Summary

Astronomers now generally agree that asteroids are probably the cause of smaller impact craters on earth consisting of space junk "flung" out of the Oort cloud, while comets are truly visitors from deep space. Comets are probably responsible for large impact craters on earth, while the smaller craters most likely were formed by asteroids, Randall says. Comets also carry a disproportional amount of energy because of their speed, which is twice the speed of asteroids according to astronomer Gene Shoemaker. Randall says the Oort cloud ("a somewhat spherical collection of minor bodies") may extend to 50,000 times the distance between the sun and the earth and probably is a major source of comets like the one that smashed into the earth 66 million years ago.

In that region, the power of gravity between the sun and these proto-comets is...