Known as Stardust, the robotic scout has traveled some two billion miles to get to the far side of the sun so it can analyze one-thousandth of an ounce of dust from the Wild-2 comet.

The rendevous -- at exactly 2:20 p.m. EST -- will be quick, with Stardust traveling at 13,650 mph relative to its target.

Engineers programmed the probe to approach Wild-2 from above, then dip below, allowing itself, in effect, to be run over.

The maneuver has been planned to get Stardust within about 186 miles of the comet's core -- close enough to trap some particles for return to Earth but far enough away to avoid being damaged or destroyed by the comet's debris.

"This could prove to be a pivotal time for science," said Donald Brownlee, principal scientist for the Stardust mission.

"There's a museum out there in the outer solar system that has preserved our building blocks, and it is going to be an absolute thrill to have this stuff to look at."