A NASA spacecraft with a Hollywood name -- Deep Impact -- blasted off Wednesday on a mission to smash a hole in a comet and give scientists a glimpse of the frozen primordial ingredients of the solar system.

With a launch window only one second long, Deep Impact rocketed away at the designated moment on a six-month, 268-million-mile journey to Comet Tempel 1. It will be a one-way trip that NASA hopes will reach a cataclysmic end on the Fourth of July.

"We are on our way," said an excited Michael A'Hearn of the University of Maryland, the mission's chief scientist. Minutes later, the spacecraft shot out of Earth's orbit and onto its collision course.

"We'll be there July Fourth," NASA launch director Omar Baez said.

The flight was barely under way when an overheating problem -- considered slight and not at all damaging -- was detected by the spacecraft itself. Onboard computer software put Deep Impact in a protective "sleep" mode that flight controllers expected to emerge from within 24 hours, via recovery commands.

"We don't see it as a long-term threat by any means," said project manager Richard Grammier. The spacecraft is healthy, with the solar panel deployed and generating power, and the temperature increase in the propulsion-system heaters is slight and well within safety limits, he said.

Scientists are counting on Deep Impact to carve out a crater in Comet Tempel 1 that could almost swallow the Roman Coliseum. It will be humans' first look into the heart of a comet, a celestial snowball still containing the original building blocks of the sun and the planets.

Because of the relative speed of the two objects at the moment of impact -- 23,000 mph -- no explosives are needed for the job. The force of the smashup will be equivalent to 4 1/2 tons of TNT, creating a flash that just might be visible in the dark sky by the naked eye in one spectacular Fourth of July fireworks display.

Nothing like this has ever been attempted before.

Little is known about Comet Tempel 1, other than that it is an icy, rocky body about nine miles long and three miles wide. Scientists do not even know whether the crust will be as hard as concrete or as flimsy as corn flakes.

"One of the scary things is that we won't actually know the shape and what it looks like until after we do the encounter," said Jay Melosh, a planetary geologist at the University of Arizona.

The comet will be more than 80 million miles from Earth when the collision takes place -- on the sunlit side of the comet, NASA hopes, in order to ensure good viewing by spacecraft cameras and observatories. The resulting crater is expected to be two to 14 stories deep, and perhaps 300 feet in diameter.

Scientists stress that Deep Impact will barely alter the comet's orbital path around the sun and will not put either the comet or a chunk of it on a collision course with Earth. In the 1998 movie "Deep Impact," astronauts try to blow up a comet in hopes of saving the Earth, but the comet winds up being split in two and one section slams into the Atlantic, creating a huge tsunami on the East Coast.

A jagged, cratered comet like the one headed for Earth in the movie would be difficult if not impossible to hit because of all the shadows, Melosh said. Comet Tempel 1 is believed to be smoother and easier to strike, unlike that "Hollywood nightmare."

The scientists came up with the Deep Impact name independently of the movie studio, around the same time, neither knowing the other was choosing it, even though some members of NASA's Deep Impact team were consultants on the picture.

Deep Impact is carrying the most powerful telescope ever sent into deep space. It will remain with the mother ship when the copper-fortified impactor springs free the day before the comet strike, and will observe the event from a safe 300 miles away.

NASA space telescopes like the Hubble will also watch the collision, along with ground observatories and amateur astronomers. The impactor will have a camera, too, that will snap pictures virtually all the way in.

The entire mission costs $330 million, all the way through the grand finale.

(China Daily via agencies January 13, 2005)