- WASHINGTON (Reuters) - Astronomers Thursday said many asteroids come close
enough to Earth to raise alarms and, occasionally, hit us, because of gravity.
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- Resonance nudges the asteroids into orbits
that bring them close to Earth and Mars. Resonance is when orbital paths
nearly overlap.
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- A resonance occurs, for instance, when
the Earth orbits the sun in one year, an asteroid orbits the sun in precisely
two years, and thus the Earth and the asteroid always pass close to each
other at exactly the same point.
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- This allows the Earth's gravity regularly
to perturb the orbit of the asteroid.
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- "It laps it," explained Richard
Greenberg, an astronomer at the University of Arizona. "Time and time
again they line up exactly at the same position. They have a gravitational
effect at the same point, every time."
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- When this happens between Jupiter and
an asteroid, Jupiter's strong gravity gives the asteroid a little kick
-- causing it to wobble.
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- Previous models have suggested that it
takes a very strong resonance to kick asteroids out of the asteroid belt
between Jupiter and Mars and onto a collision course with Earth.
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- But writing in the journal Science, Alessandro
Morbidelli of the Armagh Observatory in Northern Ireland and colleagues
said it was found just a year ago that such strong resonances would actually
knock the asteroids right into the sun, or pitch them out of the solar
system altogether.
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- Back to the drawing table for astronomers.
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- Morbidelli, who also works at the Turin
Observatory, and his team designed a computer model that shows weaker resonances,
instead, are responsible. Their model correctly predicts the 10 known asteroids
in near-Earth orbits and 354 in orbits that bring them close to Mars.
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- Greenberg said the finding helps astronomers
explain how the population of near-Earth asteroids is replenished.
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- If new asteroids weren't constantly being
knocked into the paths of Earth and Mars, all of them would have long ago
crashed into the planets and there would be none left to write disaster
movies about.
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- Greenberg, who wrote a commentary about
the findings, said scientists had always assumed that weaker resonances
were not strong enough to affect the asteroids.
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- "It turns out that weaker resonances
are just right," he said in a telephone interview.
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- "They're not too strong that they
kick the asteroids into the sun or out of the solar system, and they're
not too weak so that the asteroids just stay in the asteroid belt,"
he added.
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- "There are at least half a dozen
of these weaker resonances. We hadn't understood their strength."
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- Greenberg said the phenomenon could also
explain meteorites, which are pieces of asteroids. The name meteorite,
he pointed out, comes from the same word for meteorology.
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- Early observers assumed they came from
the atmosphere, and were a weather phenomenon, and not from space.
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