Worlds of David Darling > Children's
Encyclopedia of Science > Comets, Meteors, and Asteroids > Chapter 1
COMETS, METEORS, AND ASTEROIDS
a book in the Discovering Our Universe series by David Darling
1. Beware, Low-Flying Space Rocks
On the morning of June 30, 1908, in a lonely Russian valley, there was a
huge explosion. Tall trees were laid flat across a wide area, and the bang
was felt hundreds of miles away. Whatever had caused the explosion had come
from outer space.
A bright comet
Could it have been an alien spaceship that lost control while trying to
land? Was it perhaps a tiny "black hole" that came to Earth from a distant
part of the universe? These are among many explanations that have been put
forward over the years. But scientists now think that the likeliest cause
of the famous Tunguska event, as it's called, was a chunk of a COMET that
burst apart as it rushed through our atmosphere.
In 1937, a mountain-sized boulder called Hermes whizzed by the Earth at
a distance of just 500,000 miles (800,000 kilometers). As quickly as it
came, it vanished again into the depths of space. More recently, on March
18, 2004, another runaway rock, named 2004 FH, about the size of an office
building, came even closer. It zipped past us just 25,000 miles (40,000
kilometers) away – about one-tenth the distance of the Moon. Both
Hermes and 2004 FH were visiting ASTEROIDS.
In almost any science museum, you'll find examples of smaller space rocks,
called METEORITES, that have crashed into the Earth. But not all meteorites
are small. If you travel to the northern Arizona desert, you can see the
great Barringer crater. This three-quarter-mile-wide pit was made thousands
of years ago by a giant meteorite. If you gaze into the sky on an clear
night, you may see bright streaks of light, or shooting stars. These are
caused by tiny METEORS burning up as they plunge into the Earth's atmosphere.
The Barringer crater in Arizona
What could be more thrilling than these strange, often unexpected visitors?
They bring with them matter that has journeyed for trillions of miles and
that is billions of years old. They have traveled through regions that no
human eye has ever seen. And the comets, asteroids, and meteors that crash
into, or pass close by, the Earth are just a few of the billions that wander
through our neighborhood of space.
The Kingdom of the Sun
The part of space in which we live is called the SOLAR SYSTEM. It contains
the Sun, an ordinary, yellow star, and all the objects that go around it.
In terms of size and mass, the Sun is the most important member of the solar
system. Because of its great mass, it has a strong pull of gravity. And
because of its strong pull of gravity, the Sun forces everything else in
its kingdom to move around it in huge curved paths called orbits.
Biggest of the orbiting objects are the planets. There are nine of these,
ranging in size from mighty Jupiter, at 88,700 miles (142,700 kilometers)
in diameter, to tiny Pluto, at just 1,442 miles (2,320 kilometers). Recently,
Pluto was demoted to the class of "dwarf planet."
Next come the moons, or satellites, of the planets. Like our own Moon, these
objects orbit about their parent worlds. The largest of them is Jupiter's
Ganymede, with a diameter of 3,278 miles (5,274 kilometers). Smallest are
the odd-shaped moons, such as Mars's Deimos, which is only 9 miles (14½
Finally, come the comets, meteors, and asteroids. They are small in size,
ranging from an asteroids 623 miles (1,003 kilometers) in diameter to tiny
meteors too small to be seen. But they are huge in number. Scientists already
know of hundreds of comets and thousands of asteroids. Countless more are
waiting to be discovered. Some have been visited by spacecraft and there
are plans for even more daring missions to these small, wandering bodies
to uncover the secrets that are locked within them.