Meteors and Meteorites

Meteorite Facts

• Meteorites are bits of the solar system that have fallen to the Earth. Most come from asteroids, including few are believed to have come specifically from 4 Vesta (picture 6); a few probably come from comets. A small number of meteorites have been shown to be of Lunar (11 finds) or Martian (12) (picture 5) origin.
• Though they are just boring looking rocks, they are extremely important in that we can analyze them carefully in our labs. Aside from the few kilos of moon rocks brought back by the Apollo and Luna missions, meteorites are our only physical evidence about the universe beyond the Earth.
• Meteorites are classified as:
• carbonaceous chondrites: very similar in composition to the Sun less volatiles; similar to type C asteroids (picture 4);
• ordinary chondrites: by far the largest number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets (picture 1, above);
• irons: primarily iron and nickel; similar to type M asteroids (picture 3);
• stony irons: mixtures of iron and stoney material like type S asteroids;
• achondrites: similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites (picture 2)
• A very large number of meteoroids enter the Earth's atmosphere each day amounting several hundred tons of material. But they are almost all very small, just a few milligrams each. Only the largest ones ever reach the surface to become meteorites.
• The average meteoroid enters the atmosphere at between 10 and 70 km/sec. But all but the very largest are quickly decelerated to a few hundred km/hour by atmospheric friction and hit the Earth's surface with very little fanfare. However meteoroids larger than a few hundred tons are slowed very little; only these large (and fortunately rare) ones make craters.
• A good example of what happens when a small asteroid hits the Earth is Barringer Crater (a.k.a. Meteor Crater) near Winslow, Arizona (picture 7). It was formed about 50,000 years ago by an iron meteor about 50 meters in diameter. The crater is 1200 meters in diameter and 200 meters deep. About 120 impact craters have been identified on the Earth, so far.
• There are probably at least 1000 asteroids larger than 1 km in diameter that cross the orbit of Earth.
• The impact of a comet or asteroid about the size of Hephaistos or SL9 hitting the Earth was probably responsible for the extinction of the dinosaurs 65 million years ago.
• Calculations based on the observed number of asteroids suggest that we should expect about 3 craters 10 km or more across to be formed on the Earth every million years. This is in good agreement with the geologic record. It is more difficult to compute the frequency of larger impacts like the one 180 km in diameter near Chicxulub in the Yucatan Peninsula; once per 100 million years seems like a reasonable guess.

Pictures

1.  Chondrite Meteorite 186k gif
2.  Achondrite Meteorite 178k gif
3. (above) Iron Meteorite 193k gif
4.  part of the Allende fall, a carbonaceous-chondrite 30k jpg
5.  Martian meteorite 212k gif
6. a piece of 4 Vesta 78k jpg; 376k gif; caption
7. Barringer Crater 308k gif; 107k jpg; 2000k tif

More about Meteorites

• from LANL
• from JPL
• from RGO
• near-Earth objects from RGO
• Micrometeorites from LANL
• Meteorite and Impacts Advisory Committee, includes info and more images
• an essay on Antarctic meteorites and Antarctic Meteorite News
• 1995 Meteor Shower Calendar from the International Meteor Organization
• Gary Kronk's Meteor Showers page
• Dr. Cain's class notes

Impacts

• Asteroid and Comet Impact Hazard from NASA Ames
• The Impact Catastrophe (ExInEd hypercard stack)
• Essay about the K-T Event by Calvin Hamilton of LANL
• The Probability of Collisions with Earth (also from LANL)
• text and pictures of Terrestrial Impact Craters (also from LANL)
• a teacher's guide about Cosmic Collisons by Sally Stephens of PASP
• Comet Busters by Peter Tyson (from Technology Review)

Open Issues

• Was it an small asteroid or something else that caused the Tunguska fireball over central Siberia in 1908?
• Was it a comet or an asteroid that caused the Chicxulub crater in the Yucatan (and probably caused the extinction of the dinosaurs)?