Confirmed by science: our species is doomed.

[jeff47]

81. There is no star close enough to us to cause a deadly gamma ray burst.

None of the stars that are close enough for a gamma-ray burst to be dangerous are capable of producing a gamma ray burst.

As for an asteroid impact, that may or may not wipe out humanity depending on the kinetic energy involved. A massive enough asteroid would cause extinction.

You'd need an object large enough to re-liquefy the Earth's surface. Such as the impact that created the moon. An object that big is relatively easy to find, and there's no object that big close to Earth.

A full scale nuclear conflict also could kill us all off. The initial exchange would not. Subsequent radiation effects would kill off a large number of those left and weaken the rest. The nuclear winter created by the blasts would destroy vegetation on most of the planets surface for years. Few would survive that, and those who did would face long odds of creating a successful future generation.

Face long odds because........?

Radiation in such an event will not be distributed all over the Earth. Some parts will be "hot", others will not. People already live in the areas that are unlikely to be "hot".

Nuclear winter will definitely harm food production, but we don't need to feed billions for the species to survive. Getting down to thousands would still leave an easily sufficient "breeding population".