[Klaustrophobia]

Small moons / asteroids are the most obvious places to start.  There are any number of NEAs that would be ideal.  Eros comes to mind.  There have been quite a few science fiction stories involving that particular asteroid.  The moons of Jupiter and Saturn are of particular interest to me just because that would provide so much variety in a relatively small area.  The biggest problem there is going to be radiation.  Jupiter's magnetic field is truly frightening.  I'm given to understand that Saturn's is more benign, but I am not sure how much more benign.

We have been able to enjoy trips into space thus far with relative impunity because we have (almost) always been traveling no further out than low-earth orbit, well inside the van Allen Belts.  In other words, all human beings except those on the Apollo moon missions have been safely protected by the earth's magnetic field from the freakish vagaries of charged particle radiation sleeting through space, both from the sun and from elsewhere.  As of yet, I have not heard any good plan to protect humans from this radiation during long periods of exposure.

But, assuming you can harden your ships against this radiation, small moons and asteroid can give you protection so long as you build your colonies underground.  Also has the benefit of giving you a source for raw materials for your bioplants.

If you are really interested in this sort of thing, I recommend Paul McAuley's "The Quiet War" and "Gardens of the Sun."  I was a bit disappointed by the stories themselves, but the biologist's perspective on intrasystem colonization was very interesting and very practical.  The books highlighted a lot of the problems we're going to have to overcome, and some of how to do it.

charged particle radiation is not hard to sheild for.  betas, the most penetrating, only need a few cm of relatively dense material.  heavy ions are stopped by a sheet of paper.

[Bob-san]

Not 1/10 having a planet.  1/10th having a habitable planet.

Not every system will have a planet, and not ever planet will be habitable. In just our solar system, 1/8th of the planets are habitable. Look outside, we've not even identified a single planet that is habitable as-is (or with some plant growth; we should probably be looking for a nitrogen&carbon dioxide planet with water and ~earth gravity, as that'd be easiest to grow know plants and crops in and could quickly provide a source of oxygen. Besides, calling it 1/10th may be a bit more accurate as there are a lot of systems that have already gone (super)nova or are in a multiple-star system. You don't necessarily want to give a binary or trinary star system double or triple (or more) the number of planets; if anything, there'll probably be less planets, as more material is going into the stars or split among cooler stars (Alpha Centauri AB-C is a binary system but star C is a dwarf; the material in that Dwarf is probably as much as our entire solar system has). As far as I know, other than location relative to a volatile star, there's nothing stopping a habitable planet from appearing in a binary+ system. Though almost surely, the planet itself will either orbit one star or will orbit the central point of multiple close stars.

[watsisname]

Another interesting thing to note is that we observe many systems with some truly bizarre planetary orbits, and it's unlikely that habitable worlds could exist in such systems.
We don't yet know how common star systems that "look like ours" (terrestrial worlds close in, gas giants farther out, orbits with low eccentricity) really are in the universe, but hopefully the Kepler mission will help answer that question.

[Klaustrophobia]

Not every system will have a planet, and not ever planet will be habitable. In just our solar system, 1/9th of the planets are habitable.

fixed   pluto will always be a planet!

[karajorma]

fixed   pluto will always be a planet!

Only to people who don't understand science.

Feel free to carry on going on about phlogiston instead of oxygen and mentioning how if we could push the aether out of the way we could get faster than light travel while you're at it though.

[perihelion]

charged particle radiation is not hard to sheild for.  betas, the most penetrating, only need a few cm of relatively dense material.  heavy ions are stopped by a sheet of paper.

Only if those ions etc. have a relatively low energy level.  If you are referring to the alpha and beta particles kicked off by radioactive decay, then typical kinetic energy is only going to be on the order of 5 MeV or less most of the time.  The minimal shielding you describe would be adequate.

However, a significant portion of the charged particle radiation in space is cracking along at relativistic and ultrarelativistic velocities.  I'm seeing 40 MeV being quoted by Wikipedia as a threshold beyond which damage is likely to occur.  The Sun can whip out particles with KE in the GeV range.  Galactic cosmic rays have been observed with energies of as much as 10²⁰eV.  And the real kicker is that unless your shield is a veritable sponge for low AND high energy radiation, the thicker it is, the more secondary radiation you are going to create from spallation.

I do not think it is an unsolvable problem, but it is not a trivial one.  Many solutions have been considered.  The ones that are most likely to be effective are some of the most difficult to implement because of our extremely limited surface-to-orbit lifting capacity.

[General Battuta]

fixed   pluto will always be a planet!

Only to people who don't understand science.

Feel free to carry on going on about phlogiston instead of oxygen and mentioning how if we could push the aether out of the way we could get faster than light travel while you're at it though.

Are you telling me my ether plow isn't going to work?

[StarSlayer]

fixed  pluto will always be a planet!

Only to people who don't understand science.

Only to people who underestimate Macgyver

[Bobboau]

fixed   pluto will always be a planet!

Only to people who don't feel like following pointless arbitrary rules that have no bearing on reality.

[castor]

Well I think he’s comparing interstellar travel to overseas travel, and pointing out how that culture was able to accomplish such travel and yet was very much violent and aggressive against the technologically inferior Americans.  Therefore we can’t expect the same to not be true of space-faring races. To be fair, it’s possible that the morality point I brought up isn’t all that valid… perhaps it is inevitable that in the search for growth and riches, better morality is often tossed aside, like what the movie Avatar hints at.

Oh, I thought your were talking about the probability of a civilization to extinct/near-extinct itself half accidentally (catastrophic failure in controlling the use of power). Among a number of civilizations with the same "level of moral", the most technologically advanced is the most capable (probable) to pull that off, I'd think? In long run, that should favor the more pacifistic civilizations to grow more advanced.

[Machaka]

I find it hard to believe that we're closer to extraterrestrial contact than technological singularity.

And if tech singularity radically changes everything we currently understand about society, humanity, and Earth, all current hypotheses about extraterrestrial contact are meaningless.

[SpardaSon21]

We are of course assuming that said race doesn't have awesome technology that allows them to defy physics and travel between solar systems in a few weeks with low energy costs, making FTL travel highly accessible.

If we humans ever get something like that, say, subspace drives, you bet we're going to be competing amongst ourselves for territory and resources with mass drivers and fusion warheads.

[Kszyhu]

That would be a very fast conflict.

[Klaustrophobia]

charged particle radiation is not hard to sheild for.  betas, the most penetrating, only need a few cm of relatively dense material.  heavy ions are stopped by a sheet of paper.

Only if those ions etc. have a relatively low energy level.  If you are referring to the alpha and beta particles kicked off by radioactive decay, then typical kinetic energy is only going to be on the order of 5 MeV or less most of the time.  The minimal shielding you describe would be adequate.

However, a significant portion of the charged particle radiation in space is cracking along at relativistic and ultrarelativistic velocities.  I'm seeing 40 MeV being quoted by Wikipedia as a threshold beyond which damage is likely to occur.  The Sun can whip out particles with KE in the GeV range.  Galactic cosmic rays have been observed with energies of as much as 10²⁰eV.  And the real kicker is that unless your shield is a veritable sponge for low AND high energy radiation, the thicker it is, the more secondary radiation you are going to create from spallation.

I do not think it is an unsolvable problem, but it is not a trivial one.  Many solutions have been considered.  The ones that are most likely to be effective are some of the most difficult to implement because of our extremely limited surface-to-orbit lifting capacity.

i hadn't considered the higher energies, no.  but how much exposure from those is there when you aren't right next to a star?  radiation intensity falls off as 1/r^2.  heavy ions of huge energies would still have relatively short ranges (i.e., not requiring feet of material), but would do a ****load more damage.  cosmic rays would be rather penetrating, but if i'm correct in my understanding, the earth's atmosphere does not sheild those and we are already subject to them as part of our natural background radiation, and that component shouldn't change in space.

[BengalTiger]

We are of course assuming that said race doesn't have awesome technology that allows them to defy physics and travel between solar systems in a few weeks with low energy costs, making FTL travel highly accessible.

If we humans ever get something like that, say, subspace drives, you bet we're going to be competing amongst ourselves for territory and resources with mass drivers and fusion warheads.

As long as there's free space, the only conflicts going on would be those of planets/star systems etc trying to get independant from Earth (or later on other planets).

Either that or fighting with aliens.

[perihelion]

i hadn't considered the higher energies, no.  but how much exposure from those is there when you aren't right next to a star?  radiation intensity falls off as 1/r^2.  heavy ions of huge energies would still have relatively short ranges (i.e., not requiring feet of material), but would do a ****load more damage.  cosmic rays would be rather penetrating, but if i'm correct in my understanding, the earth's atmosphere does not sheild those and we are already subject to them as part of our natural background radiation, and that component shouldn't change in space.

Potentially, expose could be pretty severe.  If the Apollo astronauts had been unlucky enough to be half-way between the Earth and the Moon during a solar flare, they would have been exposed to enough radiation to kill them in a matter of hours.

Even under nominal levels of solar activity, the cumulative radiation exposure is sufficient that health problems are inevitable unless some kind of protection can be implemented.  Either biological hardening (augmenting our bodies' ability to cope with higher ambient levels of radiation) or physical shielding, or some combination of the two.

Radiation intensity does fall off as the inverse square of distance, yes.  But we're dealing with individual particles here with sufficient energy to do damage just on their own.  The effect is cumulative.  The Earth's atmosphere and magnetic field do a pretty good job of protecting us from all but the most extremely energetic particles down here on the surface.  Outside of that protective envelope, there's an entire spectrum of charged particle radiation we are not normally exposed to (unless you happen to sleep in a cardboard box huddled under a nuclear reactor to keep warm).  Mostly, we're talking about very fast moving protons (hydrogen ions), but there is some helium and heavier ions in the spectrum.  It's worth reading up on if you are really interested.  Next to our pitiful surface-to-orbit capacity, space radiation is probably the biggest hurdle we need to overcome before we can really start living in space.

[General Battuta]

So here's an interesting question that even Hawking didn't seem to touch on.

What if the dominant form of life in the universe isn't remotely organic or even sentient? What if the von Neumanns have taken over?

Most biomass here on Earth is bacterial or insectoid. Neumanns would be the analogous tactic on the galactic level, except their advantages over 'standard' aliens would be even more pronounced.

I did a story once in which the galactic ecosystem was dominated by berserker-style Neumanns that were attracted by radio activity (a relic of their original programming as weapons in an early-universe war.) Our solar system seemed to have some kind of mysterious immunity, and the thrust of the story was figuring out why. Cookie for anybody who gets it.

[perihelion]

So here's an interesting question that even Hawking didn't seem to touch on.

What if the dominant form of life in the universe isn't remotely organic or even sentient? What if the von Neumanns have taken over?

Most biomass here on Earth is bacterial or insectoid. Neumanns would be the analogous tactic on the galactic level, except their advantages over 'standard' aliens would be even more pronounced.

I did a story once in which the galactic ecosystem was dominated by berserker-style Neumanns that were attracted by radio activity (a relic of their original programming as weapons in an early-universe war.) Our solar system seemed to have some kind of mysterious immunity, and the thrust of the story was figuring out why. Cookie for anybody who gets it.

Because they were programmed to ignore their system of origin?  They are what is left from the first time life evolved on Earth; all life currently on Earth, including us, is a result of a second "genesis" after a mass extinction event.

 

[General Battuta]

Good guess...but not the direction I took.

Imagine a Neumann ecology. Presumably they'd start preying on each other.

[Mongoose]

They saw what a fantastic job we humans have done of killing each other over the millennia and figured they didn't want to stick around to be offed themselves?