What if humanity never had to send a living thing between the stars?

A spacecraft carrying humans would need to survive an extraordinary journey through interstellar space. But what if we removed the most fragile part of the equation entirely — the traveller?

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In this second episode of the series, we explore one of the strangest possibilities in interstellar exploration: self-replicating probes. Machines that could travel to another star, land on an asteroid or moon, extract raw materials, build copies of themselves, and send those copies onward into the darkness.

The idea begins with a deceptively simple question: if a machine can build another machine, how far could that process eventually go?

We follow the concept from John von Neumann's original thinking about self-reproducing machines to the engineering challenges of building an autonomous probe capable of mining, manufacturing and repairing itself far from Earth. Then we follow the mathematics outward, where a single machine becomes two, two become four, and an apparently slow process begins to spread across the galaxy.

But exponential growth brings an uncomfortable question with it.

If we ever built a machine capable of reproducing itself among the stars, could we be certain we could control what happened next?

And if such machines are possible, and a civilisation had millions of years to develop them, what might the galaxy look like today?

Perhaps the strangest part of the idea isn't how quickly these machines could travel.

It's how patiently they could wait.

So settle in, turn down the lights, and come with me into the darkness between the stars.

Can machines build themselves?