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 add free, plus hours of bonus content, check out

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

Cohen theory says the dinosaurs came to an end when a giant asteroid from space crashed into Earth.

Wouldn't it be ironic then if we sent asteroids out from Earth into deep space to colonize other worlds?

Oh, me, oh, my God. A few weeks back, we were discussing interstellar colonization strategies, and next week

we'll be discussing planetary terraforming strategies for when a colony ship arrives in a new world.

One of the strategies we didn't cover last week was

the possibility of using asteroids as spaceships to travel to new systems.

Now, normally I don't think of this is a good option, for many reasons we're discussed

today, but the big one has usually been there simply a fuel hog compared to far less

massive ships made from hollowing out the asteroid and refining

its oars to make a ship.

When I was riding interstell colonization strategies though, I talked about how big or

was better when it came to colony ships, as they have more cargo, redundancy, and endurance.

Those are fuel hogs too, but you wouldn't really care because the fuel in question, hydrogen

or perhaps deuterium or helium-3, is so incredibly abundant.

So the idea of launching some asteroid massing a trillion tons into space, using trillions

of tons of fusion fuel, seems absurd at first, but the economics say otherwise.

As it turns out, the fuel part is the easy part.

Even by modern standards, the cost to pump liquid hydrogen in some tank is going to be way

...