NASA’s Deep Impact spacecraft lived up to its name early Monday when it slammed into a comet with such force that the resulting blast of icy debris stunned scientists with its size and brightness.

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

“The impact was spectacular,” said Dr. Michael A’Hearn of the University of Maryland, the projects 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 sunlit point near the bottom of the elongated comet where they collided with a force equal to 4.5 tons of dynamite at 1:52 a.m. Eastern time.

“We’ve had a far bigger explosion than we anticipated,” said Dr. Donald Yeomans, a mission scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., which controlled the flight. “It was considerably brighter and there was considerably more matter coming off than I had thought.”

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. Depending upon the composition of the comet, scientists speculated that the impact could excavate a crater as large as a sports stadium or as small as a house.

Dr. A’Hearn told an early morning news conference that the blast was so bright that initial images did not reveal the size and depth of the impact crater. This hopefully will be revealed in later images recorded by the flyby spacecraft when they are received and processed on Earth, he said.

“Obviously, it was a very big impact,” he said. “Presumably, we have a large crater in one of those images that hasn’t played back yet.”

A quick look at data streaming down to Earth indicates the best is yet to come, said Dr. A’Hearn. “There are many more spectacular images yet to be revealed,” he said.

The impact was observed by scores of telescopes at ground observatories, as well as NASA’s Hubble, Spitzer and Chandra observatories in Earth orbit, and other spacecraft.

Rick Grammier, the mission’s project manager at the Jet Propulsion Laboratory, said the encounter came off without a hitch. The flyby craft, with high and medium-resolution telescopic cameras and an infrared spectrometer for identifying materials ejected from the comet, successfully monitored the impact from 5,300 miles away. The craft also emerged undamaged after passing within 310 miles of the comet while ducking behind a set of shields designed to protect it from dust and other particles streaming from the comet.

“We have a healthy flyby spacecraft,” Mr. Grammier said at the news conference. It is particularly gratifying, he added, to have such success on July 4th, the nation’s birthday. “I actually hope it’s made America proud,” he said.

After its close approach, the flyby craft pivoted around and took more pictures of the receding comet. It then continued its playback of data it and the impactor collected during the encounter.

The battery-powered impactor separated from the flyby craft 24 hours before colliding with Tempel 1. During the last two hours, to impactor used an autonomous navigation system to pick a target point on the sun side of the comet and made three course corrections to reach it. The craft also took increasingly detailed pictures with its telescopic camera as impact closed in, 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 mile-wide craters.

Scientists are interested in comets because they are believed to be remnants of the materials that formed the solar system some 4.5 billion years ago. Astronomers believe the interiors of comets 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.

An added reason to probe comets is that they, along with rocky asteroids, pose the threat of hitting the Earth and causing cataclysmic damage. Potential planetary defense requires knowing more about these objects in hopes of deflecting or destroying dangerous ones, experts say.

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