More Asteroid Strikes in Our Future?

The Times today reported that we are likely to encounter more astroid strikes than we thought.  "That's interesting," I thought, "But how do we know?"  And "What changed?"

Turns out that they are using a new measurement that begins with empirical observations of collisions instead of predicting the number of likely collisions based on estimates of how many asteroids there are in orbit around the Sun and what their orbits are.

Because telescope surveys have counted so few of the small asteroids, Dr. Brown and his colleagues instead investigated what has actually hit the Earth. In one of the articles in Nature, they examined United States Air Force data from the 1960s and 1970s and later data from sensors verifying a ban on aboveground nuclear weapons testing. 

The recordings captured the low-frequency atmospheric rumblings generated by about 60 asteroid explosions. Most came from small asteroids, but their data suggested that the somewhat larger ones hit more frequently than would be expected based on the estimates from sky surveys. That could mean the Earth has been unlucky recently, or that the estimates on the number of Chelyabinsk-size asteroids are too low.