Asteroid/Comet Impact Event

A asteroids or comets impact both pose a risk to Earth though by immediate damage to the surrounding area of the impact and the long term effects of dust in the atmosphere.  The dust in the atmosphere if significant could block out the sunlight sending Earth into a mini or long term ice age.  Currently The Near-Earth Object Program tracks thousands of asteroids and comets and know of any large size is known to be on a path to hit Earth directly, however a collision inside the asteroid belt could send an asteroid speeding to Earth at anytime.   In addition the orbit of comets can be altered as it travels close to the sun releasing gases as the ice evaporates.