NASA probe is zooming in on heart of a comet

PASADENA, Calif. -- A NASA space probe was bearing down on its comet target yesterday in a mission scientists hoped would gain them insight into the origins of the solar system.

The 820-pound copper probe was on course to intercept the comet Tempel 1 and smash a hole in it so scientists could get their first peek at the heart of one of the icy celestial bodies.

Comets are leftover building blocks of the solar system, which formed when a giant cloud of gas and dust collapsed to create the sun and planets. Because comets were born in the system's outer fringes, their cores still possess some of the primordial ingredients, and studying them could yield clues as to how the solar system formed 4.5 billion years ago.

The ''impactor" probe separated from the Deep Impact spacecraft late Saturday and began a 500,000-mile dive toward the sunlit section of Tempel 1, a pickle-shaped comet half the size of Manhattan and 83 million miles from Earth.

Deep Impact, meanwhile, fired its thrusters to change course slightly and stake out a front-row seat 5,000 miles from the high-speed collision, which was expected to occur at 1:52 a.m. today.

Yesterday scientists released the first picture of the solo probe taken by Deep Impact shortly after separation, showing the probe as a bright dot surrounded by blackness.

The probe was to designed to shoot close-up pictures at a relative speed of 23,000 miles per hour until hitting the comet. After that, Deep Impact was to take over recording the scene through its high-resolution telescope.

''We anticipate a little bit of a bumpy ride," said systems engineer Jennifer Rocca.

Among the challenges workers in mission control at NASA's Jet Propulsion Laboratory in Pasadena faced was the probe's switch to autopilot two hours before the crash, relying on computer software and thrusters to steer itself into the path of the onrushing comet.

About 15 minutes after the probe's impact, Deep Impact was to make its closest flyby of the comet nucleus, approaching within 310 miles. Scientists expected it would be bombarded with flying debris and will stop taking pictures, turning on its dust shields for protection.

The National Aeronautics and Space Administration's brigade of space-based observatories, including the Hubble Space Telescope, Chandra X-ray Observatory, and Spitzer Space Telescope, also were pointing at the comet to record the impact.

Little is known about comet anatomy, so it wasn't clear what would happen. Scientists expected the collision to spray a cone-shaped plume of debris into space with a resulting crater ranging anywhere from the size of a large house to a football stadium and between two and 14 stories deep.

Deep Impact blasted off in January from Cape Canaveral, Fla., for its six-month, 268 million-mile journey. In what scientists say is a coincidence, the spacecraft shares the same name as the 1998 movie about a comet that hurtles toward Earth.

Discovered in 1867, Tempel 1 moves around the sun in an orbit between Mars and Jupiter every six or so years.

The idea for the Deep Impact mission came in 1986, after a spacecraft captured images of Tempel 1 as it flew by Halley's Comet in 1986. The comet had what looked to be a hard exterior, which led several scientists to wonder what was underneath the surface.

In April, the spacecraft took its first picture of Tempel 1 from 40 million miles away, showing what amounts to a celestial snowball. Last month, still 20 million miles away, scientists saw the solid core of Tempel 1 for the first time.

Americans who live west of a line extending from Chicago to Atlanta might have been able to see the impact with a telescope, and some scientists believed the debris from the collision could be visible with the naked eye. Those east of that line could not see it, because the impact would have occurred below the horizon. Residents farther south and west had the best opportunity to see the event.

The impact could have been be too small to see without a telescope, especially in large cities with light pollution.

NASA was to show the impact by live-feed television on the Internet, at www.nasa.gov.

The big question is: What kind of astronomical fireworks can sky-gazers expect to see this Fourth of July?

Scientists do not know yet. But if the probe hit 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.