# Solar Sails III (because two just isn’t enough)

One thing that I’ve wanted to quantify since reading *Intelligent Life
in the Universe*, an outstanding book by Carl Sagan and I.S. Shklovskii,
is the idea of exogenesis. Exogenesis is the hypothesis that life formed
elsewhere in the universe and was somehow transferred to earth in the
form of some small seed or spore. Now since E.T. E. coli presumably do
not have little tiny jetpacks or other means of active transport, they
would need to traverse the cosmos in some passive way. One such way
would be solar sailing.
Way back in Solar Sails I, we derived equations describing the maximum
speeds and time-of-travel for various distances for a given solar sail.
Each of these equations was a function of the surface mass density of
the sail, which is just the amount of mass per unit cross-sectional
area. All we need to know is the cross-sectional area and mass of a
given object and we can apply these equations to just about anything!
Assume we have some spherical blob with the density of water (1g/cm^3).
The effective sigma of this blob would just be the mass divided by the
cross-sectional area. In other words,
$$ \sigma = \frac{m}{Area} = \frac{\frac{4}{3}\pi r^3 \rho}{\pi
r^2} = \frac{4}{3}\rho r .$$
Rearranging to get r in terms of the other variables, we have
$$ r = \frac{3\sigma}{4\rho} . $$
Plugging in our density of 1g/cm^3 and a suitable sigma (10^-4
g/cm^2), we get
$$ r \le 0.75 \times 10^{-4} cm = 0.75 \mu m .$$
Check out
this fun
site to see what kind of critters can fit in this blob.
From the previous post, we saw that for a sigma of 10^-4 g/cm^2, our
sail would get to the nearest stars on a timescale of order 10,000
years. Thus if our blob has a radius of less than about a micron, it
could spread to hundreds of stars in around 10,000-100,000 years. Even
if it would take millions of years, that would be almost no time at all
on the cosmic scale. Just based on this calculation it all seems fairly
feasible.
In making these calculations I have neglected several important aspects
of the problem. First, in no way have I actually calculated any sort of
probability of this happening. Additionally, I would have to see how
likely it is for some blob to reach planetary escape velocity
(presumably just by riding that tail of the Boltzmann distribution).
Finally, and perhaps most important of all, I have not given any sort of
motivation or mechanism by which some living body could survive hundreds
of thousands of years in the vacuum of space with constant radiation
exposure. But I have heard that some forms of life are totally
extreme (especially if they
drink this).
Even though such a process seems possible, it certainly doesn’t seem
like the easiest way to get life on earth. I prefer the much more
satisfying “amino acids + lightning + magic =
life” model. But it does
offer some interesting possibilities. Suppose we as people think that
people are super awesome and therefore people should be everywhere. We
do some bio magic and put whatever DNA we want into viruses, which we
then pack into as many micron spheres as we can make. We then point them
at the nearest stars and have them disperse.
What would the probabilities be that they land somewhere habitable? Are
there any ethical considerations in doing this? Is it a galactic faux pas?

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