PARIS, June 13: 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 asteroid that collided with another space rock around 5.8 million years ago.

The work could throw 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.

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 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 breakups 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.—AFP