Asteroid deflection. Artwork of astronauts installing nuclear bombs on an asteroid. Only a small portion of the asteroid is vaporised. This is thought to alter the trajectory of the asteroid. Asteroids are minor planets with diameters of less than 100 kilometres. A collision with Earth would cause massive tsunamis if it hit the oceans and block sunlight due to the huge quantities of dust thrown in to the stratosphere. It is thought that the extinction of the dinosaurs around 65 million years ago was caused by an asteroid impact. An asteroid, comet or other large body posing a danger to Earth is known as a Near- Earth Object (NEO).