In memory of Carl Sagan

My work is inspired by an idea left to us by a hero of mine, Carl Sagan, who said:
Since, in the long run, every planetary civilization will be endangered by impacts from space, every surviving civilization is obliged to become spacefaring--not because of exploratory or romantic zeal, but for the most practical reason imaginable: staying alive... we have a basic responsibility to our species to venture to other worlds."