When you think of sand, you might think of something brown, or white, if you're lucky. Maybe black if you're on a beach on Hawaii's Big Island. But the reality is sand is a melange of colors as varied as the rainbow.
Frank Drake developed an equation in 1961 that was designed to give us a formula to estimate the number of intelligent and communicative civilizations in the Milky Way. Now 55 years later, the equation remains popular — and Drake is still looking to prove he was correct.