Cape Canaveral is ready to launch NASA's Deep Impact mission today at 1847 GMT. It is due to meet up with the Comet Tempel 1 6 months later. Once at its destination, a 372kg projectile will be released which should punch a crater in to the comet.
The projectile will take 24 hours, travelling at 37,000km/h, to reach its target. The collision is planned for July 4th, and the crater left behind should reveal what is in the centre of the comet. The hole will be big enough to hide Rome's Coliseum.
Dr Mike A'Hearn, principal investigator on the mission, said: "We know so little about the structure of cometary nuclei that we need exceptional equipment to ensure that we capture the event, whatever the details of the impact turn out to be."
