Meteoroids:
"Shooting stars" or meteors are bits of material falling through Earth's atmosphere; they are heated to incandescence by the friction of the air. 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. Large pieces that do not vaporize completely and reach the surface of the Earth are termed meteorites.
Scientists estimate that 1,000 tons to more than 10,000 tons of meteoritic material falls on the Earth each day. However, 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.)
Where do they come from? They probably come from within our own solar system, rather than interstellar space. Their composition provides clues to their origins. They may share a common origin with the asteroids. Some meteoritic material is similar to the Earth and Moon and some is quite different. Some evidence indicates an origin from comets.
Several "shooting stars" or meteors per hour can usually be seen on any given night. Sometimes the number of meteors seen increases dramatically: these are termed "meteor showers". In fact, some meteor showers occur annually or at rather regular intervals. The number is greater in autumn and winter. The number always increases after midnight and is usually greatest just before dawn. Perhaps the most famous are the Perseids which peak around August 12 every year.
Meteor showers are usually named 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.
Meteorites may look very much like Earth rocks, or they may have a burned appearance. They may be dense metallic chunks or more rocky. Some may have thumbprint-like depressions, roughened or smooth exteriors. They vary in size from micrometer size grains to large individual boulders. The largest individual iron is the Hoba meteorite from southwest Africa which has a mass of about 54,000 kg. The stones are much smaller, the largest falling in Norton County, Kansas having a mass of about 1,000 kg.
Considering the vast infall of meteorites, one cannot help but wonder if anyone has been hurt or killed by meteorites. There are only a few documented cases on record. A shower of stones fell upon Nakhla, near Alexandria, Egypt on June 28, 1911, one of which allegedly 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.
Most meteoritic samples are either iron (actually nickel-iron alloy); stony,which are predominately rocky-silicates; or stony-iron.
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 or asteroid.
Sixteen 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.
Much remains to be learned about meteorites and their origins.
(Primary source material from: Dr. Harry B. Herzer, III, NASA Aerospace Education Services Project, Oklahoma State University, Stillwater, Oklahoma.)
