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.nebola.tv 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 four-scale planet

careening our way, that wouldn't be enough time either, outside of a Hollywood movie.

Indeed, there's not much you could do without centuries' advancement

and forewarning in those two cases, which you might actually have, and we'll get to them

later, but at the moment, even something a kilometer crossed would be something we need a large

...