... Astronomy 101 . ... NASA scientists find history of asteroid impacts in Earth rocks . ... Research by NASA and international scientists concludes that giant asteroids, similar or larger than the one believed to have killed the dinosaurs, hit Earth billions of years ago with more frequency than previously thought. ... By studying ancient rocks in Australia and using computer models, researchers estimate that approximately 70 asteroids the same size or larger impacted Earth 1.8 to 3.8 billion years...