1. Crash into Earth
  2. Send a rain of hell fire (and comets!) into Earth
  3. Disrupt Earth
  4. Impact life on Earth in any way whatsoever
  5. Except that it might inspire interest in science, deepen our understanding of the solar system, and teach us new and interesting things. I guess that's an impact (yuck yuck yuck).
  6. Seriously internet relax.