[ExI] Von Neumann Probes

[spike at rainier66.com]

-----Original Message-----
From: spike at rainier66.com <spike at rainier66.com> 
Sent: Saturday, 24 January, 2026 3:10 PM
To: 'ExI chat list' <extropy-chat at lists.extropy.org>
Cc: 'Ben Zaiboc' <benzaiboc at proton.me>; spike at rainier66.com
Subject: RE: [ExI] Von Neumann Probes



> On Behalf Of Ben Zaiboc via extropy-chat



The other thing I'm trying to understand is how a bacterium-sized probe
travelling at 1%c would last more than a few decades in interstellar space.
Just one single collision with a grain of dust would destroy it. I'd think,
for this reason, that von Neumann probes would need to be at least
millimetre or perhaps centimetre scale objects, capable of maintaining an
active power source to be able to self-repair. ...--- Ben

_______________________________________________


Ben it wouldn't need to be a speck of dust, which is quite unlikely to
encounter in interstellar space.  But I agree with your concept for a
different reason: the probe going at .01c would erode from encountering
hydrogen.  It is easy enough to estimate if we want to go with Wiki's
estimate of a trillion molecules per cubic meter, then recognize at .01c,
you pass thru about 3 million cubic meters per second.

If the frontal area of the probe is one square millimeter, it is hitting
about 3 trillion molecules per second, and some ions, which is significant:
accelerating the ion from the collision creates radiation.  But before we
bother pondering radiation, estimate the drag from the 3 trillion molecules
per second hitting the probe at .01c.

spike




OK I did some BOTECs and I estimate the cubic millimeter probe, if we assume
a mass of about a milligram and initial velocity of .01c will decelerate
from hitting interstellar hydrogen at about 6 m/sec^2.

It is a very unsophisticated calculation however.  If anyone would try to
check my work, I will cheerfully accept any answer within an order of
magnitude.  

Ben your point is carried however: anything that small will need to consider
deal with the drag from the diffuse interstellar medium, as well as the
erosion.

spike