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Asteroids
Quick quiz: How many planets orbit our Sun? If you said nine, you're
shy by several thousand. Scientists consider asteroids to be minor
planets - some are hundreds of miles wide (and seldom round).
Orbits
Most, but not all, orbit the sun in an asteroid belt between Mars and
Jupiter. The huge gravitational pull of Jupiter accelerated these asteroids
to more than three miles per second - too fast to prevent violent
collisions. Otherwise, they might have joined up to form "real" planets.
When asteroids collide, fragments sometimes are sent on a collision
course with Earth and become meteors.
Size and makeup
The vast majority of asteroids are small, compared with a large one
like Ida, this 32-mile-long chunk of stone and iron that was photographed
in 1993 by the Galileo spacecraft. Though we normally think of asteroids
as crater-makers, they are typically pockmarked with their own impact
craters.
Scientists divide asteroids into two groups, based on how they appear
in infrared images: light and dark. The lightest-looking asteroids are rocky
bodies with lots of iron and nickel, and they resemble lunar rocks. The
darkest asteroids have high quantities of hydrated minerals and carbon.
In the early days of the solar system (some 4.6 billion years ago)
asteroids had metallic cores, middle regions of stone and iron, and
surfaces of stone. Over time, many of them collided with others and broke
apart. The fragments, which became many of today's asteroids, are
therefore classified as irons, stony-irons or stony.
When an asteroid, or a part of it, crashes into Earth, it's called a
meteorite.
Origin
There are two hypotheses about how most of the asteroids formed.
One says they broke off of a mother planet that existed between Mars
and Jupiter. More likely, however, they represent what space was like
before the planets formed, and they are the remnants of that process -
bits and pieces that never quite joined together.
The threat of impact
Since the Earth was formed more than four billion years ago, asteroids
and comets have routinely slammed into the planet. The most dangerous
asteroids are extremely rare, according to NASA.
An asteroid capable of global disaster would have to be more than a
quarter-mile wide. Researchers have estimated that such an impact
would raise enough dust into the atmosphere to effectively create a
"nuclear winter," severely disrupting agriculture around the world.
Asteroids that large strike Earth only once every 1,000 centuries on
average, NASA officials say.
Smaller asteroids that are believed to strike Earth every 1,000 to
10,000 years could destroy a city or cause devastating tsunamis.
More than 160 asteroids have been classified as "potentially
hazardous" by the scientists who track them. Some of these, whose orbits
come close enough to Earth, could potentially be perturbed in the distant
future and sent on a collision course with our planet.
Scientists point out that if an asteroid is found to be on a collision
course with Earth 30 or 40 years down the road, there is time to react.
Though the technology would have to be developed, possibilities include
exploding the object or diverting it.
For every known asteroid, however, there are many that have not been
spotted, and shorter reaction times could prove more threatening. NASA
puts the odds at 1 in 10,000 of discovering an asteroid that is within 10
years of impact.
Two programs have been set up to actively search for Near-Earth
Objects (NEO's): NASA's Near Earth Asteroid Tracking (NEAT) program,
and Spacewatch at the University of Arizona.
Also, the Spaceguard Foundation was established in 1996 in Rome. The
international organization's goal is to protect Earth from the impacts by
promoting and coordinating discovery programs and studies of NEOs. A
January report shows that NEOs 1 kilometer or larger are being
discovered at the rate of about five a month. The combined goal of these
agencies is to find 90 percent of all NEOs 1 kilometer or larger within the
next decade.
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