It is remarkable how often the origin of things is tied to the very same phenomena that ultimately lead to their demise — a fact that is especially evident when we ponder cosmic endings, from the end of the Earth to the end of the universe. For instance, planetary scientists increasingly suspect that comets and ice-laden meteorites crashing into the primordial Earth probably provided most of the planet’s water. Organic molecules have been detected in comets, and researchers simulated those cosmic crash landings by using a gas gun to fire metal projectiles into blocks of ice containing some of the same chemicals that make up comets. The shock wave and heat generated by the impact created molecules that formed amino acids, the building blocks of proteins.

Yet the very same objects that gave this planet life could also spell its demise. Astronomers predict that a comet or asteroid large enough to cause global devastation will smash into the Earth about every 100 million years or so. Fortunately, if such a comet or asteroid were to arrive sooner than expected, we are constructing observational systems to discover and track near-Earth objects, conceivably providing us with sufficient time to attempt catastrophe. Other cosmic smash-ups, however, cannot be averted, no matter how much advance warning we have. The inexorable tug of gravity that enabled the formation of the Milky Way has also put us on a collision course with our neighboring galaxy, Andromeda. Recent observations confirm that Andromeda is heading straight toward us at about 60 miles per second, and will traverse the 2.5 million light-year distance currently separating our galaxies in about four billion years.