[meteorite-list] Violent Creation of Asteroid Families

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:47:13 2004
Message-ID: <200111240259.SAA24625_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/solarsystem/asteroid_birth_011123.html

Violent Creation of Asteroid Families
By Robert Roy Britt
space.com
23 November 2001

In this week's journal Science, three new studies explain the violent family
history of asteroids, give a new estimate for how many space rocks exist in
potentially dangerous orbits near Earth, and paint a more precise picture of
where these asteroids roam.

Asteroids known to orbit the Sun in family groups are likely the result of
tremendous collisions between two rocks, both larger than Rhode Island,
according to a new computer simulation. A huge shock wave reverberates
through the asteroids and splinters them into myriad fragments, but gravity
gathers some of the pieces together again to create somewhat loosely bound
"rubble piles."

These individual asteroids then continue orbiting the Sun, but now instead
of two asteroids there are many.

New Near-Earth Asteroid estimate

In a separate study, MIT researcher Joseph S. Stuart developed yet another
estimate for the number of asteroids 1-kilometer (0.62 miles) or larger
orbiting the Sun roughly at the same distance as Earth. These Near-Earth
Asteroids, or NEAs as astronomers call them, are a top priority for
discovery because they stand the greatest chance of colliding with Earth
sometime in the future.

None of the roughly 500 known NEAs is on a course that will hit Earth
anytime in the next century. But scientists are unsure exactly how many more
NEAs are out there. Most estimates for the total have ranged from 900 to
1,200 and have been revised many times in recent years.

The new study, based on data from the highly successful Lincoln Near-Earth
Asteroid Research (LINEAR) project at MIT, puts the count at between 1,137
and 1,397. It is based on a larger sample of known NEAs compared with
previous studies.

Donald Yeomans, an asteroid expert at NASA's Jet Propulsion Laboratory, said
the MIT estimate is the result of new data and methods and is roughly in
line with other recent estimates, though slightly higher than some. The most
widely accepted estimate in recent months has been about 1,000, plus or
minus 200 or 300, he said. Other studies have put the count as high as
1,400.

Where they roam

Stuart also looked into the locations of NEAs. Earth, the other planets and
most asteroids orbit the Sun roughly in the same imaginary plane in space,
called the ecliptic. But Stuart found that more NEAs are farther above or
below this plane than previously thought. This greater orbital
"inclination," as it is called, may be good news for Earth.

"NEAs with higher inclinations are less likely to impact the Earth," Stuart
said.

Yeomans said the result reaffirms the need to continue looking for asteroids
in the entire sky, as the LINEAR search program does.

"If all you want to do is discover the most NEAs, you look in the ecliptic,"
Yeomans said. "But if everyone does that, you miss some that are in higher
inclinations."

Colliding space rocks

Farther out in space, well beyond NEAs, some 20 families of asteroids are
known to orbit the Sun in the main asteroid belt, between Mars and Jupiter.
Millions of asteroids populate the main belt, leftovers of the solar
system's formation more than 4 billion years ago.

The study of how asteroids might collide and create family groups was led by
Patrick Michel and a colleague at Tanga at Observatoire de la Côte d'Azur in
Nice, France.

Michel explained his group's computer simulation:

Travelling at 11,180 mph (5 km/s), an asteroid 30 miles (48 km) in diameter
slams into another that is 177 miles (284 km) wide. A shock wave sends
cracks propagating through the larger asteroid. Within minutes, it shatters
into 100,000 pieces, none larger than 2 miles (3 km).

The bits are strewn through space, some heading in slightly new directions
at slightly different speeds. But the mutual gravity of the hoard of giant
boulders begins pulling some back together, a process that lasts roughly two
weeks.

The simulation could explain the developmental histories of many asteroids,
which researchers believe are loosely bound "rubble piles" rather than solid
rocks.

"Since a big majority of real asteroids with sizes above a few kilometers
should already have suffered a collision during their lifetime, our result
suggests that many should be rubble piles," Michel told SPACE.com.

And because many of the rocks got back together after being blown apart,
Michel said the study should help scientists better understand the collision
energy required to divert asteroids onto a potentially threatening
trajectory to the Earth. Researchers suspect that many NEAs may have begun
their lives in the asteroid belt and been bumped inward by collisions.

Onward to Earth

In fact, a third study provides further clues as to how some main-belt
asteroids might be bumped into near-Earth orbits.

William F. Bottke, Jr., of the Southwest Research Institute, led a team that
found that the smaller members of an asteroid family spread out and undergo
a change in orbital momentum caused by their re-radiation of solar energy.
This "Yarkovsky effect," as it is called, has been shown to send small rocks
to Earth but was previously thought to be ineffective at changing the orbits
of larger asteroid.

But Bottke's team found, again in computer simulations, that this Yarkovsky
effect leads some family members to the edges of gaps in the main asteroid
belt -- regions of the belt that have been swept clean by the gravitational
effects of Jupiter.

And what does Jupiter do with asteroids that enter these gaps? Sends them to
near-Earth orbits.
Received on Fri 23 Nov 2001 09:59:30 PM PST