March 23rd is known as Near Miss Day. On March 23rd 1989, an asteroid that could have left a crater about the size of Washington D.C., passed by the earth at a distance of 500,000 miles. That may sound like a huge distance. But in the world of asteroids, that extremely close!
Although near misses are not constantly happening, they probably happen more than you are aware of. Most of these, however, would not really do any damage to the earth because the asteroids are small enough to burn up in our atmosphere when they pass into it.
The term “near miss” is an interesting one. It is defined as a “barely missed” catastrophe. The example that I found was that commercial airliners are considered dangerously close if they are only one mile apart. If they pass at a quarter mile apart or less, that is considered a near miss.
NASA’s Near Earth Object Program lists many of these near misses and has some great information. Check out their page here.