Origins of the planets
2002-06-13 08:10
Paris - American astronomers have achieved the mathematical feat of finding a broken needle in a haystack and determining when it was snapped.
They have found a cluster of debris whirling in orbit around the
Sun and calculated that the pieces broke away from a 25-kilometre
(15.6-mile) asteroid that collided with another space rock around
5.8 million years ago.
The work could shed light on the violent origins of the
planets, the scientists report in Thursday's issue of Nature, the
weekly British journal.
The theory of the Solar System's birth is that hundreds of
millions of years ago, the planets grew from small clusters of
rocks and dust that clumped together by gravitational force.
Over time, these proto-planets grew steadily larger, swelling as
countless smaller bodies were ensnared by their gravitational pull and plummeted to their surface.
The rubble left over from this planetary building site comprises
the asteroid belt between Mars and Jupiter - a band of wheeling
orbital junk too far either planet or the Sun to be captured.
Endless collisions
At one time, these asteroids were big. But, over the aeons, in
the soundless void, they smashed up into fragments, through endless collisions with each other, and were progressively weathered by the Sun.
But little is known about the dynamics of collision among small
bodies ('planetisimals') such as these.
David Nesvorny and others at the Southwest Research Institute in
Colorado discovered a "family" of 39 fragments, ranging in size
from two to 19 kilometres (1.25 to 11.8 miles) across, that are
orbiting in a relatively tight cluster.
Crunching their trajectories through a computer model, they
believe the fragments all came from an asteroid that broke up 5.8
million years ago, with a margin of error of plus or minus
200 000 years.
Nesvorny's so-called Karin cluster was a good candidate for the
ambitious exercise because the pieces were in a remarkably similar orbital path.
Older collisions are far more difficult to plot because they
have had more years for their fragments to follow separate
trajectories and knock into other asteroids - a game of cosmic
pinball unfolding over unimaginable lengths of time and with
millions of bumpers.
The authors believe that after the collision, the Karin family
scattered at low velocity, which suggests that a big factor in
asteroid break-ups could be fragments' residual gravitational pull.
In a commentary, University of Maryland astronomer Derek
Richardson, said the findings could shed light into how
protoplanets grew from their early tiny clumps of dust and mineral splinters.
Finding out more about asteroid dynamics could also be essential
for saving the Earth from a killer space rock, of the kind that
wiped out the dinosaurs 65 million years ago, he suggested.
"Under what conditions does an asteroid break apart?" he asked.
"What happens to the debris? How do pre-existing fractures affect
the outcome? Could a 'doomsday' asteroid threatening Earth be
stopped by brute-force methods such as a nuclear blast?" - Sapa/AFP
- SAPA