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.nebola.tv slash Isaac Arthur and use my code, Isaac Arthur.

The final frontier beckons us, but for now we must proceed one small step at a time.

After we return to the moon, we set our site on Mars, but where we go after that?

We often talk about Mars as the next great Frontier, our first true home beyond Earth,

it's the poster child for space colonization, a world that's both familiar and alien,

with rust-red soil, polar ice caps, and sunsets that turn blue.

But why Mars? Why does it so often come second in line after the

moon, or sometimes even first? Some of it is cultural. We've been dreaming of Mars for over a century,

from H.G. Wells and Ray Bradbury to Rovers and Modern Efforts from SpaceX and the Mars Society.

It captured our collective imagination. But beyond the romance, Mars does offer practical advantages. It has seasons, a day

only slightly longer than Earth's at 24.6 hours, 38% of Earth's gravity, which is strong

enough to stand in, but low enough to escape from,

and a surface that could be walked on. It's also got water, at least a little bit of nitrogen,

and enough sunlight to run solar power or focus with mirrors. Still, if we're serious about

becoming a space-faring civilization, we have to ask the bigger question, not just why Mars, but

what comes after it, because Mars is not the finish line, it's just the beginning.

In this episode, we'll explore the many destinations that might follow, some nearby, some

far-flung, and how they each serve different roles in the grand expansion of humanity.

We'll look at which ones help support Mars itself, and which ones point the way toward an

...