NASA can defend Earth
In a blog post published by NASA, APL writer Ajai Raj said that the “kinetic impact” technique, which he describes as “knocking one thing against another”, is indeed an effective method for planetary defense. NASA’s DART mission did just that; The spacecraft collided with the asteroid and its orbit was changed. The asteroid known as Dimorphos posed no threat to Earth, but the purpose of the experiment was to test feasibility.
It has been confirmed that relatively small (170 meters in diameter) asteroids like Dimorphos can be successfully deflected by this method and no prior reconnaissance missions are needed. However, scientists also underline that they should learn about such a threat to the Earth at least a few years beforehand. Additionally, according to the APL, there are no asteroid threats to Earth for the next century, at least as far as is known.