Cosmic events are astronomical events which take place in the vast outer space, but of great significance to the earth also. The conjunctions of planets, meteoric showers, etc., are examples of cosmic events. Cosmic also means great, vast, or very important.
A disaster of cosmic dimensions means a disaster on a very large-scale causing great suffering to a lot of people. In the course of their orbits, certain stars and planets come very close to the Solar System or the Earth, and they are cosmic events, which are of great significance to the earth.