A handy guide to growing your own spaceship
Terran life-forms need not apply
It was that thought, coupled with the report from National Academies, which reminded me of a wonderful SF trilogy by John Varley: Titan, Wizard and Demon. The three books are one story, describing humanity's discovery of a giant wheel near Saturn, which contained a biosphere.
The "wheel" is both hollow, and alive - sentient. At its hub is the main intelligence, controlling several subordinate intelligences around the circumference, all operating to maintain an environment which supports several intelligent life forms, and millions of vegetable, insect, animal, and bacterial ones. And the intelligence in the centre is a bit loopy, but even in its old age and short quite a few marbles, it is capable of maintaining the biosphere.
That's the one bit of technology which, so far, no human engineer has been able to develop - a way of avoiding the disaster of evolution.
People travelling across the stars (check Stross for the sums) have to find a way of keeping their civilisation stable (another trick we haven't managed) for not just decades, but centuries, and probably millennia. To quote Douglas Adams: "You may think it's a long way down to the chemist shop..." but interstellar space is, literally, beyond imagination. We have to retreat into arithmetic - actually finding a place to visit might take millions of years. During that time, nothing at all can prevent the life-forms inside the spaceship from mutating - and eventually, changing.
Varley's story gives us an idea of how to deal with this.
OK, he doesn't deal with trivial engineering problems like "how do we take the Sun with us?" and "how does the thing move?" because his story discovers the giant wheel simply orbiting the planet. It's been there for thousands of years, and it's potty, and doesn't remember, or want to tell, how it got there. But if you read the book with this thought in mind, it becomes clear that it would make a great starship. It has the ability to design life-forms and balance ecologies.
In other words, it can do the equivalent of spotting cancers spreading inside its own body and fix them.
Now, you don't have to be a rocket scientist to see that you aren't going to build such a being out of hide and bone. But why limit your definition of life to "carbon-based, water-powered" biologies?
Actually, there are lots of good answers to that, but the Academies report lists them in full. Chief among them is the need for biosolvents. Water is about as good as it gets, for lots of reasons - but it's hardly unique. Others you might look into: Ammonia, Dihydrogen, Dinitrogen, Ethane, Formamide, Helium, Hydrazine, Hydrogen cyanide, Hydrogen fluoride, Hydrogen sulphide, Methane and even Neon are all possible candidates. Some of them might work as life even in interstellar cold. Others might work at ferociously high temperatures of the sort you find in fission plants.
In other words, it's possible to conceive of a living being capable of growing and healing itself in space.
Varley's solution to the intelligence problem is another one which is ideal for interstellar travel - making sure the motivation of the spaceship is in some way governed by the desires of the passengers. He has a kind of "elevation to God" process whereby the consciousness of a human (or something else) can take over as the consciousness of the spaceship - which would be handy if the people decided to change their itinerary, for example, and the spaceship was programmed not to.
OK, the technology doesn't exist. It's not going to solve any sort of problem with today's blue, cloudy Terran planet any century soon (we do have to solve global climate change on our own), and it certainly isn't going to provide remote colonies to trade with. But it might allow a human being, many centuries from now, to watch a different star rise as the sun above a different planet.
In the meanwhile, forget SETI@home. It won't find anything, and it uses masses of mains electricity (roughly, doubling the power used by your PC when just sitting there). And the reason it won't find anything is simply that as a civilisation gets sophisticated with wireless (like, within a century or two) it quickly discovers cellular radio and stops shining its transmitters wastefully out into the sky. It uses techniques like ultra-wide band (UWB), which are indistinguishable from background noise if you don't have the key. Actually, UWB is not only the same as noise, but quieter.
And the most likely source of interstellar invaders is ourselves. Just suppose some idiot did manage to build a light-speed drive and set out to explore the galaxy. After 60, 200 or so years, they'd come back home, to where a couple of million years would have gone by - only to discover a civilisation of alien monsters had apparently conquered Earth. And Earth's newly evolved master race would find this weird group of hostile bipeds trying to claim they owned the place. ®