Comets visible to the naked eye are a rare delicacy in the celestial smorgasbord of objects in the nighttime sky. Scientists estimate that the opportunity to see one of these icy dirtballs advertising their cosmic presence so brilliantly they can be seen without the aid of a telescope or binoculars happens only once every five to 10 years. That said, there may be two naked-eye comets available for your viewing pleasure this year.