Threat from the Skies: Will a Comet Strike?

We can afford to be blasé about most of the cosmic catastrophes we have been discussing. Many of them, like the impact that created the Moon, happened long ago, when the solar system was young; similar calamities cannot happen again now that the planets have evolved. Other kinds of catastrophes affect worlds other than our own; we have nothing to fear from sulfurous, Ionian-style volcanism. Even the death of the Sun, which will certainly consume our planet in the distant future, is not a hazard to modern-day civilization. If scientists understand anything at all about stellar evolution, the Sun can no more become a red giant in the near future than a four-year-old girl can enter menopause: Both require maturation and aging. And the Sun, with its inadequate mass, is no more capable of becoming a supernova than a man is capable of becoming pregnant. Climatic changes are of great moment, if not for us then for our grandchildren.