Lindley Johnson, NASA's Planetary Defense Officer and program executive of the Planetary Defense Coordination Office.

Lindley Johnson, NASA’s Planetary Defense Officer and program executive of the Planetary Defense Coordination Office. (Image credit: Lonnie Anderson/Sandia National Laboratories)

It’s a cosmic roll of the dice. There’s no doubt that a major asteroid or comet strike could cause extensive devastation and profoundly affect life on Earth. 

The largest hit in recent times was the object that exploded over Tunguska, Siberia, in June 1908 with an energy impact of five to 15 megatons. Then there was that spectacular and destructive airburst in February 2013 over the Russian city of Chelyabinsk. The Chelyabinsk explosion generated a shock wave that shattered windows on the ground, and the resulting flying glass shards injured more than 1,000 people.