1. Qu’est-ce qu’un bolide ?
Un bolide est le terme désignant un météore très brillant, généralement de magnitude inférieure à -4, ce qui correspond à peu près à la même magnitude que celle de la planète Vénus dans les cieux du matin ou du soir. Il est possible qu’un bolide se termine par un intense sursaut de luminosité, et qu’il se fragmente.
Si vous avez la chance d’observer un événement aussi remarquable, n’hésitez pas à faire un rapport d’observation à l’International meteor organization, en décrivant autant d’informations que possible. Ces dernières concernent notamment la luminosité, la longueur de la trajectoire, la couleur et la durée du bolide. Il est très utile que l’observateur note les points de début et de fin de trajectoire du bolide par rapport aux étoiles et constellation du fond de ciel, ou avec une boussole en notant bien la hauteur angulaire de ces deux points au-dessus de l’horizon.
Le tableau ci-dessous peut aider les observateurs à estimer la magnitude du bolide observé :
|Premier Quartier de Lune||-9.4|
2. A quelle fréquence apparaissent les bolides ?
Plusieurs milliers de météores suffisamment lumineux pour être qualifiés de « bolides » apparaissent quotidiennement dans l’atmsophère terrestre. Cependant, la plupart d’entre eux ont lieu au-dessus des océans ou de régions inhabitées, et une autre grande partie a lieu en plein jour. Ceux qui apparaissent de nuit n’ont également que peu de chances d’être observés au vu du faible nombre de personnes généralement dehors pour pouvoir les observer.
Il est également d’autant plus rare d’observer un bolide que celui-ci est brillant. En règle générale, il y a 3 fois plus de bolides de magnitude m observables que de bolides de magnitude m-1, suivant ainsi une tendance exponentielle. Les observateurs confirmés peuvent espérer observer un bolide de magnitude -6 ou plus lumineux toutes les 200 heures d’observation environ, tandis qu’un bolide de magnitude -4 peut en moyenne être observé toutes les 20 heures d’observations.
3. Peut-on voir des bolides en plein jour, et est-ce qu’un bolide peut laisser une traînée ?
Les bolides peuvent être observés en plein jour, mais ces derniers doivent alors être de magnitude inférieure à -6 pour pouvoir être observés à grande distance du Soleil. Et doivent donc être plus brillants pour pouvoir être détectés plus près de notre étoile.
Les bolides peuvent laisser derrière eux deux types de traînées : des traînées persisitantes, et des traînées de fumée. Une traînée persistante est brillante et est associée à l’ionisation des molécules d’air excitées par le passage à grande vitesse du météoroïde. La plupart des traînées persistantes durent quelques secondes, mais elles peuvent parfois rester visibles pendant plusieurs minutes. Dans ce dernier cas, la traînée change généralement de forme avec le temps au fur et à mesure qu’elle est soufflée dans différentes directions par les vents d’altitude. Les traînées persistantes sont localisées à haute altitude dans l’atmosphère, généralement au-dessus de 85 km d’altitude, et la plupart sont associées à des météores rapides. Elles sont très souvent visibles la nuit, mais très rare la journée.
Le deuxième type de traînée est la traînée de fumée, qui est plus souvent observée en plein jour que de nuit. Son altitude de formation est d’environ 80 km d’altitude, ne sont pas lumineuses et sont composées de particules expulsées pendant le processus d’ablation. Ces dernières sont semblables aux traînées de condensation des avions, et peuvent avoir une apparence claire ou plus sombre.
4. J’ai vu un météore très brillant. Quelqu’un d’autre l’a-t-il vu, et à qui dois-je envoyer un rapport d’observation ?
L’IMO (International Meteor Organization) collecte les rapports d’observation de bolides en provenance des observateurs du monde entier. Les personnes ayant observé un météore brillant sont encouragées à envoyer un rapport d’observation. Si l’objet a été observé par plusieurs personnes, il devient dès lors être possible de reconstituter sa trajectoire.
La manière la plus simple de faire un rapport d’observation est de répondre aux questions de l’application de l’IMO.
Une copie est envoyée automatiquement au REFORME (REseau Français d’ObseRvation des MEtéores).
En cours de traduction… Adapté de la page de l’American Meteor Society: http://www.amsmeteors.org/fireballs/faqf/
5. Est-ce qu’un bolide peut apparaître coloré ?
Vivid colors are more often reported by fireball observers because the brightness is great enough to fall well within the range of human color vision. These must be treated with some caution, however, because of well-known effects associated with the persistence of vision. Reported colors range across the spectrum, from red to bright blue, and (rarely) violet. The dominant composition of a meteoroid can play an important part in the observed colors of a fireball, with certain elements displaying signature colors when vaporized. For example, sodium produces a bright yellow color, nickel shows as green, and magnesium as blue-white. The velocity of the meteor also plays an important role, since a higher level of kinetic energy will intensify certain colors compared to others. Among fainter objects, it seems to be reported that slow meteors are red or orange, while fast meteors frequently have a blue color, but for fireballs the situation seems more complex than that, but perhaps only because of the curiosities of color vision as mentioned above.
The difficulties of specifying meteor color arise because meteor light is dominated by an emission, rather than a continuous, spectrum. The majority of light from a fireball radiates from a compact cloud of material immediately surrounding the meteoroid or closely trailing it. 95% of this cloud consists of atoms from the surrounding atmosphere; the balance consists of atoms of vaporized elements from the meteoroid itself. These excited particles will emit light at wavelengths characteristic for each element. The most common emission lines observed in the visual portion of the spectrum from ablated material in the fireball head originate from iron (Fe), magnesium (Mg), and sodium (Na). Silicon (Si) may be under-represented due to incomplete dissociation of SiO2 molecules. Manganese (Mn), Chromium (Cr), Copper (Cu) have been observed in fireball spectra, along with rarer elements. The refractory elements Aluminum (Al), Calcium (Ca), and Titanium (Ti) tend to be incompletely vaporized and thus also under-represented in fireball spectra.
6. Can a fireball create a sound? Will the sound occur right away, as you watch the fireball, or is their some delay?
There are two reported types of sounds generated by very bright fireballs, both of which are quite rare. These are sonic booms, and electrophonic sounds.
If a very bright fireball, usually greater than magnitude -8, penetrates to the stratosphere, below an altitude of about 50 km (30 miles), and explodes as a bolide, there is a chance that sonic booms may be heard on the ground below. This is more likely if the bolide occurs at an altitude angle of about 45 degrees or so for the observer, and is less likely if the bolide occurs overhead (although still possible) or near the horizon. Because sound travels quite slowly, at only about 20 km per minute, it will generally be 1.5 to 4 minutes after the visual explosion before any sonic boom can be heard. Observers who witness such spectacular events are encouraged to listen for a full 5 minutes after the fireball for potential sonic booms.
Another form of sound frequently reported with bright fireballs is “electrophonic” sound, which occurs coincidentally with the visible fireball. The reported sounds range from hissing static, to sizzling, to popping sounds. Often, the witness of such sounds is located near some metal object when the fireball occurs. Additionally, those with a large amount of hair seem to have a better chance of hearing these sounds. Electrophonic sounds have never been validated scientifically, and their origin is unknown. Currently, the most popular theory is the potential emission of VLF radio waves by the fireball, although this has yet to be verified.
How bright does a meteor have to be before there is a chance of it reaching the ground as a meteorite?
Generally speaking, a fireball must be greater than about magnitude -8 to -10 in order to potentially produce a meteorite fall. Two important additional requirements are that (1) the parent meteoroid must be of asteroidal origin, composed of sufficiently sturdy material for the trip through the atmosphere, and (2) the meteoroid must enter the atmosphere as a relatively slow meteor. Meteoroids of asteroid origin make up only a small percentage (about 5%) of the overall meteoroid population, which is primarily cometary in nature.
Photographic fireball studies have indicated that a fireball must usually still be generating visible light below the 20 km (12 mile) altitude level in order to have a good probability of producing a meteorite fall. Very bright meteors of magnitude -15 or better have been studied which produced no potential meteorites, especially those having a cometary origin.
No. At some point, usually between 15 to 20 km (9-12 miles or 48,000-63,000 feet) altitude, the meteoroid remnants will decelerate to the point that the ablation process stops, and visible light is no longer generated. This occurs at a speed of about 2-4 km/sec (4500-9000 mph).
From that point onward, the stones will rapidly decelerate further until they are falling at their terminal velocity, which will generally be somewhere between 0.1 and 0.2 km/sec (200 mph to 400 mph). Moving at these rapid speeds, the meteorite(s) will be essentially invisible during this final “dark flight” portion of their fall.
Probably not. The ablation process, which occurs over the majority of the meteorite’s path, is a very efficient heat removal method, and was effectively copied for use during the early manned space flights for re-entry into the atmosphere. During the final free-fall portion of their flight, meteorites undergo very little frictional heating, and probably reach the ground at only slightly above ambient temperature.
For the obvious reason, however, exact data on meteorite impact temperatures is rather scarce and prone to hearsay. Therefore, we are only able to give you an educated guess based upon our current knowledge of these events.
How frequently do meteorite falls occur?
Our best estimates of the total incoming meteoroid flux indicate that about 10 to 50 meteorite dropping events occur over the earth each day. It should be remembered, however, that 2/3 of these events will occur over ocean, while another 1/4 or so will occur over very uninhabited land areas, leaving only about 2 to 12 events each day with the potential for discovery by people. Half of these again occur on the night side of the earth, with even less chance of being noticed. Due to the combination of all of these factors, only a handful of witnessed meteorite falls occur Each year.
As an order of magnitude estimation, each square kilometer of the earth’s surface should collect 1 meteorite fall about once every 50,000 years, on the average. If this area is increased to 1 square mile, this time period becomes about 20,000 years between falls.
Meteorite finds range in size from particles weighing only a few grams, up to the largest known specimen: the Hoba meteorite, found in South Africa in 1920, and weighing about 60 tons (54,000 kg). As with the magnitude distribution of meteors, the number of meteorites decreases exponentially with increasing size. Thus, the majority of falls will produce only a few scattered kilograms of material, with large meteorites being quite rare.
Meteorites are known to fall as single, discreet objects; as showers of fragments from a meteor which breaks up during the atmospheric portion of its flight; and (rarely) as multiple individual falls. The initial mass and composition of the meteoroid primarily determine its eventual fate, along with its speed and angle of entry into the atmosphere.
How fast are meteorites traveling when they reach the ground?
Meteoroids enter the earth’s atmosphere at very high speeds, ranging from 11 km/sec to 72 km/sec (25,000 mph to 160,000 mph). However, similar to firing a bullet into water, the meteoroid will rapidly decelerate as it penetrates into increasingly denser portions of the atmosphere. This is especially true in the lower layers, since 90 % of the earth’s atmospheric mass lies below 12 km (7 miles / 39,000 ft) of height.
At the same time, the meteoroid will also rapidly lose mass due to ablation. In this process, the outer layer of the meteoroid is continuously vaporized and stripped away due to high speed collision with air molecules. Particles from dust size to a few kilograms mass are usually completely consumed in the atmosphere.
Due to atmospheric drag, most meteorites, ranging from a few kilograms up to about 8 tons (7,000 kg), will lose all of their cosmic velocity while still several miles up. At that point, called the retardation point, the meteorite begins to accelerate again, under the influence of the Earth’s gravity, at the familiar 9.8 meters per second squared. The meteorite then quickly reaches its terminal velocity of 200 to 400 miles per hour (90 to 180 meters per second). The terminal velocity occurs at the point where the acceleration due to gravity is exactly offset by the deceleration due to atmospheric drag.
Meteoroids of more than about 10 tons (9,000 kg) will retain a portion of their original speed, or cosmic velocity, all the way to the surface. A 10-ton meteroid entering the Earth’s atmosphere perpendicular to the surface will retain about 6% of its cosmic velocity on arrival at the surface. For example, if the meteoroid started at 25 miles per second (40 km/s) it would (if it survived its atmospheric passage intact) arrive at the surface still moving at 1.5 miles per second (2.4 km/s), packing (after considerable mass loss due to ablation) some 13 gigajoules of kinetic energy.
On the very large end of the scale, a meteoroid of 1000 tons (9 x 10^5 kg) would retain about 70% of its cosmic velocity, and bodies of over 100,000 tons or so will cut through the atmosphere as if it were not even there. Luckily, such events are extraordinarily rare.
All this speed in atmospheric flight puts great pressure on the body of a meteoroid. Larger meteoroids, particularly the stone variety, tend to break up between 7 and 17 miles (11 to 27 km) above the surface due to the forces induced by atmospheric drag, and perhaps also due to thermal stress. A meteoroid which disintegrates tends to immediately lose the balance of its cosmic velocity because of the lessened momentum of the remaining fragments. The fragments then fall on ballistic paths, arcing steeply toward the earth. The fragments will strike the earth in a roughly elliptical pattern (called a distribution, or dispersion ellipse) a few miles long, with the major axis of the ellipse being oriented in the same direction as the original track of the meteoroid. The larger fragments, because of their greater momentum, tend to impact further down the ellipse than the smaller ones. These types of falls account for the “showers of stones” that have been occasionally recorded in history. Additionally, if one meteorite is found in a particular area, the chances are favorable for there being others as well.
How can I recognize a meteorite, and where should I hunt for them?
The classic concept of a meteorite is a heavy, black rock. This stereotype is true in some cases, but many, many more meteorites resemble nothing more than mundane terrestrial rocks. These will attract attention only by being different from all others around them.
To understand what a meteorite might look like on the ground, we must first examine the numerical distribution of the three major types of meteorites. Of the known meteorite classes (combining falls and finds):
- stones (Aerolites) comprise about 69 per cent;
- irons (siderites) comprise about 28 per cent;
- and stony-irons (siderolites) comprise the remaining 3 per cent.
First of all, if a meteorite is found fairly quickly after it falls, most will exhibit an overall blackened surface, called a fusion crust. This fusion crust is a souvenir of ablation heat from the meteorite’s rapid atmosphere transit. Depending on the composition of the meteorite, the fusion crust may appear glassy, or dull. Irons develop a fusion crust consisting of magnetite, and having the appearance of a fresh weld on steel.
Once a meteorite is on the surface, all the normal weathering effects that erode earthly rocks affect meteorites, too. A fusion crust will weather, and on a stone, lighten in color to a brownish hue. Chemical weathering, or oxidation, will attack meteorites. Irons will quickly rust. Stones will lose their fusion crusts entirely. Water will seep into the interior, and chemically alter the minerals. Mechanical weathering, by frost, sun, and wind will reduce the meteorite further. This is why most ancient meteorites found are irons, most able to resist these processes.
Most suspected meteorites, by the percentages above, are stony, and the finder’s attention was drawn to them by their contrasting appearance with their surroundings. The indisputable identification of a stony meteorite requires chemical tests which are beyond the scope of this article.
Iron meteorites may frequently be recognized by their shape. The melting of the exterior of the body will sometimes cause iron meteoroids to arrive at the surface carved into fantastic shapes. Complete rings and segments of arcs have been found. An iron will be pitted, as portions of the alloy with a lower melting temperature will be scooped out by the heat and pressure. There will sometimes be sharp points surrounding these pits, an ablation effect. Positive identification of an iron requires a grinding and acid etching process that is again, beyond the scope of this article.
Anyone with a serious interest in searching for meteorites should arrange a visit to a large museum with a meteorite collection, in order to view not the spectacular specimens on display, but the more “ordinary” specimens kept in the institutions’ collection. By examining many specimens, the seeker will gain a good understanding of the varied appearance that meteorites may present.
The most successful areas for hunting for meteorites are open, flat, arid regions, usually having a light background color. Such regions have the lowest rates of mechanical and chemical weathering, preserving the meteorite for much longer periods of time. Some irons and stony-irons have been found in desert regions more than 10,000 years after the fall which produced them. Arid regions also offer great advantages in visual searches due to the relative lack of vegetation or bodies of water, as well as a light contrasting background color.
The best areas for meteorite searching (although rather impractical for most persons) are the regions of the earth covered by continental glaciers, such as Greenland and Antarctica. These ice packs offer the highest degree of preservation of a meteorite after its fall, high background contrast, and few competing terrestrial rocks. Many of the meteorites used in research today were recovered during Antarctic expeditions.
For those without access to arid deserts or continental glaciers, perhaps the best place to do meteorite hunting is in freshly plowed farmer’s fields, especially following a recent rain. Native-American arrowhead hunters frequently employ this technique as well. Farmers have plowed up many of the more famous meteorite finds in history. Iron meteorites are the easiest to recognize and are most frequently found. Stony meteorites are more difficult to recognize and to differentiate from terrestrial rocks, such as (ice age) glacial erratics.
The majority of meteorites, including the stone varieties, contain sufficient amounts of iron (Fe) and nickel (Ni) to cause them to be paramagnetic. Meteorite hunters often employ metal detectors, or very strong magnets attached to a walking stick, to aid them in their searches. Meteorites have been known to literally “jump” out of loose soil in the presence of a strong magnet.
Where can I get a potential meteorite authenticated?
Below is a brief list of academic institutions and museums which might be contacted about authenticating a potential meteorite find.
Readers are highly advised to first contact the institution and obtain information about their individual policies regarding such testing and potential fees prior to shipping any actual material. Since the American Meteor Society does notÂ deal in meteorites, we cannot make recommendations or give advice on the selection of a testing facility. Readers must use their own discretion in this matter.
Center for Meteorite Studies Arizona State University Temple, AZ 85281
Institute of Geophysics and Planetary Sciences University of California Los Angeles, CA 90024
Institute of Meteoritics Department of Geology University of New Mexico Albuquerque, NM 87131
Lunar and Planetary Laboratory Space Sciences Building University of Arizona Tucson, AZ 85721
The American Museum of Natural History Central Park West at 79th St New York, NY, 10024
The Field Museum of Natural History S. Lake Shore Dr. Chicago, IL 60605
National Museum of Natural History Dept. of Mineral Sciences Smithsonian Institution Washington, DC 20560
What do fireballs and meteorites tell us about their origins?
Most of our current knowledge about the origin of meteoroids comes from photographic fireball studies (meteors > magnitude -4) done over the last 50 years or so. This may sound like a long time, but good data has been collected on only about 800 fireballs so far. Of these, only 4 have been recovered on the ground as meteorites. A meteorite-causing fireball is very rare and must be at least magnitude -8 to have sufficient mass to survive the trip. Even with an accurate photographic or video trajectory, it is still a matter of finding a needle in a haystack once the meteorite is on the ground. In recorded scientific history, un-photographed (eye-witnessed) falls have resulted in only about 900 meteorite finds.
Studies of meteoroid parent bodies, comets and asteroids, have been more successful, using space probes and infrared telescope studies to greatly increase our knowledge of these objects. What we have found is that, rather than distinct differences between these two smaller solar system members, there exists an entire spectrum of parent bodies, ranging from low-density comets to large differentiated asteroids. The similarities between asteroids and comets is made more apparent by the recent discovery of a coma (a distinctly cometary phenomena) around the asteroid Chiron, at its perihelion.
At the present time, meteoroid parent bodies can be roughly divided into the following classes:
By far the most prevalent parent body of meteoroids, cometary meteoroids form about 95% of the total meteor population, and include nearly ALL of the shower meteor population. These parent bodies are composed of frozen methane (CH4), ammonia (NH3), water (H2O), and common gases (such as carbon dioxide, CO2), carbon dust and other trace materials. As a comet passes near the sun in its orbit, the outer surface exposed to sunlight is vaporized and ejected in spectacular jets and streams, freeing large amounts of loosely aggregated clumps of dust and other non-volatile materials.
These freshly generated cometary meteoroids, often called “dustballs” will roughly continue to follow the orbit of the parent comet, and will form a meteoroid stream.
Based upon photographic fireball studies, cometary meteoroids have extremely low densities, about 0.8 grams/cc for class IIIA fireballs, and 0.3 grams/cc for class IIIB fireballs. This composition is very fragile and vaporizes so readily when entering the atmosphere, that it is called “friable” material. These meteoroids have virtually no chance of making it to the ground unless an extremely large piece of the comet enters the atmosphere, in which case it would very likely explode at some point in its flight, due to mechanical and thermal stresses.
These parent bodies are the smaller asteroids, constructed of denser and less volatile materials than the comets. Small meteoroids of this type are produced through collisions. This class of parent bodies generate about 5% of the total meteor population, generally as part of the non-shower, or “sporadic” meteors. These meteoroids can make it through the atmosphere, and as meteorites, they make up about 84% of all falls.
Stony meteorites from this source are called Chondrites, due to the rounded nodules of material found within their structure, which are called chondrules. Chondrite meteorites have two major groupings:
The first group, the Class II fireballs, are the carbon-rich Chondrites, or Carbonaceous Chondrites, which help bridge the gap between comets and asteroids. They make up about 4% of all observed falls, and have densities of around 2.0 grams/cc. They are characterized by the presence of 2% or more carbon, partly present as complex hydrocarbons, and of considerable hydrogen (hydroxyl groups, OH-1, and water, H2O).
The second group, the Class I fireballs, are what is called the Ordinary Chondrites, making up about 80% of all observed falls. They have an average density of 3.7 grams/cc, and generally fall into two general types: Olivine-Bronzite Chondrites (about equal amounts of bronzite and olivine) and Olivine-Hypersthene Chondrites (less pyroxene than olivine).
These asteroids are physically the largest parent body for meteoroids, but generate only a small fraction of the overall meteor population: less than 1%, and have no fireball classification. Due to their hardier composition, however, they make up about 16% of the observed falls. A differentiated asteroid is one with sufficient size to cause internal temperatures high enough to melt and stratify the asteroid. The higher density materials (mainly iron) gather in the core, the lighter basalt/silicate materials gather in the outer layers, with thinner layers of various concentrations of other materials stratified in between. Small meteoroids of these types have been produced by what must have been spectacular collisions, breaking up even the iron core of the asteroid.
The three major groups for these meteors are:
1. Achondrites (Basalt/Silicate non-chondritic stones); with a 3-4 grams/cc density, and comprising about 8% of observed falls. These formed in the outer and crustal layers of the asteroid.
2. Siderolites (Stony-Irons); with a 5-7 grams/cc density, and comprising about 2% of observed falls. These formed a thin layer between the core and outer layers of the parent bodies. They generally consist of round, translucent green crystals of olivine imbedded in a matrix of iron.
3. Siderites (Irons); with a 7.9 grams/cc density, and comprising about 6% of observed falls. These are the remains of the core of a differentiated asteroid, and show signs of extremely slow cooling (1-10 deg C per million years), and extremely high shock stresses, presumably from collisions. These meteorites weather so well once on the ground, they make up 54% of all meteorite finds despite their small percentage of the fall population.
The very rarest of meteorites are those which are thought to have originated from large differentiated bodies, such as moons and planets. These Achondritic stones (basalt/silicate) are believed to have been ejected from a moon or planet’s surface, due to the impact of another very large meteoroid. One sub-class of Achondrites show a very similar composition to that of the earth’s moon, and are believed to be Lunar meteorites. Another class, the SNC (shergottite-nakhlite-chassignite) meteorites, are believed to have been ejected from the crust of the planet Mars.
Author’s note on fireball / meteorite statistics.
Readers of this FAQ will notice that those particles which make up the majority of the meteoroid population are those which are the least likely to make it to the ground as a meteorite. Conversely, those particles which make up a minority of the meteoroid population are the most likely to reach the ground as a meteorite. This disparity becomes even more skewed when weathering conditions on the ground are considered. Thus, the meteors which are most often seen are not found on the surface, and the ones which are most often found are uncommon in the sky.
It took scientists many years to realize this disparity, and published texts frequently seem to conflict with one another with regard to the percentile breakdown of meteorite types. This is especially true if the author has combined old meteorite finds with fresh, observed falls. In an attempt to help alleviate this confusion, we present a current breakdown of the different meteoroid/meteorite types, in their various stages:
Overall Meteor Population:
As a general rule, the smaller (fainter) is the meteoroid population under consideration, the more likely is a cometary origin. As a very rough estimation, the visible meteor population is composed of about 19 cometary meteors for every 1 asteroidal meteor. This yields the following breakdown:
- Cometary meteoroids: ~95%
- Chondritic meteoroids: ~5%
- Non-chondritic meteoroids: <1%
When only the population of meteors of > -4 magnitude are considered, the more sturdy asteroidal meteoroids begin to make up an increasingly higher percentage when compared to fainter magnitudes. There are four basic fireball classes which are divided as follows:
- Cometary meteoroids: 38%
- Type IIIb fireballs, low density comets: 9%
- Type IIIa fireballs, high density comets: 29%
- Chondritic meteoroids: 62%
- Type II fireballs, Carbonaceous Chondrites: 33%
- Type I fireballs, Ordinary chondrites: 29%
- Non-chondritic meteoroids: <1%
- No fireball class
Observed Meteorite Falls / Fresh Finds:
When only very fresh meteorite falls are considered, it becomes instantly apparent how important the density and sturdiness of the meteoroid material is to its likelihood of reaching the ground. The cometary meteoroid population disappears, and the carbonaceous chondrite population is greatly reduced. Thus, the ordinary chondrites and non-chondritic meteorites become the primary constituents of this population:
- Cometary meteoroids: 0%
- Chondritic meteoroids: 84%
- Carbonaceous chondrites: 4%
- Ordinary chondrites: 80%
- Non-chondritic meteoroids: 16%
- Achondrites: 8%
- Siderolites: 2%
- Siderites: 6%
Once they are on the ground, meteorites instantly begin to undergo mechanical and chemical weathering. Again, those meteorites which are more sturdy and dense tend to withstand these processes much better. In this case, the iron meteorites (siderites) fare the best, despite their very small proportion of the overall meteoroid population:
- Cometary meteoroids: 0%
- Chondritic meteoroids: 37%
- Carbonaceous chondrites: <1%
- Ordinary chondrites: 37%
- Non-chondritic meteoroids: 63%
- Achondrites: 3%
- Siderolites: 6%
- Siderites: 54%
This is an active field of study, and readers are reminded that all of the above numbers are estimates, and subject to revision as our knowledge level increases. We have attempted to select the most representative values for each.