Comet-busters left with empty feeling

WASHINGTON - The comet Tempel 1, the target of Nasa's Deep Impact probe, is mostly "fluffy" and fragile, with no more substance than snow, scientists said.

Tempel 1 was fired on by Nasa on July 4, to release a plume of primordial material from its nucleus, the first time astronomers have been able to glimpse a comet's interior.

Comet specialist Michael A'Hearn said initial findings show it "is mostly empty, mostly porous".

Material on the comet's surface and several dozen metres down is "unbelievably fragile, less strong than a snowbank," he said. "Probably all the way in, there is no bulk ice. It's in the form of tiny grains."

The comet's dust and ice grains form a fluffy structure of fine particles held together loosely by a weak gravitational pull.

Tempel 1's surface is pocked with apparent impact craters, features that have not been detected on close-up observation of two other comets.

The smash-up with Deep Impact's washing-machine-sized probe was monitored by another part of the Nasa spacecraft that flew above the comet, with a European spacecraft called Rosetta, and by more than 70 ground-based telescopes.

Scientists hope research into Tempel 1 will help unlock the secret of how life arrived on Earth. Variously described as dirty snowballs or snowy dirtballs, comets are prime candidates for seeding planets, including Earth, with water and organic material.

An analysis of material in the strike plume showed a huge increase in the amount of molecules that contain carbon, suggesting comets like Tempel 1 contain a substantial amount of organic material.

Scientists speculate they might have brought such material to Earth in its early years.

Further findings from the study will be published later this week in the journal Science.

- REUTERS