One need only look up at the Moon on a clear night to see that its surface is pockmarked from a long history of collisions both large and small. Asteroids, comets, meteoroids, and smaller debris fly around our Solar System on orbits that occasionally intersect with each other or with larger bodies. When a large rock smashes into the Moon, the energy of the impact melts both it and the lunar surface, spraying debris into space that then rains back down on the Moon’s surface, leaving a crater surrounded by a blanket of ejecta. Younger craters layer on top of older craters, telling the story of eons of bombardment. But it’s not only the Moon that experiences such impacts. Other moons and planets, including the Earth, do too.
On Earth, unlike on the airless, lifeless Moon, the evidence of past impacts is blurred by the actions of erosion from air and water, by continental drift, and by the presence of life. Furthermore, our atmosphere causes smaller objects to burn up before they even hit the ground. Nevertheless, many meteorites have been observed to fall from the sky and are later recovered. Their chemistry is subtly different from that of terrestrial material and preserves the stories of their origins and journeys through space. Sometimes, we also directly observe the destructive power of these impacts. The Tunguska explosion, which flattened large areas of Siberian forest in 1908, as well as the more recent 2013 Chelyabinsk event that blew out windows in thousands of buildings and injured hundreds of people in Russia, are evidence that the collision of bodies in our Solar System is far from over.
View a YouTube video of a 2013 lunar impact.
Learn more about impact craters on Earth.
View a YouTube video of the Chelyabinsk Meteor Shockwave Compilation
Learn about Sutter's Mill meteorite
Are You Alone? (In The Universe)
Learn more about meteorite impacts on the Understanding Global Change site.