Responses to an Imminent Doomsday

We will consider the hypothetical scenario of a planet-killing asteroid or comet. What would be possible and likely responses to an expected extinction event on a known date? (At present, we do not possess the ability to detect such threats far in advance, nor can we accurately predict the exact date of occurrence.)

We will ponder reactions (desirable, undesirable, expected) by government, by society, and individually to such a discovery by the scientific community while incorporating variables such as length of lead time.

An expected apocalypse has been the subject of numerous Hollywood productions but they have focused on attempts to prevent the catastrophe; we will operate under the assumption that the end of our planet is a fait accompli.