Meteoroids, Meteors and Meteorites
It is estimated that 1,000 tons to more than 10,000 tons of interplanetary material falls on the Earth each day. Most of this material is very tiny in the form of micrometeoroids or dust-like grains a few micrometers in size. These particles are so tiny that the air resistance is enough to slow them sufficiently that they do not burn up, but rather fall gently to Earth.
"Shooting stars" or meteors are bits of material falling through Earth's atmosphere at altitudes of 50-100km. They are heated to incandescence by the friction of the air. The surface melts and vaporizes leaving behind an ionized trail of gas and dust. The bright trails as they are coming through the Earth's atmosphere are termed meteors, and these chunks as they are hurtling through space are called meteoroids. As the object penetrates to 20km it will have slowed enough that heating no longer makes it glow and it will continue to fall to Earth. Large pieces that do not vaporize completely and reach the surface of the Earth are called meteorites.
On occasions a large meteoroid will strike the earth and at 120km glow as bright as the sun. These fireballs are called bolides and often are accompanied by sonic booms.
Meteorites share a common origin with the asteroids. Some meteoritic material is similar in composition to the Earth and Moon and some is quite different.
While most meteors burn up before reaching the Earth's surface, many meteoroids break apart in the upper atmosphere and become "fluffy meteors". This "fluffy" nature indicates a loose structure or vapor grown crystal aggregates. This gives rise to theories that some meteoroid material was aggregated, some subjected to heating-vaporization-condensation. This contrasts with the idea that meteoroids originated from an exploded planet or planetoid.
Showers
Several "shooting stars" or meteors per hour can usually be seen on any given night but sometimes the number of meteors seen increases dramatically: these are termed "meteor showers". Some meteor showers occur at regular intervals and are given names after a star or constellation which is close to the radiant (the position from which the meteors appear to come). Many of the meteor showers are associated with comets. The Leonids are associated with Comet Tempel-Tuttle; Aquarids and Orionids with Halley, and the Taurids with Encke. To my knowledge no meteorites result from meteor showers indicating a different origin between shower and sporadic meteors.
Finds and Falls
About 500 meteorites per year (not counting micrometeorites which never heat enough to melt) land on the earth. Meteorites look very much like Earth rocks to an untrained observer. They may be dense metallic chunks or more rocky. Some may have thumbprint-like depressions, roughened or smooth exteriors. They vary in size from pebbles to large individual boulders. We distinguish between
Finds: "Funny looking rocks" on the ground, and
Falls: "Funny looking rocks" on the ground shortly after an observed fall.
Scientists much prefer to get Falls, especially if they are carefully handled and quickly taken to a "clean room". These Falls are less likely to be contaminated by Earth material.
Meteorite mines
If you see a rock on the ground it is not remarkable. But it could be a stony meteorite, a find! But you just walk on because you are not an astrogeologist. An iron meteorite is more likely to be noticed but not much more so. But if you see a rock in antarctica it must have fallen from the heavens! Any rock there is a find or fall. Such rocks gradually sink through the antarctic ice which is silently transporting ice and sinking meteorites to the ocean. But the sinking meteoroids tend to collect in valleys and if they become exposed you have an unusual concentration mechanism for meteorites! Several such locations have been discovered.
Impact velocity
Earth orbital speed is 28.8km/s, orbital escape speed is the square root of two times this, 42.1km/s, and surface escape speed is 11.2km/s. The slowest meteor would be one that just catches up with the earth then "falls", 11.2 km/s. Don't expect anything less than 12km/s. But a retrograde body in parabolic orbit (the maximum speed for a body at a distance of 1AU that is a member of the solar system) might close with earth at a speed of 42.1+28.8=70.9km/s plus the 11.2 "fall" for a total of (total2=70.92+11.22) 72km/s. So we expect a range of 12-70km/s with the slower ones in the evening ("catch up" and prograde, no retrograde) and fast ones in the morning where we do the catching-up and collect all the retrograde rocks.
Impact Energy
That's just one half emm vee squared... a two kg mass coming in at 20 km/s should deliver (20000)2J. That's a lot, but people like to express this as "kilotons of TNT" (as if they knew what that was)... Well, the energy equivalent of one kg of Acme(TM) TNT is 4.2 megaJoule so how fast does a mass have to travel before its KE equals its mass equivalent of TNT? 4.2E6=0.5v2 (for m=1kg) or v=2898m/s so if v=2.9km/s the energy is the same as detonating the same mass of TNT. Since E goes as v-squared if v=29km/s each kg delivers 100kg TNT equivalent. A 1km cube of water has a mass of 1000x(1000)3=1012kg and if that hit the earth at 29km/s would deliver 1015 kg or a million megatons of TNT. (Remember, this is over the entire impact process and tides may tear the thing apart before it hits the atmosphere -- we hope).
Orbits
Where do meteorites come from? The orbits of many meteorites were obtained from the Prairie Network of Baker-Nunn cameras designed to do this job. Most had orbits similar to the Apollo earth-crossing asteroid family. Of course the shower meteorites (never oids) are associated with their parent comet.
Classification scheme
Meteorites are classified according to their composition:
Iron (Fe/Ni) are the most likely finds (they look more unusual),
Stones (C/O/Mg/Si) are the most likely falls (the most common type) and
Stoney Iron (Si/Fe/Ni -pretty rare).
The stoney meteorites are further subdivided into chondrites and achondrites. Most meteorites are chondrites, so named because of the presence of small rounded grains called chondrules. Thes grains are a few mm diameter and rich in olivine and pyroxine.
A few chondrites have high contrations of Al, Mg and Ca, rich in O and have almost no metallic Fe-Ni. These meteorites are dark in color and called carbonaceous chondrites. I call these genesis rocks since the composition matches the non volatile composition of the sun. If you took the primordial material of the solar system as we suspect the sun is composed then condense it, discarding volatiles such as H, He, you would end up with such material. This mixture is called "chondritic".
Carbonaceous chondrites are some of the most complex of all meteorites. They are rare, primitive and contain organic compounds. Most importantly they contain water-bearing minerals which is evidence of water moving slowly through their interiors not long after formation.
One from Allende, Mexico represents some of the oldest known matter. The meteorite formed 4.56 billion years ago and contains interstellar grains within calcium/aluminum rich inclusions. Interstellar grains are remnants of a prior star that lived out its life and exploded before the formation of our Sun. It is possible that this explosion was the trigger for the formation of our solar system.
Tektites are blobs of natural glass, usually no larger than a golf ball, and variously colored black, brown, yellow or green. They commonly have raindrop or dumbbell shapes, such that they clearly flew through the air in a molten state. Theories of their origin include volcanic ejecta from the earth or moon, and the result of meteorites hitting the earth and throwing molten terrestrial material from the crater. However, tektites do not chemically match terrestrial sediment. Nor do they match moon rocks brought back by the Apollo missions, and a lunar volcano is unlikely to be able to eject a particle fast enough to escape the Moon's gravity. It has been proposed that tektites are silica meteorites.
Widmanstatten figures are a pattern of interlocking iron-nickel minerals that is exposed when an iron meteorite is cut then polished and etched by acid. The minerals are formed after the molten iron-nickel solidifies and the atoms slowly migrate through the solidified mass in a process called exsolution. For iron and nickel to exsolve like this, the mass cannot cool off from its melting point any faster than three degrees per one million years. Thus, you will never see Widmanstatten figures in a man-made object.
Killer Meteorites
There are only two documented cases on record. The only incident of a fatality was a shower of stones which fell upon Nakhla, near Alexandria, Egypt on June 28, l911. One of these stones killed a dog. On November 30, 1954, Mrs. Hewlett Hodges of Sylacauga, Alabama was severely bruised by an 8 pound stony meteorite that crashed through her roof. This is the first known human injury.
Martian Meteorites
Thirteen meteorites have been found in Antarctica that are believed to have originated on the planet Mars. Gases trapped in these meteorites match the composition of the martian atmosphere as measured by the Viking spacecraft which landed on Mars in the mid-1970s. Controversy continues about whether structures found in one of these meteorites, known as ALH 84001, might be fossil bacteria or geologic structures.
The possible evidence of life in ALH 84001 was all found in and around carbonate mineral globules. In this microscope view (~0.55 mm across), the cores of the globules are orange-brown, fading outward to colorless, and are surrounded by black and clear bands. Other minerals in ALH 84001, which surround the globules, include orthopyroxene and plagioclase. Both are colorless in this view. Photo ©Allan Treiman, Lunar and Planetary Institute.