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.

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66 million years ago, the world was changed forever when an asteroid only the size of a

mountain struck Earth and wiped

out the dinosaurs. It could happen again any day, so what could we do to stop it?

The primary current theory for what ended the dinosaurs was a massive asteroid, and it wasn't

the first or last time one struck our world with earth-shattering consequences.

Indeed, current theory also says a truly massive asteroid or dwarf planet early in our planet's

history struck us and threw much of our crust and mantle into orbit, debris from which later

formed our a typically large moon.

When thinking of calamities, it is worth remembering that we ourselves probably

would not exist except for that moon-generating event, and we'd still be rodents if not for that

one that got the dinosaurs, so they can be beneficial for everyone but the folks on the receiving

end, though that might not be the case for more advanced civilizations who may consider

an incoming asteroid

quite a boon. Those two events, the dinosaur killer and the moonmaker, are hardly the only

major impacts we've had. The latter was so enormous there is no crater for it, it erased

...