Worlds of David Darling > Children's
Encyclopedia of Science > Comets, Meteors, and Asteroids > Chapter 5
COMETS, METEORS, AND ASTEROIDS
a book in the Discovering Our Universe series by David Darling
5. Meteorites: Stones From the Sky
The biggest space rocks, comets
and asteroids, hardly ever hit the Earth. The smallest meteors, burn up
in the atmosphere. But medium-sized space rocks quite often crash into the
Earth's surface. These are called meteorites.
About 500 meteorites strike the Earth every year. Since most fall in out-of-the-way
places, or into the sea, they are difficult to find and study. But a few
are found. These are looked at carefully to discover what they are made
of and where, in space, they may have come from. To scientists, they are
very important. Apart from rocks brought back from the Moon, they are the
only samples we have of matter from beyond our planet.
Meteorites can range in size from small pebbles to huge boulders. The largest
ever found weighs 60 tons, measures 10 feet across, and landed thousands
of years ago in Namibia in southwest Africa. It has never been moved from
where it fell. Another, discovered in Greenland, weighs about 34 tons. It
is now on display in the American Museum of Natural History in New York.
Even larger meteorites have hit the Earth in the past. They have left behind
huge IMPACT CRATERS – hollows in the ground surrounded by steep, rounded
The most famous crater, thought to be about 50,000 years old, is near Winslow,
Arizona. Called the Barringer crater, it measures 4,000 feet across and
was probably caused by a giant meteorite weighing around one million tons.
Older, more weathered craters, known as ASTROBLEMES, have been found in
remote parts of Canada, the Soviet Union, and South Africa. The most striking
one of all is the Vredefort Ring in the Transvaal of South Africa. Measuring
30 miles across, it is the giant scar left by a mile-wide object that smashed
into the Earth about 250 million years ago.
Such big impacts are rare. But there have been one or two recent near misses.
In 1908, a large space rock exploded over the Tunguska river valley in Siberia.
Scientists now think that it was probably not a meteorite. Instead, they
believe, this object was a small comet that burned up as it raced through
In 1972, a 1,000-ton meteoroid streaked across the skies of the western
United States. It flew safely by at a distance of just 36 miles (58 kilometers)
above the Earth before disappearing back into space.
Even when meteorites do hit the Earth they usually land far away from people.
A few, though, have paid unexpected visits. On April 25, 1969, a one-pound
space rock ended its journey by dropping through the roof of the Sprucefield
police storehouse in Northern Ireland!
Meteorites are the best understood of all space rocks. They come in three
main types: stony meteorites, stony-iron meteorites, and iron meteorites.
Of these, the stony variety is the most common. Only about one in twenty
meteorites is an iron, which is made of an iron-nickel mixture. Rarest of
all are the stony-irons.
Some of stony meteorites, known as chondrites, are among the oldest objects
ever found. Their age may be more than 4½ billion years – greater
than that of the planets. To scientists, chondrites are exciting because
they have been found to contain organic matter. This discovery proves that
at least some of the substances on which life is based have also been formed
No one is really sure where all of the 2,000 known meteorites have come
from. It seems likely that many of them are pieces of asteroids that have
broken off and found their way to earth. But the link between meteorites
and asteroids isn't as strong as that between meteor showers and comets.
Perhaps new clues to help solve this mystery will come from Antarctica.
For here, in the ice of the great southern continent, scientists have uncovered
a vast number of frozen meteorites. The cold, dry climate has kept them
exactly the way they were when they fell.
A meteorite from Mars known as ALH84001
Among the Antarctic meteorites are several that have come, not from an asteroid,
but from the planet Mars! Altogether about 30 Martian meteorites have been
found on Earth. Scientists have compared the makeup of gas trapped in these
unusual space rocks with that of the Martian atmosphere as measured by the
Viking spacecraft, which landed on the planet in 1976, and found a near
perfect match. It seems that these rocks were hurled into space by asteroids
that crashed into Mars long ago, eventually finding their way to our own
world. Other meteorites have been found on Earth that came from the Moon.
There are still many unanswered questions about the rocks that speed through
space and sometimes come close to or collide with our world. The mysteries
of comets, asteroids, and meteors will be solved only when more spacecraft
are sent to explore them.
In years to come, spacecraft will chase comets around the Sun, fly through
the heart of meteor storms, and land on the surface of distant asteroids.
All these missions will add to our understanding of the mysterious rocks