Today is Asteroid Day, and it's a reminder that as our planet follows its path around the sun, it encounters a lot of stuff. Science tells us that every day, Earth's atmosphere is hit with roughly 100 tons of dust and particles the size of a grain of sand. And every year, at least 30 small asteroids enter the atmosphere, only to burn up before touching the ground. NASA says it's pretty much guaranteed that at least one of them will be about the size of a car. As time progresses, the likelihood increases that even larger celestial rocks will hit the ground and cause significant damage.