No matter how “tucked away in some forgotten corner” of the universe we are, to use the astronomer Carl Sagan’s phrase, the universe still finds us. That was evident to people near the Russian city of Chelyabinsk Friday morning, when a meteor exploded across the sky with the power of an atomic bomb. Even as astronomers were tracking an asteroid that was cruising to within 17,000 miles of Earth, a 10-ton meteor — a completely different object — hit Earth’s atmosphere with no warning at 33,000 miles per hour. The shock wave of its explosion shattered enough windows to injure perhaps 1,000 people in Chelyabinsk.
It was an extreme reminder that when we look up at a sky that seems mostly empty, cosmic dust is falling all around us, to the tune of tens of thousands of tons a year. We may see a few pea-sized rocks burn up in annual meteor showers such as the Perseids, but most of what falls to earth is in the form of invisible grains.