Exploratorium     Exploring Space, Page 4 of 6


Meteoroid: Any small or rocky object that orbits the sun. The earth's atmosphere protects us from many of the smaller bodies that fall toward it; they burn up before ever reaching the surface.

Asteroid: A large meteoroid, usually found orbiting in a belt between Mars and Jupiter. Occasionally asteroids are deflected into a different orbit by gravitational interactions with planets or collisions with each other, which may bring them closer to the earth.

Comet: An orbiting object made of ice, frozen gas, and dust. Many comets have long eccentric orbits that swing them past the earth periodically. Their tails of gas and dust pick up light from the sun, creating a spectacle in the night sky.

Near-Earth Object (NEO): Any meteoroid, asteroid, or comet whose orbit brings it near the earth. Programs such as Spacewatch, from the University of Arizona, and NASA's Near-Earth Asteroid Tracking program, scan the skies looking for NEOs. The Minor Planet Center in Cambridge, Massachusetts, calculates the orbits and catalogs the asteroids. Scientists estimate that approximately 2,000 potential assailants crowd the earth's orbit; so far, only a fraction (about 1/20th) of them have been identified.

Meteor: A streak of light from any meteoroid, asteroid, or comet that enters the earth's atmosphere and "burns" because of the friction, creating meteor showers or "shooting stars." A meteor can be as small as a grain of sand or many miles across.

Meteorite: A meteor that makes it to the earth's surface. Generally, objects less than 100 feet across break up in the atmosphere and never make it to the ground intact; instead the rubble from these meteoroids rains down as scores of small stones.


S cientists estimate that monster meteorites, those above a mile in width, hit only once every 300,000 years. A ten-mile-wide impactor, like the dino-killer, is rarer still, striking the earth on the average of once every 100 million to one billion years. More frequent are smaller objects, such as the 180-foot rock that exploded over Tunguska, Siberia, in 1908, flattening trees for about 15 miles in all directions. Objects of this size invade our atmosphere every few centuries on average, causing extensive local damage. If they hit the ground, they create large craters a mile or so in width. The most common meteors, however, are created by dust particles that have an average size of about 0.2 millimeters. Most of them never make it through the atmosphere intact. Instead, they melt and burn high above the earth in spectacular displays commonly known as shooting stars.

Micro-meteoroid Crater
An impact crater on the window of the space shuttle Challenger , possibly caused by a micro-meteoroid. Another possible explanation is that the crater was formed by a speck of paint that had flaked off another spacecraft. [ Click for a larger image.] Photo: NASA.

Meteorites that land on earth can provide researchers with valuable information. Scientists track down larger meteorites and impact sites, searching for clues about the birth of the solar system and the earth's geologic past. A popular hunting ground is Antarctica, where the dark objects are easy to spot against the icy white background.

By chemically analyzing meteorites, scientists can determine their makeup, which is primarily iron, nickel, and silicate rock. Determining the composition of a meteorite can help us infer the composition of the earth's interior. Analysis also reveals a meteorite's point of origin and its age. The oldest space rocks are about 4.6 billion years old, leading scientists to conclude that that's the age of the solar system as well.

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