Raoul,

I believe interstellar colonization is technically feasibly but transport of
DNA based life across interstellar distances is probably infeasible. This
sounds like a contradiction but isn't.

The way I think interstellar travel will be achieved is through plasma
propulsion systems where the propulsive plasma is directly driven by a
controlled nuclear fusion reactor running a deuterium-deuterium reaction.
Obviously it is necessary to get as close to the speed-of-light as possible
which means the vehicle must have an initially tiny mass fraction. I
suspect the fastest velocity any practical technology could produce
is slower than 5 percent the speed-of-light. The deuterium-deuterium
reaction is really the only option because it has a practical ignition
temperature with a fuel that is stable in terms of nuclear decay (unlike
tritium). He-3 propulsion systems are nonstarters because of the high
ignition temperatures. Deuterium-deuterium nuclear fusion produces a
fair amount of fast neutrons due to side reactions. Therefore the payload
of this sort of rocket would get a significant radiation dose both from
the propulsion system and cosmic rays. Having thick radiation shields is
not practical because the vehicle must have a tiny mass fraction. The
dual requirement that the vehicle be in interstellar space for centuries
and carry minimal radiation shielding excludes biological payload as a
practical option. However this does not prevent colonization. We human
beings like all life forms can be completely describe as information stored
in a computer, viz. our DNA sequence. The starship must have a self-aware
autopilot to handle in-flight repair and starship reconstruction for when
it arrives in a new star system. The starship autopilot could contain
the complete sequence of the human genome and upon arrival in the vicinity
of a viable planet, construct human beings from local organic material.
The autopilot could then raise the first generation of colonists using
human form puppets acting as surrogate parents. This sort of starship could
repair or duplicate itself and then proceed to new star system(s). Self
duplicating starships are classic "von Neumann machines" and could conceivably
colonize the entire galaxy in a few million years. Once our species
survival was assured would galaxy wide colonization be desirable? One
could argue that colonization beyond a certain radius from the Earth might
be counter-survival because of possible encounter with a pathological
extra-terrestrial civilization that found our outward expansion a threat.

Another aspect to galactic colonization has a theological dimension. If
one starts from the proposition that the universe is an artifact and that
we human beings play a role in the universal creator's plan (a huge leap
of faith!) then our continued survival might be part of the creator's plan,
i.e. our civilization must survive long enough that we acquire sufficient
intelligence or knowledge to enable us to perform our designed function.
However once the designed function was performed then we might have
no further role (the first stage of a multistaged rocket lands in the
ocean after stage separation). This sort of line-of-reason has no real
value because there are too many leaps-of-faith. However it seems to me
that we aren't players in any sort of bigger game if our entire civilization
and species dies with the Earth. Therefore colonization at least to Mars
seems reasonable. Maybe in a few thousand years our descendants might have
a better understand of why we exist and proceed from there.

Gary