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Asteroid resonance - Reuter's

Hello Ron and List,

Below is an article from 1998 which raises more questions than it answers.
Now, given Ron's recent posts regarding 1999AN 10,  it might be helpful if
we had some clarity on exactly what resonance is.
The article below is misleading
is several regards, not the least of which is that it refers to resonance as
'nudging' an asteroid out of it's orbit, or 'knocking' it out, rather than
'pulling' it out?   What precisely is the distinction between resonance and
gravity?  Their net effect seems to be the same.


September 25, 1998
Web posted at: 12:00 PM EDT

"Gravity Explains Why so many Asteroids Scare Us


WASHINGTON (Reuters) -- Astronomers said on Thursday they had explained why
so many asteroids come close enough to Earth to raise alarms and,
occasionally, hit us.

They said it is all due to a coincidence in their orbits known as resonance,
which nudges the asteroids into orbits that bring them close to Earth and
Mars.

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.

This allows the Earth's gravity regularly to perturb the orbit of the
asteroid.

"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."

When this happens between Jupiter and an asteroid, Jupiter's strong gravity
gives the asteroid a little kick-- causing it to wobble.

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.

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.

Back to the drawing table for astronomers.

Morbidelli, who also works at the Turin Observatory, and his team designed a
computer model that shows weaker resonances are instead responsible. Their
model correctly predicts the 10 known asteroids in near-Earth orbits and 354
in orbits that bring them close to Mars.

Greenberg said the finding helps astronomers explain how the population of
near-Earth asteroids is replenished.

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.

Greenberg, who wrote a commentary about the findings, said scientists had
always assumed that weaker resonances were not strong enough to affect the
asteroids.

"It turns out that weaker resonances are just right," he said in a telephone
interview.

"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.

"There are at least half a dozen of these weaker resonances. We hadn't
understood their strength."

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.

Early observers assumed they came from the atmosphere, and were a weather
phenomenon, and not from space."


Best Regards,
Julia












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