Question: Hypothetical question: We hear about what would happen when the Earth is struck by comets and asteroids; the extinction of the dinosaurs for example or the end of life on our planet for us humans. In its infancy the Earth was struck by another protoplanet and survived the impact that formed our Moon. How large would the object have to be to cause the Earth to disintegrate having been struck by it? – David
Answer: First, lets set the scale of impact events. The object that collided with the Earth about 65 million years ago which resulted in the extinction of the dinosaurs was about 10 km is diameter. Clearly an impact of this magnitude would lead to mass extinction of most species on Earth. Also, as you pointed out, the most likely theory which explains the formation of our Moon involves a collision with a Mars-sized object early in the formation of the solar system. Mars is about 3500 km in diameter. Compare this to the diameter of the Earth, which is about 12,700 km. So, an object which is anywhere from 5,000 to 10,000 km in diameter contains enough energy to seriously damage, and possibly even destroy, the Earth.