Comets Incoming

Scientists have understood for a few decades that a shell of comets orbit the Sun far beyond the planets. That group is called the Oort Cloud, after Jan Oort, the developer of the concept.

Comets from that region occasionally blaze past Earth. Because the objects in the Oort Cloud are so far away, the Sun's gravity has only a tentative grip on them. Any little nudge can send one or several Sunward. A recent study reveals that even tiny shifts in the gravitational influence of the galaxy can alter a comet's orbit.

That type of comet is the wild card in the survival of life on Earth. Danger from near-Earth asteroids can be predicted decades in advance-- if the resources are put to that end. There would likely be time to build a capability to deflect such bodies away from Earth. But a comet comung into the inner Solar System for the first time would give us no time to plan. Only a mature spacefaring civilization could deal with such a threat. For now, our defense is that Earth is a tiny, fast moving target in the vastness of space.