When I started writing about astronomy 35 years ago, astronomers knew of just a few thousand asteroids. Now they know and have cataloged half a million asteroids! That's one reason we hear so often nowadays about asteroids passing near Earth, like the one that will pass on Friday, February 15, 2013. This video is an awesome depiction of the increasingly fast discovery rate for asteroids, over the past three decades. Scott Manley, formerly of Armagh Observatory, created it.

Or, try another graphical representation, also on Armagh Observatory's website. These are are maps of the inner solar system for the years 1800, 1850, 1900, 1950, 1990, 2000 and 2007 showing the increasing rate of discovery of asteroids.

Bottom line: Video from astronomer Scott Manley shows the increasingly fast rate of discovery of asteroids in our solar system, and hence of asteroids whose orbits cross that of Earth.