If you think of asteroids, meteors and comets as potentially destructive and life-threatening forces, you're correct. After all, the arrivals of these celestial bodies are typically associated with catastrophic impacts on earth, profound environmental changes, impact winters, plagues and famines. But that's not all. Over the last century, a number of scientists have speculated that they might also bring life to the barren outreaches of the galaxy, such as our solar system. Not only that,...