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. This episode is brought to you by World Anvil.

It takes a lot of energy, precision, and effort to hang a satellite in orbit above our atmosphere,

but it takes even more to hang one inside our atmosphere.

So today we'll be looking at placing satellites and even non-moving objects in ultra-low

orbits, or even beneath the carbon line, the edge of outer space in the main atmosphere,

and to do that, we need to start by explaining why we don't now and why we want to.

Many folks mistakenly believe there is no gravity on the space station because it's way out in space,

an enormous distance from Earth.

In reality, the ISS and most of our satellites are as close to Earth as we can possibly put them

before atmospheric drag slows them down too much,

for them to remain in orbit.

Gravity is an inverse square force, meaning it's proportional to the square of the distance

from the center of the planet, and the ISS is only 254 miles or so up, just 6% farther

from the Earth's center than you are, so the gravity there is only 13%

weaker than what's holding you down right now.

So if the ISS is at .87G, why does it fall?

Well, it is falling, but as Isaac Newton famously illustrated, if you travel fast enough

over the surface of a planet, you'll fall toward it at the

same rate it curves away from you, so you'll keep falling around it indefinitely, a situation

...