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.

In science fiction, we often see immense starships attacking planets, crushing or besieging them,

but in our own future, we may deploy powerful orbital fortresses to defend our world.

Way back in the late 90s, when I was just starting college, one of my favorite sci-fi

TV shows, Babylon 5, was wrapping up its fourth season with the universe story arc about a

human civil war that had an attack on Earth.

CGI was just starting to let us have grand space battles between large fleets instead of a

handful of physical spaceship models,

and so Babylon 5 and Deep Space 9 and some others were featuring episodes with dozens or

even hundreds of ships flying around. In this climatic episode, the good guys managed to trick

the defending fleets out of position and temporarily out of play, and jump to Earth directly to begin the assault.

Classically in sci-fi, one or two ships are evenly matched against the planet, or even

have the upper hand, but in the episode we suddenly see a whole bunch of big orbital stations

flick on, rotate into play, and begin firing high-energy beams and volleys of missiles, and just start

beating the heck out of the fleet.

...