offer a window into the early history of the solar system. Meteorites also represent some of the rarest material on Earth.

Until the advent of the space age meteorites were the only extraterrestrial material available for study here on Earth. Both the scarcity and the scientific importance of meteorites leads collectors and researchers alike to seek them out. Types Of Meteorites Meteorites form three main groups based upon their composition. Types of Meteorites Mixture of iron and stony material. Similar to type S asteroids. Vaca Muerta, Altacama, Chile (MES) Iron Meteorites Iron meteorites are probably what most people picture as "typical" meteorites. Iron meteorites consist almost entirely of a mixture of metallic nickel and iron. They are easier to spot on the ground because their highly unoxidized iron content stands out from background rocks. The outer surface of iron meteorites often melts during their passage through the atmosphere resulting in a dark fusion crust. Primary fusion crust forms while the meteoroid is incandescent. Secondary fusion crust forms on the broken surfaces of fragments which break free from the main mass during incandescent flight. They may also exhibit flow markings and interesting molten metal shapes. The interior of some iron meteorites displays a criss-cross pattern of different iron-nickel minerals.

Iron meteorites may originate in the cores of differentiated parent bodies at least 100 km in diameter. The composition of some main-belt asteroids called M-type asteroids resembles that of iron meteorites. These M-type asteroids may be the source of iron meteorites. Iron meteorites with weights of 50 to 100 kg are not uncommon. The Hoba meteorite, at 60 tons, is the largest known iron meteorite to have landed without exploding. It still lies where it was found. Stony Meteorites Stony meteorites are the most common, making up about 94% of observed falls. They are composed of 75-90% rocky silicates including familiar minerals such as pyroxene, olivine, and plagioclase, and 10-25% nickel-iron metal and iron sulfide. (Silicates are minerals containing silicon, oxygen, and one or more metals.) Stony meteorites are difficult to find because they look like terrestrial rocks. The best places to find stony meteorites are in deserts or on the ice sheet of Antarctica. The meteorites stand out against the background of ice or sand.

Like iron meteorites, stony meteorites often exhibit a dark fusion crust. There are three major subgroups of stony meteorites, Chondrites, Carbonaceous Chondrites, and Achondrites. Chondrites are the most common type of stony meteorite. About 86% of all recovered stony meteorites are chondrites. Chondrites are composed of small spherical chondrules. Chondrules are millimeter to centimeter sized glassy mineral spheres. Chondrules are composed of silicate material that has melted and then resolidified. Chondrules formed early in solar system history. They were the most primitive "building blocks" of the solar system. Over time chondrules accreted to form larger and larger objects including asteroids, moons, and planets. In some chondrites the chondrules are separated by patches of iron metal. Different types of chondritic meteorites contain different amounts of metal. They have been heated to varying degrees. Chondrites are called primitive because they have changed very little since their initial formation early in the history of the solar system. Their composition resembles that of the Sun except that the lightest gases Hydrogen and Helium are missing from the meteorites.

Ordinary chondrites are the most common type of meteorite, representing 87% of all recovered specimens. The parent body or bodies are unknown, but asteroids 6 Hebe and 3628 Boznemcová have been suggested as possible sources. Enstatite chondrites are metal rich meteorites in which the primary mineral is Enstatite. Enstatite chondrites may be fragments of asteroid 16 Psyche. Some scientists have suggested Mercury as the originating body. Carbonaceous Chondrites are essentially just pieces of chondrules stuck together. They are very black because of their high carbon content. Some of their mineral grains predate the solar system -- probably fragments blown out from distant stars that became supernovae. Carbonaceous chondrites also contain water and amino acids. Some types of carbonaceous chondrites may be cometary material. The building blocks of life on Earth may have been seeded by comets and carbonaceous meteorites early in Earth's history. For example, the Murchison meteorite, a fragment of which appears in the table above, was found in 1969.

This carbonaceous chondrite contains 16 amino acids, 11 of which are rare on Earth. It may represent the type of cosmic visitor which early in the Earth's history brought the raw materials needed to jump start life. In some stony meteorites called achondrites the chondrules have been partially or completely destroyed by metamorphic processes. This took substantial time and pressure. Such meteorites must be fragments of the interior of larger bodies on which the weight of the overlying rock created enough pressure to obliterate the chondrules. About 7% of recovered stony meteorites are achondrites. Some achondrites resemble terrestrial igneous rocks and formed during volcanic eruptions on planets and asteroids. Some asteroids like Vesta heated up enough that their interiors melted and erupted lava onto their surfaces. The lava hardened into a rock called basalt. The Mt. Egerton meteorite (see table above) is a type of achondrite known as an aubrite. Fragments of DAG 476 Mars Rock Some achondrites are composed of rock fragments broken and fused back together during an impact event. Meteorites believed to originate from the Moon and Mars are achondrites that formed during impact events.

The achondrite Dar al Gani 476 (see photo of fragments at right) is a type known as a Shergottite. It probably originated as a fragment of the planet Mars blasted off the surface during a large impact event. The heaviest known stony meteorite was Jilin which weighed 1.8 tons. It fell in Jilin, China on March 8, 1976 as part of a meteorite shower (see below) which produced about four tons of meteoric material altogether. Witnesses of Jilin report a spectacular daytime fireball and several explosions. Jilin is classfied as an olivine-bronzite chondrite (H5). Because the Chinese leader Mao Zedung died three days after this fall, many Chinese took Jilin as an omen. Stony Iron Meteorites Stony Iron meteorites are the rarest type of meteorite, making up about 1 to 2% of all recovered meteorites. Stony Iron meteorites consist of a mixture of rocky silicates and metallic nickel/iron. There are two main groups of stony iron meteorites.

Pallasites are composed of olivine crystals set in a nickel-iron matrix. They probably formed in the boundary layer between the iron core and the stony mantle of an asteroid. Pallasites are very popular as jewelry when cut and polished. Mesosiderites are conposed of pyroxene, olivine, plagioclase, and metal grains. They probably formed when a metal-rich asteroid collided with a silicate-rich asteroid. Cosmic Dust A tiny meteoroid of whatever composition which is smaller than 0.1 mm in diameter is called a cosmic dust particle. The frictional heating which occurs during the descent through the Earth's atmosphere does not melt cosmic dust particles because of the large surface to mass ratio of such particles. Cosmic dust radiates away the heat which burns up larger meteorites. The Earth accumulates about 10,000 tons of cosmic dust each year. The Zodiacal Light, a faint pyramid-shaped glow extending away from the Sun along the plane of the ecliptic, is caused by sunlight scattered off cosmic dust particles. The gegenschein or counterglow which appears as a faint spot of light opposite the Sun is also caused by cosmic dust.

Both are most visible early in the morning a couple of hours before sunrise. The cosmic dust particles causing the Zodiacal Light and the Gegenschein form a very low density cloud in the same plane as the planets. These particles slowly spiral into the Sun over time and are replaced by new particles emitted from comets and asteroid collisions. Meteorites from Comets? As noted previously, no meteorite has ever been known to fall from a meteor shower. This leads astronomers to disagree about whether any cometary material can survive passage through the Earth's atmosphere to reach the ground. However, the composition of some carbonaceous chondrites appears so similar to the expected composition of comets that such meteorites may be pieces of a comet. In particular, the class CI meteorites are often cited as potential comet fragments. There are only seven known CI class meteorites: Ivuna (see fragments at right), Alais, Orgueil, Revelstoke, Tonk, Yamato 82162, and Tagish Lake. For many years some believed these meteorites may have come from dormant comets masquerading as near-Earth asteroids.

The entry speed would be low enough for meteoroids derived from such carbonaceous near-Earth "asteroids" to produce meteorites, even though these would be ultimately of cometary origin. Some suggest CI meteorites have a Martian origin, making them the oldest known Martian meteorites. Both of these origin hypotheses remain controversial. The most commonly held hypothesis is that CI meteorites are of asteroidal origin because of their similarity to CM class meteorites, indicating a similar origin process. Meteorite Impacts and Showers Most meteorites range in size from fist to head size. Occasionally larger specimens -- over 50 kg -- land. All of these are stopped and often fragmented by contact with the Earth's atmosphere which acts as a "brick wall" to most meteorites. (Think about how the air resists you when you ride a bike, and consider that the meteorite is travelling thousands of times faster.) The faster the meteoroid enters the atmosphere, the more the atmosphere slows it down. The effects upon landing depend primarily upon the mass and composition of the meteorite. As noted above, cometary material usually strikes the Earth's atmosphere at speeds between 30 and 55 km/sec, sometimes up to 72 km/sec.

Most asteroidal material arrives at slower speeds, between 11 and 20 km/sec. This is one reason why cometary material rarely (if ever) reaches the ground. The higher entry speed of cometary material results in its more thorough destruction by atmospheric heating. Another reason is that cometary material is much more fragile than asteroidal material. The light phenomena of the meteor cease once the meteoroid is stopped by the atmosphere. Any remaining meteorite fragments not vaporized during incandescent flight fall subsonically in dark flight to the ground. When a meteoroid fragments in the atmosphere, the surviving fragments may rain down as a meteorite shower. (Do not confuse this with a meteor shower!) The fragments land in an elliptical region call the fall ellipse. Stony meteoroids are more likely to produce meteorite showers because they break up more easily than iron or stony iron meteoroids. For example. the Jilin, China daytime meteorite shower of March 8, 1976 was caused by the fragmentation of a stony body.

However, the famous Sikhote-Alin fall of February 12, 1947 was a daytime meteorite shower caused by the fragmentation of an iron body with a mass of 70 to 100 tons. Witnesses reported a fireball brighter than the Sun around 10:38 a.m. The trail of smoke and dust stretched out about 35 kilometers and remained visible for several hours. The fireball flash and the sound of the fall were seen and heard over 300 kilometers away from the point of impact. The speed of entry was estimated to be about 14.5 kilometers per second. The meteoroid fragmented as it descended until it reached an altitude of about 5.5 kilometers. At this height the main mass of the meteoroid exploded violently. The fall ellipse formed by the meteorite fragments covered an area between one and two square kilometers. Bigger fragments produced at least 122 small craters and pits, the largest of which measured 28 meters in diameter and 6 meters deep. The table above shows a small fragment from the Sikhote-Alin meteorite shower. See Roy Gallant's article about the Sikhote-Alin meteorite shower for more information.

Stony meteorites feel lukewarm at best when they reach the ground. Stony iron and iron meteorites may retain more heat. Eye witnesses have reported some stony iron and iron meteorites have scorched the earth around the spot where they landed, but most meteoriticists discount these reports. A giant meteorite too large to be stopped completely by the atmosphere may strike the Earth with much of its cosmic velocity intact. Such a meteorite may explode in the air near the ground causing an airblast or airburst, or explode upon contact with the ground forming a crater . The Tunguska event of 1908 was caused by the powerful airburst of a stony body or comet fragment several kilometers above the Earth's surface. The much smaller Sikhote-Alin impactor also exploded violently in an airburst. The famous Meteor Crater in Arizona resulted from the impact and explosion of a large iron meteorite. The Earth's atmosphere cannot prevent ground impact damage for stony meteorites greater than 200 meters in diameter. For iron meteorites that impact at greater than 20 km/sec, the critical diameter is about 40-60 meters.

Stony bodies greater than 60 meters and less than 200 meters can cause significant airburst damage. How Dangerous are Earth-Crossing Objects? discusses the damage caused by giant meteorite strikes. Asteroids, meteoroids, meteors, meteorites, comets Asteroids, meteoroids, meteors, meteorites, comets Asteroids, meteoroids, meteors, meteorites, comets What's the difference between asteroids, meteoroids, meteors, meteorites and comets? Contents A celestial body bigger than 10 m orbiting the Sun, mainly between Mars and Jupiter Meteoroid Similar to an asteroid, but significantly smaller. Mostly debris of comets, sometimes debris of asteroids. Meteor A bright tail of light caused by a meteoroid during its atmospheric flight, also called a shooting star or falling star. Fireball A very bright meteor (brighter than the planet Venus). Bolide A fireball that explodes during its atmospheric flight, often with visible fragmentation. Meteorite The part of a meteoroid or asteroid that survives the passage through our atmosphere and reaches the Earth's surface. Comet A smaller celestial body mainly composed of ice and dust.

If a comet approaches the Sun it can generate a tail of gas and/or dust. Asteroids An asteroid is a celestial body - composed of rock, metal or a mixture of both - that is orbiting the Sun. Most of them are in the asteroid belt between Mars and Jupiter. Even though there are millions of asteroids with sizes up to more than 500 km (like Pallas and Vesta) they are of no danger to the planet Earth. The biggest body in the asteroid belt - Ceres - is officially not called an asteroid anymore but a dwarf planet . If you try to envision the asteroid belt don't get fooled by some science fiction films: travelling around in the asteroid belt with your spacecraft doesn't require constant steering in order to avoid crashes with asteroids. The scale of the solar system is so immense that even inside the asteroid belt the average distance between two asteroids is above one million km - or three times the distance between Earth and the Moon. Asteroid Itokawa, an Apollo asteroid with a length of 500 metres. Credit: JAXA Some asteroids have very elliptical trajectories, crossing the orbits of the inner planets Mars, Earth or Venus.

The cause of these elliptical trajectories could be collisions within the asteroid belt or the gravitational influence of the massive planet Jupiter changing the orbits of some asteroids gradually over time (see orbital resonance ). All asteroids with orbits so eccentric that they cross Earth's orbit are called 'Apollo asteroids', 'Amors' approach the Earth but do not cross Earth's orbit. Apollo asteroids are doomed to sooner or later collide with one of the inner planets, usually within a few million years of their orbit becoming so eccentric. The largest Apollo asteroid - 1866 Sisyphus - has a diameter of about 9 km, similar to the asteroid that caused the Chicxulub event , the giant meteorite impact that caused the extinction of the dinosaurs. Anyhow, Sisyphus and none of the other big Apollo asteroids will collide with Earth in the next millennia; which doesn't mean that smaller bodies can cause local damage. Many of you will remember the Chelyabinsk event which took place on the 15th of February 2013. Fortunately no people were killed during this event and today you can even buy a Chelyabinsk meteorite .

Meteoroids and meteors Generally speaking, meteoroids are all the smaller objects in orbit around the Sun. Most of them originate from comets that lose gas and dust when they approach the Sun. Other meteoroids are basically small asteroids. There is no exact diameter that distinguishes an asteroid from a meteoroid. Wikipedia states 10 metres; other trustworthy sites call anything smaller than 1 km a meteoroid. Anyhow, the vast majority of all meteoroids are just a few millimetres and less in size. The smallest and by far the most numerous ones have sizes of small dust particles and are called micrometeoroids; they do not leave any visible trace behind when they enter the Earth's atmosphere. Perseid meteor shower on August 12, 2012. Image taken by David Kingham in Snowy Range (Wyoming). The ones about the size of a pebble leave behind a flash of light when they completely vaporise. Most people call this flash a "shooting star" or a "falling star", but more accurately spoken this is a meteor. A meteor is the light that you can see when a small meteoroid enters the Earth’s atmosphere.

This normally happens with speeds between 11 and 73 km/s and at altitudes of about 75-120 km. Under a clear sky an observer can see 5 to 10 meteors per hour, especially after midnight when the Earth has rotated so far that the observer's part of the sky is positioned in the direction of the Earth's motion around the Sun. During so called meteor showers the rate of observable meteors per hour can increase significantly. Meteor showers are caused when the Earth crosses higher than usual concentrations of particles that are themselves in an eccentric orbit around the Sun. Since the orbit of these particles is fixed, we encounter this stream every year at the same time - just its density cannot be foreseen. This sometimes leads to sparse meteor showers and sometimes very intense meteor showers with more than 1000 meteors per hour, also called meteor outbursts or meteor storms. The meteors we see can be debris from a comet (> 90% of all meteors we see) or an asteroid.

The most famous meteor showers are the Perseids in mid-August (caused by Comet 109P/Swift-Tuttle ) and the Leonids (mid-November). The meteors during these meteor showers almost all emerge from the same section of the sky; indeed the meteor showers are named for the constellations from which the meteors appear to originate. Leonid meteor, image taken during the peak of the 2009 Leonid Meteor Shower (17th November). Credit: Wikipedia.org, user: Navicore. But what causes the light path of the meteor that we can see in the sky? Smaller meteoroids will be heated by adiabatic compression until the point when they completely disintegrate. However, the light emission we observe is mainly caused by interactions between evaporated and detached components of the fast moving meteoroid and air molecules. Both the meteoroid atoms and the air molecules ionize during this encounter. When the free electrons recombine with the ionized atoms in the tail of the meteoroid they emit the light that we can observe. The light track can have a length of up to several tenths of kilometres and an initial diameter of a few metres.

The colour of the meteor is an indicator of the material of the meteoroid; e.g., a yellow colour is caused by iron, a blue-green colour by copper and a red colour by silicate material. A meteor that is larger and brighter than normal is called a fireball; brighter than the brightest planet in our night sky (Venus). If these fireballs also break apart or explode during their atmospheric flight - sometimes accompanied by considerable audible sounds - they are called a bolide. Meteorites Finally, every asteroid or meteoroid that survives its passage through Earth's atmosphere (and this is the rare exception) can be advanced to be called a meteorite. Meteorites are made of rock ( stony meteorites ), metal ( iron meteorites ) or a mixture of these two materials ( stony-iron meteorites or pallasites). Pallasites form beautiful olivine crystals that are embedded into a metal matrix. Scientists are eager to study meteorites since they are the very first material that was formed in our early solar system, almost 4.6 billion years ago. Sun.org offers genuine meteorites for sale in our meteorite shop . The three main types of meteorites.

Credit: Sun.org - released under CC-BY-SA 3.0 Comets Comet Hartley 2 has a length of about 2 km and a short orbital period of just 6.46 years. CreditNASA/JPL-Caltech/UMD. Comets are asteroid-like objects which are composed of ice, dust and rocky particles; that's why they are also called 'dirty snowballs'. The sizes of their nuclei vary between a few hundred metres to tens of kilometres in diameter; their visible tails can extend to above 150 million km in length. They originate from outside Neptune's orbit and - like many asteroids and meteoroids - are unmodified remnants of the formation of our solar system about 4.568 billion years ago. When comets approach the Sun the solar radiation and solar winds cause particles to sublimate and detach from the comet, forming a tail of particles which often makes them visible in the night sky even to the naked eye. We say 'sublimate' (a direct phase transition from the solid to the gas phase) since with zero pressure in space, water will not exist in the liquid phase.

Anyhow, below its surface there can also be reservoirs of liquid water which can vaporise and feed jets of water vapour. Comets orbit as around the Sun on elliptical orbits until all of their volatile material has evaporated away. The orbital periods vary between a few years (like comet Encke ) and tens of millions of years. While we can observe Halley's Comet every 75 years we need to wait 106 000 more years until we see Comet Panstarrs (C/2011 L4) , our guest in 2013, the next time. The Kuiper belt is the region from where most of the short-period comets derive. Credit:NASA. Short-period comets mainly originate from the Kuiper belt, a region in the solar system with many millions of icy bodies extending from about 30 AU (about the orbit of Neptune) to 50 AU. If some of these icy bodies get too close to Neptune during their orbit they may be deflected and enter a new, eccentric orbit which will make them become short-period comets.

Long-period comets normally originate from the Oort cloud, a region between 2000 AU and 50000 AU (or about one light year) away from the Sun. The Oort cloud consists of trillions of icy objects with diameters above 1 km. With these huge numbers we can be sure that there will be no shortage of comets visiting the inner part of the solar system in the future. But what causes these icy objects in the Oort cloud to leave their stable orbit and approach the inner part of the solar system? Without any "push" they would certainly continue orbiting in the Oort cloud forever. But gravitational perturbations of nearby passing stars and the galactic tide can cause these comets to change their trajectory around the Sun and approach the inner parts of the solar system. The star Gliese 710 will approach within a distance of just 1 light year from the Sun in about 1.4 million years, scratching the Oort cloud and causing many objects to change their trajectories around the Sun. Comet Lovejoy. This image is taken from the International Space Station (ISS) on December 22, 2011.

At the bottom of the image you can see Earth's atmosphere. Credit: Dan Burbank (ISS Expedition 30, NASA) Comets from the Kuiper belt tend to orbit the Sun within the plane of the solar system because the Kuiper belt itself is aligned with the plane of the solar system. Comets from the Oort cloud can arrive from all different directions since the Oort cloud has a spherical shape. A comet's tail is caused by gas and dust particles that are sublimated and/or vaporised by sunlight and then blown away by the solar wind. The tail always streams out in the direction opposite to the Sun, but it doesn’t arise until the comet enters the inner parts of the solar system (somewhere between Mars and Jupiter), so that the sunlight can sufficiently heat up the comet. All text and articles published by Sun.org are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. Komet 67P/Tschurjumow-Gerassimenko A Collection of Asteroids and Comets Ida and Dactyl Meteor FAQs : American Meteor Society American Meteor Society Below are some relatively concise answers to the above questions.

If you need further clarification or have further questions, please feel free to contact us via electronic mail. 1. What is the difference between a meteor, a meteorite, and a meteoroid? Download a printable version of this poster for FREE. Meteoroids are the smallest members of the solar system, ranging in size from large fragments of asteroids or comets, to extremely small micrometeoroids. Whenever a meteoroid plows into the Earth’s atmosphere, it will create a brief flash of moving light in the sky, called a meteor. Meteors were once thought to be a purely atmospheric phenomena, and the study of these and other atmospheric effects, especially weather, spawned the science of meteorology. It was not until the mid-1800’s that the extra-terrestrial nature of meteors was widely recognized. If remnants of the parent meteoroid survive the trip through the atmosphere to reach the ground, then these remnants are called meteorites. Back to top 2. How high up do meteors occur? Most meteors occur in the region of the atmosphere called the thermosphere.

This “meteoric region” lies between about 80 km and 120 km (50 to 75 miles) in altitude. This is a general guideline only, since very fast meteors may first become visible above this height, and slow, bright meteors may penetrate below this band. Back to top 3. How big are most meteoroids? How fast do they travel? The majority of visible meteors are caused by particles ranging in size from about that of a small pebble down to a grain of sand, and generally weigh less than 1-2 grams. Those of asteroid origin can be composed of dense stony or metallic material (the minority) while those of cometary origin (the majority) have low densities and are composed of a “fluffy” conglomerate of material, frequently called a “dustball.” The brilliant flash of light from a meteor is not caused so much by the meteoroid’s mass, but by its high level of kinetic energy as it collides with the atmosphere. Meteors enter the atmosphere at speeds ranging from 11 km/sec (25,000 mph), to 72 km/sec (160,000 mph!).

When the meteoroid collides with air molecules, its high level of kinetic energy rapidly ionizes and excites a long, thin column of atmospheric atoms along the meteoroid’s path, creating a flash of light visible from the ground below. This column, or meteor trail, is usually less than 1 meter in diameter, but will be tens of kilometers long. The wide range in meteoroid speeds is caused partly by the fact that the Earth itself is traveling at about 30 km/sec (67,000 mph) as it revolves around the sun. On the evening side, or trailing edge of the Earth, meteoroids must catch up to the earth’s atmosphere to cause a meteor, and tend to be slow. On the morning side, or leading edge of the earth, meteoroids can collide head-on with the atmosphere and tend to be fast. Back to top 4. How many meteors can I expect to see if I go out to observe for them when no meteor shower is occurring? The number of random, or “sporadic” meteors that can be seen in the night sky is quite variable, depending upon such factors as the time of night, time of year, light pollution, and cloud conditions.

Perhaps the most important factors necessary in order to observe meteors are to have a clear, unobstructed view, out in the open, and under as dark sky conditions as possible. Over the course of a night, it will be noticed that more sporadic meteors can be seen in the hours before sunrise than in the hours after sunset. This is due to the motion of the Earth as it revolves around the sun, with the leading edge (morning side) of the Earth encountering more meteoroids than the trailing edge (evening side). In general, 2 to 3 times as many meteors can be seen in the hour or so just before morning twilight, than can be seen in the early evening. Additionally, the numbers of random, or sporadic meteors will also vary from season to season, due to the tilt of the Earth on its axis and other factors. As a general rule, about 2 to 3 times as many sporadic meteors can be seen in the early fall (September) as can be seen in the early spring (March).

Together, these two effects can generate a fluctuation in the hourly rate of sporadic meteors by a factor of 4 to 9 times, over the course of the year. Under good conditions, only about 2-4 sporadic meteors can be seen per hour in the early evening in March, with this rate increasing to about 4-8 sporadic meteors per hour by morning twilight. These rates will then slowly increase throughout the spring and summer. By the month of September, the evening sporadic rate will be up to about 4-8 meteors per hour, increasing up to about 8-16 sporadic meteors per hour by morning twilight. Throughout the remainder of the fall and winter, these rates will slowly drop off, returning to the March levels again. Note that these rates are rough guidelines only, with random statistical fluctuations, observing conditions, and personal perception all playing a role in the actual number of meteors seen. Back to top 5. What is a meteor shower? Does a shower occur “all at once” or over a period of time? Most meteor showers have their origins with comets.

Each time a comet swings by the sun, it produces copious amounts of meteoroid sized particles which will eventually spread out along the entire orbit of the comet to form a meteoroid “stream.” If the Earth’s orbit and the comet’s orbit intersect at some point, then the Earth will pass through this stream for a few days at roughly the same time each year, encountering a meteor shower. The only major shower clearly shown to be non-cometary is the Geminid shower, which share an orbit with the asteroid (3200 Phaethon): one that comes unusually close to the sun as well as passing through the earth’s orbit. Most shower meteoroids appear to be “fluffy”, but the Geminids are much more durable as might be expected from asteroid fragments. Because meteor shower particles are all traveling in parallel paths, at the same velocity, they will all appear to radiate from a single point in the sky to an observer below. This radiant point is caused by the effect of perspective, similar to railroad tracks converging at a single vanishing point on the horizon when viewed from the middle of the tracks. Meteor showers are usually named for the constellation in which their radiant lies at the time of shower maximum.

Thus, the Perseid meteor shower (peaking about August 12) will appear to radiate from the constellation of Perseus, while the Leonid meteor shower (peaking about November 18) will appear to radiate from the constellation Leo. Meteor shower rates are highly variable, with the number of shower meteors seen following a curve of activity which usually lasts several days. Beginning at some level below the sporadic meteor background rate, the number of shower meteors seen will increase exponentially as the Earth approaches the densest portion of the stream. The rate will then peak at some maximum level, followed by a decreasing exponential decay back below the normal sporadic level as the Earth leaves the stream. The duration of peak activity can vary widely between showers. Some meteor showers (such as the Quadrantids) have very sharp maximums, displaying their best rates for only a few hours each year. Other major showers (such as the Taurids) have a broader maximum, which can span across a few nights. Meteor streams also vary greatly in strength between each other, depending upon such factors as the stream age, parent body composition, stream particle density and distribution, and how close the earth approaches to the stream core.

Of the 10 major meteor showers, the low-rate showers (such as the Taurids and April Lyrids) will produce only about 10-15 meteors per hour at their peak under good conditions, while the high-rate showers (such as the Perseids or Geminids) can produce up to 50-100 meteors per hour at their peaks. It is important to note that even the high rate showers will still produce only about 1 to 2 meteors each minute, with faster or slower periods occurring over time. Along with the major meteor showers, there are also a number of minor meteor showers which, while greater in number than the major streams, are difficult to detect above the background sporadic meteor rate. These showers will generally yield only about 1-5 meteors per hour at their maximums, with only a sprinkling of meteors produced on non-maximum nights. It usually requires many hours of observing experience in order to correctly recognize and classify minor shower meteors. Back to top 6. How can I find out when a meteor shower is occurring, where and how to look, and what to expect?

There are a variety of sources for information on meteor showers, ranging from encyclopedia articles, to amateur astronomy books, to periodicals such as Astronomy and Sky & Telescope. In addition, the Internet is a rapidly growing source for information on astronomical topics. A few meteor shower observing guidelines are included below: In order to successfully observe a meteor shower, some familiarity with the night sky is usually required, including the use of star charts to locate constellations and locations on the celestial sphere using the Right Ascension / Declination coordinate system. Plan your observing session as close to the time of shower maximum as possible. Meteor showers are usually quite disappointing under city and suburban conditions, so a dark observation site, far from city lights is preferred. Similarly, Meteor showers which occur near the time of gibbous or full moon usually do not perform well. Many meteor shower radiants do not rise before midnight, making most meteor showers best between midnight and morning twilight. Once at the observation site, ample time should be allotted for your eyes to adjust to dark conditions, as this can take over an hour for full dark adaptation. No magnification devices will be necessary. The use of all lights should be minimized, with only dim, red pen-lights or flash-lights used sparingly.

Most meteor observers observe from a reclining position, either in a lawn chair or sleeping bag, with their gaze directed about 45 degrees above the horizon, in the general direction of the shower radiant. The best portion of the sky to watch is usually an area of sky about 30 degrees away from the radiant point for the shower. Due to the effect of perspective, shower meteors which appear very close to the radiant will be quite short in length, while those which appear some distance from the radiant can be quite long. Members of the same shower, while varying greatly in brightness, will share common characteristics, such as speed, color range, and potential for leaving behind a train (a glowing wake of air left behind after the meteor has passed). It will also be noticed that the number of shower meteors seen will improve as the radiant gets higher in the sky. This is because meteors seen near the horizon are much farther away than those seen directly overhead, making them dimmer and harder to notice. Also, the light from a meteor near the horizon must pass through much more atmosphere to reach the observer than for a meteor overhead, further attenuating the light from meteors at low elevation angles.

Perhaps the key work to remember in meteor observing is patience. Most meteor showers will not produce a spectacular display, but will instead produce a steady, reliable show — sometimes with a few surprises. Meteor watching is like watching a graceful, natural fireworks display, and you never know when or how bright the next “shot” will be. Back to top 7. Does the published meteor rate for a shower really represent what I should expect to see? Many publications which list meteor shower rates will often give a corrected value, called the Zenith hourly Rate (ZHR) which standardizes the shower rate to optimum observing conditions. The shower rates listed are usually corrected for fully dark skies, and the meteor radiant point has been artificially located at the zenith, directly overhead. The actual rate of meteors seen by most observers, however, will be lower than this corrected value. Below is a table showing actual expected values for the major meteor showers, along with their corrected ZHR’s. Other publications may show somewhat different rates. These rates have been oriented to central U.S. latitudes. The quoted values are “smoothed” and do not represent those rarer times when abnormally high or low rates occur.

We have selected the better years, assuming that a sharp observable peak occurs in your longitude. Four different rates are given for each shower, under the following conditions: city sky or rural sky with full moon, suburb sky or rural sky with quarter moon, rural sky and moonless, calculated Zenith hourly Rate, ZHR. Rates 5 10 The Quadrantids more often than not will give a display in the low 20’s; you have to be fortunately placed to do better. The large difference in the rates for the Eta Aquarids is attributed to the southerly declination of the radiant. Observers in the northern tropics southward see much better rates for this shower. Back to top 8. I thought I saw a lot of meteors coming from a certain part of the sky last night, but I can’t find any shower listed in my books. What was going on? There are several possibilities here. First, it is possible that you caught the peak of a minor shower, not listed in most texts. Consulting a more extensive shower list may reveal a match. Second, random sporadic meteor activity will occasionally increase above the average level, giving rise to the suspicion that a shower may be in progress.

Third, meteor observers have, for many years, suspected the existence of small clusters or “outbursts” of meteor activity not formally associated with a recognized shower. The reason for these pockets of activity range from statistical fluctuations in the sporadic meteor distribution to isolated remnants of old extinct meteor streams. This “clustering” effect is not yet well understood. Back to top 9. What is a meteor storm, and how often do they occur? In meteor science, the month of November is best known for the meteor storms which have occasionally given us one of the most spectacular displays the night sky has to offer. On a single night, Meteors sometimes fell so thick it would appear as though the entire sky was falling, or gave the appearance of rapid forward motion of the Earth through the stars. The great Leonid meteor storm of 1833 did more to spawn the study of meteors than any other single event, along with great excitement by the general public. Meteor storms are not limited to only November, and In a historical parallel, the famous Giacobinid or October Draconid storm of 1946 also did much to spawn the study of meteors by radio methods.

Meteor storms are generally caused by young meteor streams, in which the majority of the streams’ mass is still concentrated along that portion of the orbit occupied by the parent comet. Meteor storms occur when the Earth crosses the orbit of the meteor stream, at the same time that the main mass of the young meteor stream is crossing the orbit of the Earth. For streams with a low potential for orbital perturbation, this event may occur on a periodic basis, generally at around the same time that the parent comet becomes visible in the inner solar system. Streams which tend to undergo frequent orbital perturbations may only cause infrequent and rare storms, some never occurring again. To make the possibility even more remote, these streams also tend to be very narrow, with the Earth taking only a few hours to cross the concentrated portion of the streams’ path. Being on the right side of the globe, under good weather, on the right night is very important toward seeing these events. Two meteor streams are associated with the November storms, the Andromedid (or Bielid) stream, and the Leonid stream.

The Andromedid stream is one that is subject to frequent orbital perturbations, and as such, only rarely crosses the Earth’s’ orbit in a manner favorable for producing a meteor storm. The last storm produced from this stream was on November 27, 1885; with 13,000 meteors per hour visible at the peak. By contrast, the last appearance of a shower from this stream was in 1940, with only 30 meteors per hour at the peak. The Leonid stream is much more favorable for producing storms, and generally tends to produce one every 33 years or so, although it has sometimes been disappointing. After feeble displays in 1899 and 1933, The appearance on November 17, 1966, provided the highest known rate of any meteor stream ever recorded. An approximate rate of 40 meteors per second (144,000 m/hour), was seen for about 1 hour as viewed from the western portion of North America, and the Pacific.

Unfortunately, the east coast and Midwest were enveloped in clouds that night, disappointing a lot of amateurs and professionals alike. These extremely strong rates were not seen during the 1998-2002 Leonid storm period. Rather than one year with extremely strong rates, this period provided several displays that entertained observers world-wide. The Leonid meteors represent the fastest known shower meteors, barreling in at 72 km/sec. They are well known for their bright magnitudes, and their ability to produce extremely long duration trains, some lasting up to several minutes. On the other end of the spectrum, the Giacobinids, which last produced a brief outburst in 1998, has extremely slow meteors at less than 11 km/sec. 10. Where can I get more information about the Leonid storms? Wikiepedia’s Leonid page provides a good overview of recent activity. Back to top 11. Is there a chance of a meteor from a meteor shower or storm reaching the ground as a meteorite, and is it dangerous to observe meteor storms?

The meteoroids which make up a meteor shower or storm are very fragile in nature, and are composed of a somewhat “fluffy” composite of material from which all volatile material has escaped, due to many trips near the sun. This material readily vaporizes in the upper atmosphere, and is given the descriptive name of “friable” material. While quite spectacular to watch, a meteor storm presents no real danger to the viewer, who is protected by miles of atmosphere. Back to top 12. Where can I find information on historical meteor observations? Obtaining good historical information in the area of meteor science can often prove difficult, due to the limited publication and circulation of professional texts in this field. It is highly recommended that researchers obtain access to a university or large city library which caters to astronomical and planetary science research. The below listed books are highly recommended by us, and their bibliographies can point the researcher in other desired directions: Olivier, C. P., (1925). Meteors. Baltimore: The Williams & Wilkins Company, (276 pp). Porter, J. G., (1952). Comets and Meteor Streams. London: Chapman & Hall, Ltd., (123 pp).

Lovell, A. C. B., (1954). Meteor Astronomy. Oxford, New York: University Press, (463 pp). McKinley, D. W. R., (1961). Meteor Science and Engineering. New York: McGraw-Hill Book Co., (309 pp). The SAO/NASA Astrophysics Data System also has a wealth of information on historical meteor observations. FAQ compiled by: James Richardson, AMS Operations Manager / Radiometeor Project Coordinator James Bedient, Former AMS Electronic Information Coordinator FAQ Reference Adams, M. T., (1980). “Observing Falling Stars,” Mercury (March-April, 1980). Cook, A. F., (1973). “A Working List of Meteor Streams,” Evolution and Physical Properties of Meteoroids. NASA , United States Gov. Publication. Hey, M. H., & Rea, D. G., (1986), “Solar System/ Meteors,” Encyclopedia Britannica (Vol 27, pg. 587). Jenniskens, P., (1994).

“Meteor Stream Activity I: The annual streams,” Astronomy and Astrophysics, (1994: 990-1013). Jenniskens, P., (1994). “Meteor Stream Activity II: Meteor Outbursts,” Astronomy and Astrophysics, (1994). McKinley, D. W. R., (1961). Meteor Science and Engineering. New York: McGraw-Hill Book Co. McLeod, N. (1997), AMS staff correspondence. Meisel, D. D., (1990). “Meteor,” McGraw-Hill Encyclopedia / EST 7th Ed. Olivier, C. P., (1965). “Catalogue of Hourly Meteor Rates,” Smithsonian Contributions to Astrophysics Vol. 8 Number 6,1965. Meteor Shower Facts - Interesting Facts about Meteor Showers Meteor Shower Facts Geminid Meteor Shower – What is a meteor shower? A meteor shower occurs when a number of meteors flash seem to radiate (or shoot out from) the same point in the sky. They are usually named for the constellation in which their radiant appears.

The meteoroids in a shower usually come from the trail of debris left behind by a comet. In the case of the Geminids and Quadrantids, those meteor showers come from the debris scatted by orbiting asteroids. When Earth’s orbit intersects the dust trail, we see more meteors flaring as the cometary debris encounters our planet’s atmosphere. Facts about Meteor Showers Most meteor showers are caused by debris from comets. When Earth moves through those debris trails, we see increased numbers of comets. Two meteor showers are caused by debris shed by asteroids. The Quadrantids are very likely caused by debris from the minor planet 2003 EH1. The Geminid meteor shower comes from debris shed by asteroid 3200 Phaethon. The Orionid Meteor shower (which occurs in late October each year) is created by dust and debris left behind by the passage of Comet 1P/Halley. Meteors fall to Earth during the day, although we can’t see them. It is very rare that a meteorite will strike a human being. It’s more likely that it will fall into the ocean.

The best time to view a meteor shower is in the early morning hours, preferably on a dark, moonless night. The earliest record of the Perseids meteor shower is found in Chinese annals from 36 AD. Famous Meteor Showers There are several famous and easy-to-observe meteor showers. The Perseids occur in mid-August when Earth encounters the debris trail from Comet Swift-Tuttle. They appear to radiate from a point in the constellation Perseus. The shower lasts from mid-July to late August, with a peak around August 12th each year. The Leonid meteor shower can be a very busy one. It occurs each year in mid-November, and rains debris from Comet 55P/Tempel-Tuttle. In 1833, observers estimated that hundreds of thousands of meteors flared through the sky. Observers wait for it each year, hoping for another spectacular show, emanating from the direction of the constellation Leo. The Geminid meteor shower occurs in December, when Earth crosses the path of the asteroid 3200 Phaethon.

The meteors appear to come from the direction of the constellation Gemini, and observers have noted that they move more slowly than other meteors. In late April, the Lyrids bring pieces of comet C/1861 G1/Thatcher back to Earth, which seem to radiate from the constellation Lyra. The peak of this storm is around April 22. Every 60 years or so this shower becomes more intense. When is the next meteor shower? There are 21 meteor showers throughout the year. They occur mostly between April and December. Forecasts about meteor showers can be found at: The American Meteor Society – amsmeteors.org

Question: What is the name for the meteorites that arrive at earth's atmosphere around mid November?
Answer: Leonids