Climatologists believe they know why Earth has undergone a regular sequence of ice ages beginning around 800, 000 years ago  Calculations show that Earth' s orbit around the Sun has fluctuations that coincide with the ice-age cycles  The climatologists hypothesize that when the fluctuations occur, Earth passes through clouds of cosmic dust that enters the atmosphere; the cosmic dust thereby dims the Sun, resulting in an ice age
They concede, however, that though cosmic dust clouds are common, the clouds would have to be particularly dense in order to have this effect
Two large asteroids collided 800, 000 years ago, producing a tremendous amount of dense cosmic dust that continues to orbit the Sun.