It is extremely disturbing to know that Earth is directly in the path of about 500,000 asteroids and comets. Unfortunately, NASA and other space agencies don't know the location of 498,000 of them! Yet, these killer comets are capable of causing explosions far worse than the world's largest atomic bombs. Chelyabinsk, a small city in Russia, experienced such as incident in 2013 that seriously injured around fifteen hundred people. NASA warns that an asteroid named Bennu may hit Earth within a few decades. If so, the impact would be powerful enough to destroy any one of the world's largest cities.