HOW TO STOP KILLER ASTEROIDS: Only vigilance and ingenuity stand between Earth and oblivion.

I had an idea for a science fiction story: Man travels back in time to prevent assassination of Franz Ferdinand, and successfully prevents World War I, and, by extension, World War II. The result is a century of peace and prosperity, but somewhat slower technological development, so that in 2038 we’re just seeing the earliest beginnings of a space age — as a killer asteroid, unstoppable, appears and wipes out civilization.