We now know
that the Earth is constantly bombarded by debris from space.
200 tons of ice, dust, and tiny meteorites fall into the atmosphere
each day and are vaporized. Very large impacts that penetrate
the atmosphere are less common. Scientists estimate that major
impacts with global consequences occur every million years,
and that midsize impacts, capable of wiping out a city, happen
every century or so.
The thought that large comets and asteroids are a danger to
the Earth was not widely accepted when Eugene Shoemaker first
proposed the idea in 1960. He was a geologist studying a large
crater in Winslow, Arizona, when he suddenly realized it had
been created by a meteor. His subsequent work showed that the
Earth experienced regular meteor and asteroid strikes, and would
resemble the moon if not for our dynamic natural environment.
This theory was further strengthened in 1990, when scientists
identified a giant hidden crater near the Yucatán Peninsula.
It was created by the impact of a very large object, which probably
caused the extinction of the dinosaurs some 65 million years
Comet Shoemaker-Levy 9's fiery collision with Jupiter in July,
1994 proved there was reason to be concerned. If the comet had
hit the Earth, instead of Jupiter, it would have destroyed human
civilization. Just three days after the comet began to hammer
Jupiter, Congress authorized NASA to develop a program to identify
all potentially threatening asteroids and track comets entering
the solar system.
Comets are rocky balls of ice that orbit the Sun, trailing a
gas tail as they spin through space. Asteroids, and their smaller
relatives, meteorites, are essentially rocks left behind from
the formation of planets. They tend to cluster harmlessly in
a belt between Mars and Jupiter, but collisions and gravitational
shifts can propel them towards Earth. NASA estimates that there
are some 160,000 asteroids with diameters greater than 100 meters
(328 feet) in the solar system, and of these, approximately
1,000 objects larger than one kilometer (3,281 feet).
Today, hundreds of astronomers around the world scour the night
skies. The majority of their work is coordinated by NASA, which
has committed to finding and identifying 90% of all near-Earth
objects larger than one kilometer in diameter by the year 2010.
chosen objects larger than one kilometer because those are the
most dangerous objects, and also because given the current state
of our technology, it is a feasible goal," says Dr. Steve Pravdo,
Project Manager for the Near-Earth Asteroid Tracking System
at NASA's Jet Propulsion Laboratory, in California. Dr. Pravdo
leads one of several teams scanning the sky. Others include
the Linear Asteroid Detection Project in New Mexico, the Catalina
Sky Survey in Arizona, the Loneos Program in New Mexico, and
the Spacewatch Program at the University of Arizona.
Recent advances in electronic imaging and computer programs
have created a revolution in astronomy. The system of photographic
plates and visual identification used to identify comet Shoemaker-Levy
9 became outmoded about five years ago.
Today, most astronomers use advanced electronic imaging systems
and sophisticated computers. These systems are ten times more
sensitive than photographic film, and far more efficient than
"The computer and the camera work by themselves in Maui, Hawaii,"
says Dr. Pravdo. "They have baby-sitters, but the scientists
are here at the Jet Propulsion Laboratory." Each night, the
computers track moving objects across the sky and take about
1,000 separate images. This includes three or four images of
each object taken at different times, which allows the astronomers
to begin charting the object's orbit. When new objects are discovered,
the details are passed on to the Minor Planet Center at the
Harvard Smithsonian Astrophysical Observatory for confirmation
"We get information
from about 150 different observers around the world," says Dr.
Brian Marsden, director of the Minor Planet Center in Cambridge,
Massachusetts, which catalogues all newly discovered near-Earth
objects and tries to accurately determine their orbits.
"It usually takes two or three observations of an object, on
separate nights, to gather enough information to figure out
an orbit," says Dr. Marsden. Once an orbit has been determined,
the object is given a number, and the discoverer is given credit.
The spread of electronic detectors over the last few years has
produced a dramatic increase in the number of objects being
discovered. In 1998, the center listed 10,000 objects. Two years
later, the number has risen to 15,000. Most are objects traveling
harmlessly in the asteroid belts between Mars and Jupiter, and
more are being discovered everyday.
About 1,700 listed objects are considered benign, since they
travel in orbits that pass between five and 30 million miles
from the Earth. A smaller subgroup, called Potentially Hazardous
Asteroids, includes 249 objects that are one-tenth of one mile
or larger, and pass within five million miles of the Earth.
Last year, about 100 new objects were added to this list.
these objects are not a danger," says Dr. Marsden. "But in the
last two years, there have been five objects with a significant
impact possibility during the next half-century." Intense efforts
are underway to determine the orbits of these objects. All but
one has been ruled safe. This one object, which appeared in
1998, but vanished before an orbit could be determined, is listed
as a potential threat to the Earth.
"The possibility that the Earth will be struck by a large asteroid
or comet is not insignificant," says Dr. Marsden. "It's a low
frequency event, but the destruction it could cause is quite
tremendous. It's a serious question, given that 65 million years
ago the dinosaurs and all the other life forms were extinguished
by a large impact. It's not likely to happen soon, but you just
"We need to do the watching, so we can be aware if something
is headed our way," says Dr. Marsden. "If we do discover something
that will hit us, the more warning one has, the more possibility
we have to actually do something about it. It might be possible
to send a weapon out into space to deflect or blow up the object.
As a responsible society, this is something we should bear in
-- By Micah Fink