By Pat
Durkin, National Geographic
The early space
program gave us an image of Earth as a lustrous blue pearl, serenely
sailing through space. But a more accurate metaphor might be a
goose in hunting season, flying though a hail of bullets. Earth
orbits amidst a swarm of potentially threatening asteroids, some
large enough to cause a planet-wide disaster should there be a
collision.
 Chances
of such a collision are small in the short-term, but inevitable
over time, scientists say. The asteroid strike that ended the
age of dinosaurs whalloped Earth 65 million years ago at what
is today Mexico’s Yucatan Peninsula. However, as recently
as 1986, a dangerous asteroid came within six hours of striking
Earth, although no one realized how close Earth had come to disaster
until much later.
These things
have hit the earth in the past, and they will hit the earth in
the future,” warned Eugene Shoemaker, the space-probing
geologist who first alerted the world to the danger of near-earth
asteroids (NEAs) before he died in 1997. “The catastrophe
will exceed other natural disasters by a long shot.”
The asteroid
that ended the age of dinosaurs was at least six miles (10 kilometers)
wide, but smaller asteroids can still be devastating. Scientists
estimate that the impact of an asteroid with a diameter of one
kilometer (0.6 miles) or more could kill at least a quarter of
the world’s human population, as well as many other life
forms. Less than a century ago a space rock only 330 feet (100
meters) wide exploded over Siberia. It leveled more than a half
million acres (2,000 square kilometers) of forest.
 However
remote, the possibility of the end of life as we know it has energized
the astronomical community. The search for new asteroids, once
considered the realm of space fanatics, has become serious science.
OUT THERE
Images of
the night sky, as seen through powerful telescopes at California’s
Palomar Observatory and elsewhere around the world, are systematically
sifted for evidence of yet undiscovered threats. Each object that
doesn’t look like an asteroid is carefully removed. The
process is painstaking.
So far only
about half of the estimated 1,100 asteroids with a potential for
a catastrophic impact with Earth have been discovered. British
astronomers recently urged their government to become more actively
involved in the effort.
 The United States, faced with the possibility
of having to head off a disastrous collision someday, is in the
midst of NEAR (Shoemaker-Near Earth Asteroid Rendezvous), a year-long
project to orbit Eros, the second-largest NEA, in order to better
understand the makeup and physical dynamics of asteroids.
Close-up images
of potato-shaped Eros show an asteroid about the size of Manhattan
that has been bombarded many times. Some 100,000 craters more
than 50 feet (15 meters) wide pock its surface. More than a million
boulders the size of houses or larger litter the surface of Eros.
The asteroid’s consistent color suggests a uniform composition.
This information
may be critical should it become necessary to explode an asteroid
or deflect its orbit to prevent it from striking Earth someday.
NEAR, a joint project of the National Aeronautics and Space Administration
(NASA) and The Johns Hopkins Applied Physics Laboratory, will
conclude in February.
ESCAPEES
FROM THE BELT
Earth-threatening
asteroids are strays from what is known as the “main belt,”
an elliptical ring consisting of tens of thousands of space rocks
of various shapes and sizes in orbit around the sun between Mars
and Jupiter. Smaller asteroids—less than 12.5 miles (20
kilometers) wide—sometimes migrate to unstable areas of
the main belt, known as resonances.
Once within
the resonances, asteroids are vulnerable to the gravitational
pull of nearby planets—Mars, Jupiter or Saturn—which
can elongate an asteroid’s orbit. The change is sometimes
enough to swing the asteroid onto a path that crosses Earth’s
orbit, setting up the possibility of a future collision.
Uneven warming
by the sun may play a role in moving asteroids into the resonances.
Energy reradiating from the warmed side of the asteroid delivers
a kick in the opposite direction, similar to the recoil of a rocket
spewing gas. Kicks over millions of years can move an asteroid’s
orbit into an unstable area.
 Only a small fraction of asteroids leave
the main belt, and of those only a small fraction moves into a
potential collision course with Earth. However, over billions
of years many asteroid objects have collided with Earth and left
their marks with craters that can still be seen. Arizona’s
Meteor Crater, with its uplifted rock walls and scattered beads
of glass, is an example of an asteroid impact that occurred 50,000
years ago.
However, the
history of asteroid impacts isn’t all bad news. One theory
suggests that chemical components of life, including much of Earth’s
water, arrived with asteroids and comets that bombarded the planet
in its youth.
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