Hello, SFIA audio listeners. In this month's Nebula exclusive, big alien theory,

we're asked at the reason alien civilizations might be rare is because most aliens are huge.

To hear it and every episode early and ad-free, plus hours of bonus content,

check out go.nebula.tTV slash Isaac Arthur and use my code, Isaac Arthur.

The galaxy is an immense and mysterious place we may one day explore and settle, but for

now we have only one planet, and the galaxy holds many dangers, so what can we do to defend

Earth?

One of the more fascinating things about the moon is all the craters on it, some of which

are the size of continents.

But when pondering those, it does raise the worrisome issue that Earth has probably been

hit just as hard and often, and probably by rocks even bigger than the one we tend to

say got the dinosaurs.

Indeed, the moon's

very existence is thought to be due to some small planet hitting proto-Earth and spearing

out a debris cloud from which the moon formed. Needless to say, that is not a survivable

strike. And indeed, if we detected anything like that coming our way right now, or an alien

armada, or even a number of other

potential astronomical catastrophes, our best strategy would be to drop to our knees and pray.

Should divine intercession not be forthcoming, it leaves you in an excellent position to kiss

your butt goodbye. There's essentially no defense strategy available to us if we haven't got

at least months

to prepare right now, and against an alien armada or a full-scale planet, careening our

...