Spacecraft Hits Passing Comet, Just as Planned
http://www.nytimes.com/2005/07/05/science/05comet.html?th&emc=th  (must register to view original article)

By WARREN E. LEARY
Published: July 5, 2005

WASHINGTON, July 4 - NASA's 83-million-mile shot at a comet was a bull's-eye. Its Deep Impact spacecraft slammed into its target with such force early Monday that the resulting blast of icy debris stunned scientists with its size and brightness. If you could hear sounds in space, it would have been a big bang.

With the second stage of the two-part spacecraft, known as the flyby stage, watching from a safe distance, an 820-pound copper-core "impactor" craft smashed into the nucleus of comet Tempel 1 at 23,000 miles an hour, sending a huge spray of debris into space.

"The impact was spectacular," said Dr. Michael A'Hearn of the University of Maryland, the project's principal scientist. "It was much brighter than I expected."

Culminating a six-month journey to a point 83 million miles from Earth, the impactor guided itself to a point near the bottom of the elongated comet where they collided at 1:52 a.m. Eastern time with a force equal to four and a half tons of dynamite.

Scientists had only one chance for a collision with their fast-moving target. With radio communications taking more than seven minutes each way, the spacecraft was on its own to complete the mission.

Just 24 hours before intercepting Tempel 1, springs separated the larger flyby craft from the impactor, leaving the projectile in the path of the comet as the mother craft veered away. The impactor turned on its automatic navigation system two hours before impact and made three course maneuvers to pick a well-lit spot on the sunny side of the comet to hit. It was right on target.

"We've touched a comet, and we've touched it hard," Dr. Peter H. Schultz of Brown University, another main investigator, said at one of two news conferences at NASA's Jet Propulsion Laboratory in Pasadena, Calif., which controlled the flight.

The purpose of the $333 million mission was to make the most detailed study of a comet to date, striking the mountain-sized hunk of ice and rock, and creating a crater from which would spew some of the primal material that makes up its core. The material, to be analyzed using instruments on the flyby craft, may hold clues to the formation of the solar system 4.5 billion years ago.

Depending upon the comet's composition, scientists speculated that the impact could leave a crater as large as a stadium or as small as a house. Dr. A'Hearn said the blast was so bright that initial images did not reveal the crater's size or depth.

Those are to be revealed in later images recorded by the flyby spacecraft when they are received, he said. In some pictures, Dr. A'Hearn said, scientists see a feature or shadow where the crater would be, but it will take a week or more of image processing to be sure.

Dr. Schultz said he did not want to guess the size of the crater. But he added: "I don't think it's house-sized. I think it's bigger than that."

Even as it picked a target point on the Sun side of the comet and navigated toward it, the battery-powered impactor took increasingly detailed pictures with its telescopic camera, shooting its last image just 3.7 seconds before the collision.

Late images from the impactor, the best ever taken of a comet, showed a Moon-like surface with flat plains, circular craters and a long, irregular ridge. Some of the last pictures appeared to show the impactor coming in between two milewide craters on the deeply textured surface.

Scientists are interested in comets because they are believed to be remnants of the materials that formed the solar system. Astronomers believe that comet interiors have undergone little change since then and contain the pristine ice, gases, dust and other materials from which the rest of the solar system formed.

A secondary reason to probe comets is that along with rocky asteroids, they pose the threat of cataclysmic damage if they ever strike Earth. Potential planetary defense, Dr. A'Hearn said, requires knowing more about these objects and their composition in hopes of deflecting or destroying dangerous ones.

Sequential pictures of the impact with Tempel 1 seem to indicate a two-stage explosion caused by the energy of the penetrating spacecraft, Dr. Schultz said. First, a small flash ejecting an umbrella-shaped cloud of debris was shown, then a brief delay was followed by a large flash shooting out a tall vertical column of dust and other material.

The impact was also observed by scores of telescopes at ground observatories, as well as NASA's Hubble, Spitzer and Chandra observatories in Earth orbit and other spacecraft. Pictures from the Hubble Space Telescope showed Tempel 1 appearing four times as bright 15 minutes after the collision, and the European Space Agency's XXM-Newton observatory detected evidence of water.

Tempel 1, discovered in 1867, is a dark-colored comet that moves about the Sun in an elliptic orbit between Mars and Jupiter every five and a half years. Latest observations indicate it is an elongated object about nine miles long and 3.7 miles across, about half the size of Manhattan. Despite the force of the collision, the comet shrugged it off and continued on its way, undisturbed.

Rick Grammier, the mission's project manager at the Jet Propulsion Laboratory, said the encounter came off without a hitch. The flyby craft, equipped with high- and medium-resolution telescopic cameras, monitored the impact from 5,300 miles away. It is also equipped with an infrared spectrometer, which analyzes light frequencies from the ejected material to identify it.

The craft emerged undamaged after passing within 310 miles of the comet while ducking behind a set of shields to protect it from dust and other particles streaming from the comet. "We have a healthy flyby spacecraft," Mr. Grammier said.

After its close approach, the flyby craft took more pictures of the receding comet. One image, taken from 16,800 miles behind, showed the black end of the comet silhouetted by the glow of dust being ejected thousands of miles into space by the impact. After transmitting all its collected data, the craft will be powered down and "mothballed" in space, with the possibility of activation for another mission.

"It is particularly gratifying," Mr. Grammier said, to have such success on Independence Day.

"I actually hope it's made America proud," he said.