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
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.
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.
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.