Spacecraft Hits Passing Comet, Just as Planned
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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
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
"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
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
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
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
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
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
"I actually hope it's made America proud," he said.