http://news.yahoo.com/nasas-actual-...ng-asteroid-234624320--abc-news-politics.html
Links to NASA report and charts on the page.
From the 2007 NASA report to Congress:
In the impulsive category, the use of a nuclear device was found to be the most effective means to deflect a PHO. Because of the large amount of energy delivered, nuclear devices would require the least amount of detailed information about the threatening object, reducing the need for detailed characterization. While detonation of a nuclear device on or below the surface of a threatening object was found to be 10-100 times more efficient than detonating a nuclear device above the surface, the standoff detonation would be less likely to fragment the target. A nuclear standoff mission could be designed knowing only the orbit and approximate mass of the threat, and missions could be carried out incrementally to reach the required amount of deflection. Additional information about the object's mass and physical properties would perhaps increase the effectiveness, but likely would not be required to accomplish the goal. It should be noted that because of restrictions found in Article IV of the "Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space," including the Moon and Other Celestial Bodies, use of a nuclear device would likely require prior international coordination. The study team also examined conventional explosives, but found they were ineffective against most threats.
Links to NASA report and charts on the page.