- WASHINGTON (Reuters) - One
got the dinosaurs. Another wiped out the trilobites and just about everything
else on Earth. And an asteroid or comet might get us, too, scientists say.
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- That is why dozens of centers are searching the sky for
moderate-sized asteroids or comets that might one day collide with the
Earth.
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- It appears that every 100 million years or so, something
big enough to wipe out nearly all life hits the planet, Chris Chyba of
Stanford University in California says.
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- Such impacts bracketed the dinosaur age, scientists now
think. This week's issue of the journal Science carries a report suggesting
that an asteroid or comet was responsible for the ``mother of all extinctions''
-- the Permian event 250 million years ago that wiped out 90 percent of
all marine species and 70 percent of animals and plants on land.
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- It would have been about the size of the asteroid believed
to have hit what is now the Yucatan peninsula in Mexico, sending up clouds
of dust and sparking volcanic activity that wiped out the dinosaurs 65
million years ago.
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- The first impact would have ended the Paleozoic age 250
million years ago, starting the Mesozoic, during which dinosaurs evolved
and thrived. The impact 65 million years ago ended the party for the dinosaurs,
allowing mammals and eventually humans to evolve during the present age.
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- ``Statistically, there is something like 100 million
years between impacts of 10-kilometer (6-mile-wide) objects and the Earth,''
Chyba told a news conference sponsored by the space agency NASA on Thursday.
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- That scenario would allow for a theoretical 35-million-year
buffer. But of course asteroids and comets do not operate on schedule.
And something smaller could make quite a mess, too.
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- Smaller Impact Could Kill A Lot Of Us
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- ``Smaller impacts ... even a kilometer (half a mile)
in size could also cause not mass extinctions but could strongly affect
human existence,'' Chyba said.
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- It could kick up enough dust to cause a ``nuclear winter''
that would wipe out crops and might cause tsunamis to swamp coastal areas.
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- It did not take the recent release of asteroid disaster
films to make scientists aware of this threat.
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- In 1998 NASA started what is called the Spaceguard Survey,
which aims to find 90 percent of near-Earth objects larger than a half
a mile in diameter by 2008.
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- Teams of astronomers around the world are surveying the
sky with electronic cameras to find objects, and amateur sky-watchers help
in the effort.
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- ``We think we know all of the 10-kilometer (6-mile-wide)
objects,'' Chyba said. ``There aren't very many of them that are crisscrossing
Earth's orbit. We don't have to worry about them.''
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- He said researchers are about halfway through a catalog
of half-mile objects.
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- If one is found to be on a collision course with Earth,
Chyba and other experts say there will be plenty of time to think about
what to do -- whether to launch a spacecraft to try and deflect it, or
make the best of a bad situation and move people away from coastal areas
and stockpile food.
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- If one has been missed, NASA says the first warning will
be the explosion when it strikes.
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- ``Statistically, the greatest danger is from a NEO (near-Earth
object) with about 1 million megatons energy,'' NASA says in its Web site
devoted to the threat at http://impact.arc.nasa.gov/.
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- This object would be 1.2 miles in diameter.
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- ``On average, one of these collides with the Earth once
or twice per million years, producing a global catastrophe that would kill
a substantial (but unknown) fraction of the Earth's human population. Reduced
to personal terms, this means that you have about one chance in 20,000
of dying as a result of a collision,'' NASA says.
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- Of course such impacts give as well as receive. Some
scientists believe that meteors, comets and asteroids smashing into the
Earth may have carried the very seeds of life. Evidence of amino acids
and even tiny bacteria have been found in meteorites.
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- Just weeks ago a team at the University of California
Santa Cruz said they created an artificial cell wall in space-like conditions
and said it showed living cells could have survived a trip through space.
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