The Children's Encyclopedia
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 walls.
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 the atmosphere.
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 in space.
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.
Already, an Antarctic meteorite has been found that may have come, not from an
asteroid, but from the planet Mars! Scientists have compared the makeup of this
unusual space rock with that of the Martian surface as measured by the Viking
spacecraft. When they did, they found a near perfect match. It's possible that
the rock was hurled into space, towards the Earth, by an asteroid that crashed
into Mars many millions of years ago.
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
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 in space.