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.

A space elevator might be tricky to do on Earth, but they're much easier on the

moon and may be perfect for helping with in-situ resource extraction to fuel our expansion

into the wider solar system.

The concept of a space elevator is one that has fascinated folks since the late 20th century, and one we discussed on the show quite a few times, but also has some interesting variants

worth discussing too.

One example of that is a space tower, something we recently did an episode on, and those

are enormously tall structures that rise up to low orbit rather than hanging down from geostationary

orbit, relying on compressive strength rather than tensile strength.

We also have an abridged version of elevators such as the skyhook or rotovator that

a spaceship meets halfway up into the sky and uses to get the rest of the way into orbit.

But another example is an elevator that is not on Earth, but is on planets or moons with

very different gravities and day lengths, both of which really change the dynamics of an elevator

and what you need to construct it,

and indeed whether or not it would make sense to use one of those other approaches instead,

like a lunar skyhook or a space tower, or a mass driver or space cataport,

to simply launch ships down a very long track up to high speed and into space.

Of course that's our focus for today, the particular

case of a space elevator on the moon. This may turn out to be the first place we end up

...