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

we're asking 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.tv slash Isaac Arthur and use my code, Isaac Arthur.

Before we build cities among the stars, we'll build factories on the moon,

our first real step from earthbound to interplanetary.

Building tomorrow's industry under an alien sky.

It's one thing to dream about gleaming starships or bustling motion metropolises, but before

we ever get there, we have to master the far more humble art of building useful things

off Earth.

Otherwise, we're forever paying an invisible yet immense tax on every product we loft into the

void, tribute to the tyrannical rocket equation, which demands staggering amounts of fuel just to send

up tiny payloads, and even more to land them on another world. On top of that, anything we build

on Earth has we sturdy not to withstand our gravity, making it bulkier and heavier, and tough enough

to survive the punishing gauntlet of atmospheric exit at blistering speeds need

to reach orbit. There's only so large or delicate we can make such things.

So every kilogram of material we can mine, forge, or assemble in a place with low gravity and

no atmosphere saves us dearly, and each lunar weld or extrusion brings us one bold step closer to truly opening the final frontier.

A satellite I build on the moon from structural materials made there

does not need to be able to survive Earth's gravity or a high G launch through Earth's atmosphere,

just the gentler conditions on the moon, and later in space,

making it cheaper to build and even cheaper

...