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.

Comets have odd, amazed, and even terrified humanity

for thousands of years, with many believing they hailed at the end of the world, but could they

be the key to reaching and creating new worlds? Comets have a long and mixed history with humanity, often frightening, often seen as omens

of change, but always seen as astonishing. Even in the 20th century, there were fears

even among scientists that passing through a comet tale might kill us, and many a scientific

theory then and now contemplate them as bringers of death,

or even life, to this world.

They are our most popular candidate for how we might have gotten oceans back on Earth after

the apocalyptic encounter of early Earth, with a smaller planet that we believe tore off

the surface of the Proto Earth and from which we think the moon later formed.

That would have taken

a lot of comets, around 10 million the size of Haley's comet, the best known one, and there

was major doubts if this was or could have been the method that got us our oceans, but

the impact of such bodies would not have been trivial affairs.

We estimate the asteroid that possibly kill the dinosaurs

as only five times more massive, and Haley's comet is not the largest one out there.

Today we will be looking at comets from three perspectives. First, tapping existing comets that

periodically and potentially conveniently come in close to Earth and the inner solar system.

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