I am often asked this question by members of the general public, and, since I am known to most people who meet me as an enthusiast for space exploration and colonization as well, the question is put to me even more forcefully. Let us face this question head on.
             Firstly, there are several lessons to be learned from astronomy and space science for our life on Earth. In science, learning from extreme cases usually gives one insights into the ordinary situation; in medicine, we often learn about diseases and treatments from studying other species. So, in the case of Earth's climate and history, we can learn from other planets.
             The danger of all-out nuclear war, mercifully now receding after the end of the Cold War, was unique in that, alone of all the perils we face, it endangered not just civilization, but even our biological existence. Other threats, like Ebola virus plagues, global warming, or even a new Ice-Age, may destroy civilization, perhaps irreversibly, but would not wipe out the whole race. The reason given in the mid 1980's was the phenomenon of nuclear winter. In this picture, the exchange of more than a certain number of bombs (thought to be 100-200 by many authorities), would cause so much dust and smoke to enter the stratosphere that 99% of sunlight would be blocked from the Earth's surface for 6-12 months. Average temperatures would fall 20-25° Celsius, and all agriculture would cease. It is apparent that most creatures larger than an insect would perish, and that humanity would die of cold, hunger, disease, insanity and mass suicide. Not surprisingly, even the superpowers flinched from this prospect of desolation, and Messrs. Gorbachev and Reagan began to talk their way out of it. The result is that, although the world still has many problems, nuclear annihilation is much less likely.
             Where did this idea of Nuclear Winter come from The answer, surprisingly enough, was the planet Mars. In 1971, the American space pr...