Halley makes its appearance every 76 years. Visitors from the Abyss The sudden appearance of these mysterious objects in ancient times was often viewed as a bad omen and a warning of disaster to come. We know know that most of the comets are found in a dense layer at the very edge of our solar system. Astronomers call this the oort cloud. They believe that gravity from the occasional passing of stars or other objects can knock some of the comets out of the oort cloud and send them on a journey to the inner solar system.
Comets can, and have, struck the Earth.
In June of 1908, something exploded high in the atmosphere over Tunguska in Siberia. The explosion had the force of 1,000 Hiroshima bombs and leveled trees for hundreds of miles. The lack of any traces of meteorite fragments have led some scientists to believe that this may have been a small comet that exploded on impact with the atmosphere. A comet impact may also have been responsible for the extinction of the dinosaurs, and many astronomers now believe that ancient comet impacts may have brought most of the water to our planet. Although the possibility exists that the Earth could be hit by a large comet again in the future, the odds of this event happening in our lifetime is more than a million to one.