Celestial Fireworks On The Fourth

NASA To Blow Up A Comet On Independence Day

(AP)  Not all dazzling fireworks displays will be on Earth this Independence Day.

NASA hopes to shoot off its own celestial sparks in an audacious mission that will blast a stadium-sized hole in a comet half the size of Manhattan. It would give astronomers their first peek at the inside of one of these heavenly bodies.

If all goes as planned, the Deep Impact spacecraft will release a wine barrel-sized probe on a suicide journey, hurtling toward the comet Tempel 1 - about 80 million miles away from Earth at the time of impact.

"It's a bullet trying to hit a second bullet with a third bullet in the right place at the right time," said Rick Grammier, project manager at NASA's Jet Propulsion Laboratory in Pasadena.

Scientists hope the July 4 collision will gouge a crater in the comet's surface large enough to reveal its pristine core and perhaps yield cosmic clues to the origin of the solar system.

NASA's fleet of space-based observatories - including the Hubble, Spitzer and Chandra telescopes - along with an army of ground-based telescopes around the world are expected to record the impact and resulting crater.

The big question is: What kind of fireworks can sky-gazers expect to see from Earth?

Scientists do not know yet. But if the probe hits the bull's-eye, the impact could temporarily light up the comet as much as 40 times brighter than normal, possibly making it visible to the naked eye in parts of the Western Hemisphere.

"We're getting closer by the minute," Andrew Dantzler, the director of NASA's solar system division, said earlier this month. "I'm looking forward to a great encounter on the Fourth of July."

If the $333 million mission is successful, Deep Impact will be the first spacecraft to touch the surface of a comet. In 2004, NASA's Stardust craft flew within 147 miles of Comet Wild 2 on its way back to Earth carrying interstellar dust samples.

Scientists say Deep Impact has real science value that will hopefully answer basic questions about the solar system's birth.

Comets - frozen balls of dirty ice, rocks and dust - are leftover building blocks of the solar system after a cloud of gas and dust condensed to form the sun and planets 4 1/2 billion years ago. As comets arc around the sun, their surfaces heat up so that only their frozen interiors possess original space material.

Very little is known about comets and even less is known about their primordial cores. What exactly will happen when Tempel 1 is hit on the Fourth of July is anybody's guess. Scientists believe that the impact will form a circular depression that will eject a cone-shaped plume of debris into space.

But not to worry. NASA guarantees that its experiment will not significantly change the comet's orbit nor will the smash-up put the comet or any remnants of it on a collision course with Earth.

Discovered in 1867, Tempel 1 is a short-period comet, meaning that it moves around the sun in an elliptical orbit between Mars and Jupiter and can be sighted every six or so years.