Mitigation of asteroid impact threats

Although large asteroids and comets have impacted the Earth in the past, we do not want large ones to impact us in the future. Can we prevent this from happening? A considerable amount of work continues to be done in detecting asteroids that may pass near the Earth, but solving the engineering problem of how to actually conduct a mission to deflect an asteroid is still in its infancy. Of the many ways considered to deflect an asteroid, the methods currently in favor fall into three categories: (1) kinetic impactor, (2) nuclear standoff explosion, and (3) gravitational tractor. Other techniques such as landing machinery on the asteroid and shooting off material (mass driver), or parking a giant minor near the asteroid and focusing the sunlight on it to ablate material, are not strongly in favor at this time.