July 4, 2005

NASA probe leaves crater, debris trail on comet

By Gina Keating

PASADENA, Calif. (Reuters) - A spectacular collisionbetween a spacecraft and a comet has freed a huge plume ofprimordial material from the comet's nucleus that could unlockthe secret of how life arrived on Earth, NASA scientists saidon Monday.

The first images returned from the Deep Impact fly-byspacecraft showed a small fireball followed by a much larger,incandescent flash that engulfed one end of the comet Tempel 1as the impactor smashed into its surface at 1:52 a.m. EDT (0552GMT) on Monday.

The impactor was vaporized upon slamming into the comet at23,000 mph (37,000 kph) -- the speed it would take to fly fromNew York to Los Angeles in about six minutes.

The collision, which occurred 83 million miles from Earth,marks the first time a spacecraft has come in contact with acomet.

Observatories on the ground reported that the explosionbrightened the comet by a factor of 5 within 15 minutes ofimpact, scientists at NASA's Jet Propulsion Laboratory inPasadena said.

Scientists could not immediately determine the size of thecrater produced by the impact because of the large plume ofice, dust and gases streaming out and obscuring one end of thecomet, which is half the size of Manhattan.

"We are waiting for the outgassing to stop. It's clear it'swas still coming out for several hours ... and could go on forweeks," principal scientist Mike A'Hearn told reporters at aMonday news conference.

The Deep Impact team had estimated the washingmachine-sized impactor would punch a hole anywhere from thesize of a house to a football stadium, depending on thecomposition of the comet's surface.

"We know that we created quite a crater. We believe itpenetrated quite deeply so we know we'll get a good look at theinterior," Project Manager Rick Grammier said on Monday. "Wejust have a wealth of scientific information to go through inthe next few months."


The impact sent up twin plumes of debris, the firstappearing as a narrow column that cast a long shadow across thecomet. Another plume appeared seconds later on the heels of abrighter explosion, then fanned out in a star shape. Scientistssaid the plumes stretched for "at least thousands ofkilometers" into space.

Co-investigator Pete Schultz said the twin flashes showedthat the impactor encountered softer, layered material on thecomet's surface then hit a thick, hard crust.

A spectrometer aboard the fly-by spacecraft captured "bigchanges" in the spectra of debris flying up from the crater,indicating a variety of materials were freed by the impact,A'Hearn said.

Comets are made of gas, dust, organic material and ice fromthe solar system's farthest regions. Because they were notheated by the Sun during the formation of the solar system,comets retain the original chemical mixture from which theplanets formed 4.6 billion years ago.

Scientists think comets may be responsible for firstbringing water and organic material to Earth by crashing intoits surface during a period of heavy comet activity 3.9 billionyears ago -- around the same time as the first signs of life.

Tempel 1's rough surface, closely revealed for the firsttime in images snapped by the impactor up to 3 seconds beforeimpact, differed markedly from the two other comets scientistshave been able to observe up close, A'Hearn said.

The surface showed what appeared to be layering, craters,small bright features and smooth areas that defy physics bystretching around two sides of the comet, he said.

"There is something more going on here than we understand,"he said.

A'Hearn also said the Deep Impact team has been forced toreevaluate its ideas about Tempel 1's shape, which is more likea muffin or a loaf of bread than a pickle.

The $333-million Deep Impact mission is the eighth inNASA's Discovery Program to produce relatively cheap missions,such as the 1997 Mars Pathfinder, to explore the solar system.