Scientists at Nasa's Jet Propulsion Laboratory said that the space probe was scheduled on Friday to plunge into the 'tail' of gas and debris spewing from the comet Wild 2, passing within 188 miles of the streaking chunk of rock and ice.

The spacecraft will reach speeds of nearly 14,000 miles per hour as it snaps pictures, gathers data and scoops up dust particles destined to be the first cometary samples returned to Earth for study.

"We are literally collecting preserved samples of the building blocks of our solar system and our Earth and even ourselves," said Donald Brownlee, a University of Washington astronomer who is the mission's chief scientist. "They've been preserved for the age of the solar system out there at low temperature and are basically in a pristine state."

The historic "fly-by" will take place 242 million miles from Earth, with passage through the most intense hailstorm of particles inside the Wild 2's coma lasting about eight minutes.

At that point, debris will be pelting the bookcase-sized spacecraft at six times the speed of a bullet, but the probe's solar panels and sophisticated instruments will be shielded by two specially designed bumpers at the front of the spacecraft, NASA scientists said.

Tiny bits of matter 10 to 330 microns in diameter, up to three times the width of a human hair, will be collected in the spacecraft's "cometary catcher's mitt," a tennis racket-shaped panel lined with a super-fluffy material called aerogel, consisting of pure silicon dioxide and 99.8 percent air.

The aerogel has enough cushion to stop the particles without substantially altering them. After the fly-by, the collector panel will fold down into a clamshell-like capsule for a return flight to Earth aboard Stardust in two years.

The capsule will ultimately separate from the spacecraft and re-enter Earth's atmosphere for a landing in the Utah desert in January 2006, while Stardust veers back into space.

Scientists say the dust samples, containing particles gathered by the comet since its formation at the dawn of the solar system and during its own ancient interplanetary wanderings, hold clues to how the solar system, and even life on Earth, began.

"One of the mantras of our project is that we are stardust, as in the Joni Mitchell song, because the atoms in our bodies, before the Sun and solar system formed, were actually interstellar grains," Brownlee said.