In 1980 the physicist Louis Alvarent and his geologist son, Walter proposed that the dinosaurs were wiped out due to the impact of an asteroid . Marking the boundary between the Cretaceous and Tertiary geological periods, they discovered they discovered a layer of clay containing iridium, at the same time as the end of the dinosaurs and the rise of mammals.Because iridium is rare in earth rocks but abundant in space rocks they believe the iridium must have originated from a cosmic impact. According to this theory about 65 million years ago a 10 km wide asteroid , travelling at 15 km/second (100 times faster than a bullet hitting earth ) forced billions of tons of dust into the atmosphere . Heat generated from the crash must have resulted in fires around the world.Debris and a cloud of smoke shut out the sunlight for months.Plants died as acid rain and wildfires set in .Plant-eating dinosaurs followed because of the lack of plant life and eventually the meat-eating dinosaurs were killed off. During this lightless period temperatures dropped to below freezing for 6 to 12 months.Scientists estimate that the earth is hit by an outer space rock 1-2km wide every a million years and a rock 90m- 1km every 30 years.The 1-2 km wide strikes carry the threat of global catastrophe-as the dinosaurs experienced firsthand .