The inner solar system is a lot calmer than it was 4 billion years ago, during what’s known as the heavy bombardment period. Over the course of that violent stretch, which lasted about 500 million years, Mercury, Venus, Earth, Mars, and the moon were regularly pounded by asteroids, meteors, and other cosmic ordnance, many of the objects as big as the six-mile-wide rock that wiped out the dinosaurs. Things have gotten a lot quieter since then, but that’s not to say everything has gone entirely still. Earth still lives inside a shooting gallery, with thousands of objects—totaling about 48.5 tons per year, according to NASA—entering the atmosphere.
