I found Robert Freitas' "aerovore" scenario from
http://www.foresight.org/NanoRev/Ecophagy.html to be particularly
interesting. This depends on being able to make a replicator which is
built entirely from C, H, O and N atoms.
It is pretty questionable whether that is possible - most moderately
large nanotech designs to date require some percentage of larger atoms,
metals and the like. Also, the nanosynthesis processes described in
Nanosystems often do make use of additional elements.
But if it can be done, the aerovore can replicate from atmospheric gasses
alone: O2 and N2, of course, with carbon coming from CO2 and hydrogen
from H2O vapor. And in fact the replication rate is reasonably fast;
the paper estimates based on both energy and gas-flow considerations
that the aerovore can replicate in under an hour.
These things would therefore be a solar-powered dust which could float in
the air and make more of themselves. Incredibly, starting with a single
aerovore with Robert Freitas' design parameters (275 nm diameter, 2750
second reproduction time), the atmosphere is so filled with these guys
after 1.85 days that sunlight is dimmed by 1%; and after 2.0 days 100%
of the sunlight is blocked from reaching the surface.
This is an amazingly virulent threat. The paper's solution is to send
up a bunch of good nanites which will build a giant net around the
entire earth, then pull it inward and sieve every cubic micron of the
atmosphere in order to remove these beasties. You've also got to check
out the lungs of all the animals, where some trillions of aerovores will
reside, and get all those out too. Presumably every square micron of
the earth's surface must be checked as well.
Then of course if they miss any they have to start all over again.
All I can say is, we better hope that it's not possible to build CHON
replicators. This atmospheric net idea doesn't look very promising to me.
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