If a large asteroid or comet hit the Earth it would have catastrophic effects. The power of such a large impact would cause damage across a huge area. It would reach far wider than the crater left behind. But remember, large asteroids or comets do not pass close to our planet often. Scientists think they hit the Earth a few times each million years on average. 

So if a large asteroid or comet did collide with the Earth, what would happen?

The first danger would be from the huge shock wave made by the impact. The shock wave would move out from the impact site at very high speeds, carrying a lot of heat with it. It could lead to scorching hurricane force winds, strong earthquakes and widespread fires.

The impact would throw millions of tonnes of dust into the air. Or, if the object lands in the sea, tonnes of water vapour. Scientists have used computer models to simulate large impacts and study how much dust or water vapour they create. The results show a large asteroid would produce 1000 times its volume in dust or vapour. The models show the impact sends dust or vapour straight into the upper atmosphere via a process called the "chimney effect".

We would see a change in the weather soon after the shock wave passed. All the dust and water vapour in the atmosphere would block out the Sun's light. Global temperatures would fall due to the lack of heat from the Sun. This dust could remain in the air for many months, if not years. In the worst case, there would not be enough light for plants to carry out photosynthesis. Photosynthesis uses sunlight, water, and carbon dioxide to create oxygen. So the impact could change the chemical composition of the air. This is vital for life on Earth so there could be mass extinctions. Scientists think the extinction of the dinosaurs 65 million years ago was because of a large asteroid impact.