Pasadena, Calif. ? It’s a space mission straight out of Hollywood – launch a spacecraft 268 million miles so it can aim a barrel-sized probe toward a speeding comet half the size of Manhattan and smash a hole in it.

But that’s what NASA expects its Deep Impact mission to do this weekend, with a goal of viewing the icy core of a comet that may hold cosmic clues to how the sun and planets formed.

It’s not without challenges. To ensure a bull’s-eye hit – and a spectacular Independence Day fireworks display in space – several things must happen just right.

Just before 1 a.m. today, the Deep Impact spacecraft must release the 820-pound copper “impactor” on course for a collision expected 24 hours later with the comet Tempel 1.

Late Saturday, the spacecraft made its final move to place the probe 500,000 miles from the comet.

“We’re right in the position where we expect to be,” said Monte Henderson of Boulder, Colo.-based Ball Aerospace & Technologies Corp., which built Deep Impact.

Comets are blobs of ice and dust that orbit the sun and were born about 4.5 billion years ago – nearly the same time as the solar system itself. When a cloud of gas and dust condensed to form the sun and planets, comets formed from what was left over.

Scientists hope studying them will provide clues to how the solar system formed.

Tempel 1, their specimen, is a pickle-shaped comet that travels in an elliptical orbit between Mars and Jupiter.

After springing the probe, the mothership must slightly change course and stake out a prime seat 5,000 miles from the collision, which is expected around 12:52 a.m. Monday.

The comet, hurtling through space at a relative speed of 23,000 mph, will run over the probe with energy similar to exploding nearly 5 tons of dynamite. All the while, a camera on the impactor will be shooting pictures as it heads toward its doom, as will the mothership from afar.

Little is known about comet anatomy, so it’s unclear what exactly will happen when Tempel 1 is hit. Scientists expect the collision to spray a cone-shaped plume of debris into space.

“We still don’t know what this comet holds in store for us,” said Rick Grammier, Deep Impact project manager.

Launched in mid-January from Cape Canaveral, Fla., Deep Impact sent images of the comet’s nucleus for the first time last month from a distance of 20 million miles away.