Asteroids were formed when the space between Jupiter and Mars did not allow the formation of any planetary bodies. The objects in the space broke and fragmented giving rise to asteroids which can have a diameter of over 500 miles or as small as 20 feet. Asteroids travel around the sun in an elliptical orbit and due to their irregular shape can bounce off course, tumble and change orbit. Changing orbit sometimes means that an asteroid can encounter other planets. One of those is our home planet.
Historically we have little to go on when it comes to asteroid near misses in history. The Comet of 1491 came dangerously close, possibly less than four times the distance to the moon. In 1972, a comet bounced off the surface of the atmosphere streaking through the sky as a fireball over Utah. Only 14 meters in diameter, this near miss could have delivered an impact about half the strength of the atomic bomb that hit Hiroshima. Comet Hyakutake passed very close in 1996 and was discovered only two months before it started its earth approach.