One involves a space tug, which would use the mutual gravitational attraction between it and an asteroid to slowly change the NEO’s velocity so that it reaches the predicted collision point too early or too late. But it’s a decades-long process that would be effective only for asteroids no larger than about 300 feet across, scientists say.
Another approach involves the use of impactors, launched from a nearby spacecraft to slow or speed the NEO’s pace. NASA and the European Space Agency are studying the possibility of a demonstration mission dubbed AIDA (Asteroid Impact & Deflection Assessment). This would involve sending a spacecraft with an impactor to a binary asteroid, Didymos. The impactor would strike the smaller of the two asteroids, and the spacecraft would then measure the object’s velocity.
As a last resort, researchers have considered detonating a nuclear device to deflect objects up to a couple of miles across. For anything larger, no feasible defense is currently available, researchers say.