Meteors - Matter that falls through Earth’s atmosphere. Often called “shooting stars”. A few can be observed every hour.

Meteoroid is the term used for a piece of matter before it enters the atmosphere.A meteorite is a meteor that hits the ground.

The difference in meteoroids and asteroids is size. The distinction is fuzzy: meteoroids are generally less than 100 km in diameter.

This size distinction only exists in space. When an asteroid hits the Earth’s atmosphere, it becomes a meteoroid.

Cometary fragments can be dislodged when a comet is near the Sun. These dust or pebble-sized particles form a meteoroid swarm.

Over time these micrometeoroids are spread out over the entire orbit of the comet.

When the Earth crosses the orbit of a young, relatively undispersed cluster of meteoroids, a meteor shower occurs.

The intersections occur at the same time each year. Meteor showers are named for their radiant (the constellation from which they appear to radiate).

As far as we know, no meteor shower particle has ever reached the ground (none is large or dense enough). However, some pieces have been collected by high-altitude U2 aircraft flights.

These particles resemble burned newspaper or charred toast.

Larger meteoroids, ones more than a few cm in diameter, are usually not associated with swarms of cometary debris.

These larger meteoroids are responsible for most of the cratering of the Moon, Mercury, Venus, Mars, and all the Jovian moons.

On Earth, meteors have a typical entry velocity of approximately 20 km/s. They produce “shock waves” in the air. Some even produce sonic booms.

Some produce bright sky streaks that may last several seconds and can be seen for several hundred miles. These are often called “fireballs”.

More massive meteors, 1 ton and 1 meter across, produce craters. The Barringer crater in Arizona is one example.

The meteorite that produced the Barringer Crater was 50,000 tons.

The Earth has 100 craters larger than 0.1 km in diameter. Many are only visible from satellites.

The following is an underground resonance image of the Yucatan Peninsula crater that was formed by the asteroid impact that eventually killed the dinosaurs.

This is how that collision may have appeared to the dinosaurs.

On June 30, 1908 a meteor exploded above the Tunguska plain in Siberia. This was equal to a 1 megaton nuclear explosion.

When asteroids’ orbits are reconstructed, most intersect the asteroid belt; so this is probably where they originated.

Meteorites - several thousand have been found. Two basic types are finds and falls.

Finds are meteorites that people “find” on the ground, without seeing them fall.

Finds tend to be unusual looking. Many are found in Antarctica. (Both of these conditions make the meteorites easier to see.)

Falls are meteorites that are found after they are seen to fall to Earth.

CompositionFireballs tend to be 0.5 g/cm3 in density (like comets). Meteorites average 5 g/cm3 (like asteroids).

Another way to distinguish meteorites is by their composition:Irons - 7 g/cm3Stones - 3 g/cm3Stony-irons - a mixture of the two.

Stones make up 95% of all meteorites, but only half of these have been found.

Differentiated Meteorites are meteorites that show evidence of strong heating in their past.(prior to entering Earth’s atmosphere)

Primitive Meteorites show no evidence of heating in their past. Primitive meteorites are all stony-type asteroids.

Carbonaceous Meteorites are the oldest of all. Their composition as like that of the C-type asteroids.

Some carbonaceous meteorites contain chondrules (rocky spheres), in addition to ice and organic materials.

All the meteorites tested are extremely old, 4.4 to 4.6 billion years. This is the same as the oldest lunar rocks.

Eugene Shoemaker's passion was Astrogeology. He dreamed of going to the Moon. Credited with inventing the branch of Astrogeology within the U.S. Geological Survey, his contributions to the field and the study of impact craters, lunar science, asteroids, and comets are legendary. Though his own career as an astronaut/geologist was sidelined by a health problem, he helped train the Apollo astronauts in geology and the investigation of the lunar surface.

Seen here at Meteor Crater, Arizona in the mid 1960s, Shoemaker was killed in a tragic car accident in July 1997. He is survived by his wife and professional colleague, Carolyn, and children. In a fitting tribute conceived by a former student, Eugene Shoemaker's ashes were placed on-board the Lunar Prospector spacecraft which has now successfully reached a polar mapping orbit around the Moon. After completing its scientific mission, the spacecraft will ultimately impact the lunar surface.

The Lunar Prospector crashed to the Moon’s surface in late 1999.

Gene Shoemaker traveled to the Moon as he wished. He ultimately became what he had spent his life studying, a crater.