NASA scientists are preparing the ultimate Independence Day firework -- a copper missile shot into the heart of a giant comet.
After a voyage of more than six months and 431km, the Deep Impact spacecraft will intercept the 4km-wide Tempel-1 comet travelling at 37,000kph and fire a 1m copper projectile into it.
Astronomers hope the explosive encounter on July 4 will smash a hole in the comet's icy exterior and show what lies inside.
Michael A'Hearn, chief scientist on the project, said: "The last 24 hours of the impactor's life should provide the most spectacular data in the history of cometary science. We know so little about the structure of cometary nuclei that almost every moment we expect to learn something."
Scientists think the core of comets are made of primordial material left over during the formation of the solar system more than 4 billion years ago. Cameras on the projectile and the Deep Impact mothership will record the collision and sensors will analyze material dug out of the comet's interior and beam the data to Earth.
"We will be capturing the whole thing on the most powerful camera to fly in deep space," A'Hearn said.
Nobody knows how hard the comet's outer layers are, or the size of the resulting impact crater. It could produce anything from a small dent to a cavern. Tempel-1 is hurtling through space at about 10km per second. Two hours before impact, the impactor will be instructed to steer itself into the comet's path using automatic navigation.
The project manager, Rick Grammier, said: "The autonav is like we have a little astronaut on board. It has to navigate and fire thrusters three times to steer the wine cask-sized impactor into the mountain-sized comet nucleus closing at [37,000kph]. We are threading the needle with this one."
The mission is named after the 1998 Hollywood film when astronauts try to stop a massive comet on a collision course with Earth. But Nasa insists its act of celestial vandalism will do nothing to deflect Tempel-1 from its five-year orbit around the sun.
"This is the equivalent of a 767 airliner running into a mosquito," said Don Yeomans, a scientist on the project.
Another craft went through the coma (head) of another comet, Wild-2, in January, and will bring samples back to Earth in 2006.