That is a great question, in Science; comets are often referred to as NEO’s, or near Earth objects. Throughout the evolution of the Earth, these objects actually have been known to hit Earth. These impacts have been the cause of many mass extinctions in Earth’s history, meaning that many species of plants and animals have been wiped out. If you take a look at the moon for example, you can see examples of what these impacts look like, the craters of the moon are formed from asteroid and comet impacts.

On Earth, the scars from these impacts have long been wiped away through geological processes such as weathering of the Earth’s surface, which has erased much evidence of these collisions. Currently, there are actually many asteroids and comets that pass close by the Earth and a group of scientists at NASA have been tasked with tracking these objects. Presently there is no threat of these large objects hitting Earth. They often hit the sun or Jupiter, for example, instead of Earth because they both have much larger masses and greater gravitational pulls that attract large objects, like comets and asteroids. So, comets have the potential to hit Earth, but it is very rare and many scientists believe that another comet impact on Earth will not occur for many thousands of years.

Collisions of asteroids and especially comets are very, very rare. This is because the trajectories need to be finely tuned. Impact is in general a rare event. Hit would be like throwing a basketball towards a basketball hoop from 30 yards away. Even a professional basketball player cannot do that too often...even if she tries 1000 times!

So, although impacts are rare they DO OCCUR because in geologic time of 4,500,000,000 years, even an event that is RARE will occur. The last big impact was 65,000,000 years ago when an asteroid about 6 miles in diameter collided with Earth at what is now the Yucatan peninsula.

They do, and it's devastating when it happens - even a small comet would cause an explosion like that of a very large nuclear bomb, and big comets are much, much worse. Fortunately, it doesn't happen very often.