Deep Impact probe shows comets are fragile snowballs

The target of NASA's Deep Impact probe, comet Tempel 1, turns out to be quite fragile, with no more substance than a snowbank, scientists said on Tuesday.

"The comet is mostly empty, mostly porous," said Michael A'Hearn, a comet specialist at the University of Maryland. "Probably all the way in, there is no bulk ice. The ice is all in the form of tiny grains."

The material on the comet's surface, down to a depth of several dozen yards (meters) is "unbelievably fragile, less strong than a snowbank," A'Hearn said in a telephone news briefing to release early findings from the mission.

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

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

According to Washington Post "This will be the biggest contribution we will make," said University of Maryland astronomer Michael A'Hearn, lead scientist for NASA's Deep Impact mission. "Many hydrocarbons are in higher abundance than one would expect, and there are many others we haven't identified yet."

A'Hearn spoke during a telephone news conference to announce the first research findings from the spectacular July 4 rendezvous in space between Tempel 1 and the 820-pound copper-tipped "impactor" that slammed into it just before 2 a.m. Eastern time on Independence Day. The findings are to be published Thursday in the journal Science.

A'Hearn said data transmitted from the "flyby" spacecraft that accompanied the impactor offered several mildly surprising results, but "nothing that knocked my socks off." The flyby spacecraft dropped the impactor into the path of the comet, then photographed the encounter from a safe distance.

Horst Uwe Keller of Germany's Max Planck Institute said the impact threw about 5,500 tons of water into space, along with other types of ice including carbon dioxide, carbon monoxide and ammonia ice. But the impact tossed up an even greater quantity of dust.