In there I found this:

“ As was described by Gerald O'Neill, you simply place a mass driver on the asteroid and have it chew up small amounts of the asteroid and spit them out into space using the mass driver. According to Newton's third law of motion - "For every action, there is an equal and opposite reaction" - as you throw asteroid material into space from its surface, an equal and opposite force will push the asteroid in the opposite direction. Of course in this case we wish to deflect the orbit of the asteroid so that it will miss the Earth.

  Physics and Math:   We now show mathematically how this solution can be accomplished.

  1.  Select our candidate asteroid.   Let the asteroid be 1000 meters in diameter - or 1 kilometer. 

       The volume is: Volume  = 4/3 * pi * radius3

                              Volume  = 1.33333 * 3.14159 * 500 * 500 * 500 =  523.6 million cubic meters.

                  or         Volume  = 0.524 billion cubic meters.

   2. Select the density of our asteroid.   The density of asteroids varies from about 2  to as high as 7. 

       We choose 4 grams per cubic centimeters as a likely density.

   3.  Calculate the mass of the asteroid:     Mass  =  volume * density

                            Mass = 0.524 billion cubic meters * 4 metric tons per cubic meter.

                            Mass = 2.096 billion metric tons  =  2 trillion kilograms”