Mission to comet finds complexity


WASHINGTON -- Comet Tempel 1, little more than a fuzzy spot of light in astronomers' telescopes, has turned out to be a complex little world whose surface has the consistency of dry powder snow.

Scientists poring over data sent back by NASA's Deep Impact last July told colleagues yesterday that the comet is also unexpectedly active, belching clouds of water vapor and carbon dioxide into space as often as once a week.

A science team, led by University of Maryland professor Michael A'Hearn, has just begun to plumb the voluminous data sent back by the spacecraft and the 820-pound "impactor" it dropped into the path of the speeding comet on Independence Day.

"There's more than enough to keep us busy until well after I retire," said A'hearn, 64, at the 207th meeting of the American Astronomic Society.

The $330 million Deep Impact mission was launched from Cape Canaveral a year ago and blasted the comet on July 4. Scientists wanted to study the resulting crater, and the material blown out of it, to learn more about what comets are made of and how they are put together.

Astronomers believe comets are made of dust and ice that have changed little since the birth of the solar system 4.5 billion years ago and hold clues to the material and processes that formed the sun and planets.

As it approached Tempel 1, Deep Impact's cameras and instruments found that the comet was hurling quick burps of gas into space more often than scientists had seen from Earth.

The eruptions lasted 10 to 20 minutes. They seemed to be correlated with the comet's spin, and often with sunrise at the erupting site, A'Hearn said. That suggests they're triggered by solar heating and not by meteorite impacts, as some theorists have proposed.

Astronomers knew that comets hold water ice, but Deep Impact imaging revealed it in several patches on the surface. "It's the first detection of water ice on the surface of any comet," said Jessica Sunshine, of the Science Applications International Corp.

A'Hearn and his team expected the initial impact - with energy equal to 5 tons of TNT - to produce a bright flash as the impactor vaporized itself and part of the comet. That should have been followed by a symmetrical plume of debris.

Instead, they saw a subdued and delayed flash and a remarkably asymmetrical plume that flowed downrange from the impact site, said Peter H. Schultz, a planetary scientist and team member from Brown University.

That, he said, suggests a highly porous, fine-grained and compressible surface like fluffy snow or perlite, a mineral additive that lightens garden soil.

The projectile apparently buried itself and vaporized underground. "Afterward, the shock wave moves through the material and almost like a shovel, throwing stuff into space," Sunshine said.

The debris was so fine-grained, A'Hearn said - with pieces less than 10 percent the diameter of a human hair - that it created an opaque, fog-like curtain that blocked Deep Impact's view of the crater.

It's all consistent with the idea that the solar system condensed from microscopic grains of dust forged in ancient stars and that comets are pristine relics of that time.


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