Looking further back in time, we see evidence for even larger, rarer impacts that have profoundly affected life on Earth. The mass extinction 66 million years ago that ended the era of the dinosaurs (the end-Cretaceous or K-Pg extinction) has been convincingly linked to a giant asteroid impact, which created the Chicxulub crater now under the Gulf of Mexico. Evidence of higher impact rates than at present come from earlier in the Earth’s history. A period of enhanced impact activity known as the Late Heavy Bombardment occurred about 600 million years after the Solar System’s formation and left its mark on multiple bodies in the Solar System, including our Moon. In fact, the Moon itself probably formed when a Mars-sized body hit the proto-Earth, blasting huge amounts of material into space, some of which later coalesced to form the Moon.

Our Solar System seems to have begun as a rotating cloud of gas and dust. Collisions between clumps of material formed larger and larger bodies, some of which became large enough to form planets. A central concentration of gas collapsed and ignited hydrogen fusion to form the Sun, our star. Much of the matter in our Solar System is now “locked up” in the form of the Sun and planets, but a substantial amount exists as asteroids (monolithic chunks or loose aggregates of rock), comets (city-sized “dirty snowballs”), smaller rocks, and particles of dust. Sometimes these cosmic leftovers collide with each other or with planets — for example when Comet Shoemaker-Levy 9 collided with Jupiter in 1994, leaving giant bruises on the planet big enough to be seen in detail by telescopes from Earth.