Von Neumann probes
A von Neumann probe is a spacecraft that builds copies of itself. Even well below light speed, a fleet could reach every star in the Galaxy in a sliver of cosmic time. So why is none here?
Sending one spacecraft from Earth to each star in the Galaxy is a plan no one could finish. A probe that builds copies of itself changes the arithmetic: each copy can build more, so the fleet grows exponentially. In simulations, self-replicating fleets finish exploring the Galaxy up to 1,000 times sooner than probes that cannot copy themselves. That speed is the problem. A fleet could have reached us long ago, yet none has turned up. One proposed reason: copying errors pile up until every fleet breaks down.
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Rarely. Ellery argues that searching our own Solar System for signs of technology is one of the most neglected corners of SETI. He expects self-replicating probes to move through the Galaxy by following raw materials, favouring systems rich enough in metals. That makes the Moon a good place to look: he picks it as the ideal base for a probe's manufacturing.
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Its power plant might give it away. Ellery suggests a probe working on the Moon could build nuclear reactors from lunar raw materials alone, with the Magnox reactor as a model of what is feasible. The traces to look for are isotope ratios in the lunar rock: thorium-232 against neodymium-144, or thorium-232 against barium-137.
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