The spacecraft is called Deep Impact just like the 1998 movie about a comet headed straight for Earth. NASA's goal is to collide a part of the spacecraft called the impactor with Comet Tempel 1. Scientists expect the collision to blast a crater into the comet to analyze the ice, dust and other primordial stuff hurled out of the pit. […]

NASA guarantees that no matter how powerful the punch or how big the crater, Deep Impact will barely alter the comet's orbital path around the sun and will not — repeat, not — put the comet or any part of it on a collision course with Earth.

CNN