Their mission is to make sure there will be advance warning of asteroid or cometary impact in time for defensive action, by helping to obtain support for astronomers in their discovery of hazardous Near–Earth Objects.

Interactive tool for estimating the environmental consequences of an impact on Earth by a large object. It can estimate the ejecta distribution, ground shaking, atmospheric blast wave, and thermal effects of an impact, plus the size of the crater produced.

Lists the predicted minor-planet and comet encounters to within 0.2 AU of the earth between 1900 and 2178.

Overview of the reality of this type of threat, and proposals for protecting us.

From NASA: 140+ Near Earth Objects that could potentially collide with Earth

From NASA.