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NASA set for a crash with comet

Space probe seeks view of celestial core

As fireworks displays go, this one will be out of this world.

Early on the morning of July 4, a NASA probe traveling 6 miles a second will slam into a comet 83 million miles from earth. The crater it creates will give scientists their first glimpse into the core of a comet, and access to pristine celestial material that could help answer long-asked questions about the solar system's formation.

The cosmic collision could produce an explosion powerful enough to be visible to the naked eye from the southwestern United States. Dubbed Deep Impact, as was a 1998 movie about a doomsday comet, the $330 million mission ranks among NASA's trickiest endeavors: To be successful, scientists must pull off a series of navigational maneuvers likened to hitting a speeding bullet with another speeding bullet. Despite all the preparations, scientists don't even know if the probe will hit a surface that is sludgy, powdery, crumbly or hard, or how big a hole it will blast out of the comet.

''This is a daring mission," said Peter Schultz, a Brown University planetary geologist who is a key investigator for the Deep Impact mission. Schultz has run dozens of experiments simulating what could happen when the probe hits the Tempel 1 comet, a practice that has earned him the nickname ''Master Blaster."

''We are trying to dig below the surface because we can't take a shovel," Schultz said. ''But we still don't know exactly what will happen when it hits."

If all goes perfectly, scientists will be able to gaze into the comet's nucleus -- believed to be composed of pristine leftovers from the formation of the solar system.

Launched in January after more than five years of planning, the spacecraft is designed to split into two parts -- hurling the 820-pound copper-fortified probe at the comet while the mother ship gets ready to capture everything on camera. After the hit, the SUV-sized mother ship has about 13 minutes to take images of the impact and gather data from the event through other sensors before it has to put up shields to protect itself from the blizzard of crater debris that will be ejected from Tempel 1. The impact could produce a crater anywhere from two to 14 stories deep and perhaps as big as a football field.

''It's a very tight schedule, everything has to work on the right schedule at the right time," said Michael A'Hearn of the University of Maryland, the lead investigator of the mission.

Professional astronomers will staff 60 observatories in 20 countries to catch the explosion when the probe hits the comet at 1:52 a.m. on Monday. Scientists hope to learn basic information about comets, such as how they formed and evolved. By examining the size and depth of the crater, researchers will be able to draw conclusions about the comet's strength, density, composition, and how porous its surface is.

The Deep Impact mission, the eighth in a NASA solar system exploration program that also included a landing on an asteroid and an ongoing attempt to bring interstellar dust back to earth, comes at a crucial time for the agency. Later this month, the agency is expected to send a shuttle into space, the first since the Columbia shuttle broke up on reentry in February 2003, killing all seven astronauts on board. The next mission is primarily aimed at testing safety procedures put in place since the Columbia disaster. Since then, NASA has had a tumultuous 2 1/2 years that included a successful robotic landing on Mars, a partially failed mission to bring back solar particles from space, and a new presidential mandate to return to the moon.

Often called ''dirty snowballs" because they are filled with frozen gases, ice, rock, and dust, comets crashing into earth may have delivered water and other building blocks of life to the planet. They are also believed to have caused widespread destruction in the distant past, often cited as a possible cause for dinosaurs' extinction.

Comets have long fascinated -- and frightened -- because they glow as they near the sun and produce long tails of dust. When Halley's comet appeared in 1910, there was a run on gas masks and ''comet pills" because many people believed earth would pass through its tail. In 1997, cult leader Marshall Applewhite and 38 followers committed suicide in California in the belief they were to rendezvous with a spaceship flying in the Hale-Bopp comet's wake.

Tempel 1 hasn't caused any mass hysteria, although it has prompted a lawsuit against NASA from a Russian astrologer who claims the crash will disrupt the balance of the universe -- and destroy some of her personal memories. Marina Bai says her family's history is based on the comet: Her grandparents' romance began when her grandfather showed it to her grandmother. A lower court threw out the case, but a higher Moscow court reinstated it, Russian and US news reports said.

NASA officials say the comet, half the size of Manhattan, won't budge from its orbit because of the hit, equating the force of the probe's impact to a mosquito running into a 767 airliner.

Tempel 1, named for Ernst Wilhelm Leberecht Tempel, the astronomer who discovered it in 1867, doesn't have any special properties that make it a particularly good target. It just happens to be big enough to sustain a solid hit and is in the right place at the right time for scientists to reach it. Scientists don't even know if it's a typical comet, because they know so little about the heavenly bodies.

''The essence of the experiment is we don't know what we'll see," said A'Hearn.

Beth Daley can be reached at bdaley@globe.com.

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