Preventing an impact from an asteroid or comet
Once a NEO has been identified and it has been calculated that it is probably or definitely going to impact the Earth, there are possibilities to prevent the impact.
The prevention of the impact is in all cases achieved by deflecting the NEO from its orbit so that it passes the Earth in front or behind instead of impacting.
In their 2010 report Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies, the US National Research Council (NRC) identified four broad mitigation options and classified them by NEO impactor diameter and the warning time available before impact.
(Image by Tim Warchocki, Copyright © National Academy of Sciences from NRC report available at http://www.nap.edu/catalog/12842.html)
The borderlines between options are deliberately blurred, as there is a lot more that goes into making the final decision, however the general regions are interesting in the above graph. The principle finding, however, is that for small objects civil defence (evacuating specific areas) will be sufficient, and for large objects or a short warning time, a blast deflection of some kind will be required. In between, a kinetic impactor that imparts momentum onto the NEO could be employed, or a gravity tractor that pulls the NEO over a long period could be used if there is enough warning time.For details of the various mitigation methods, see the sections:
Another breakdown of the various mitigation methods was presented by the NASA HQ 2006 Near-Earth Object Survey and Deflection Study, and is shown here.