A Starship Today?
Posted by hyperpat on December 5, 2006
Some earlier comments here got me to wondering about just what we would really need to build, staff, and launch an interstellar multi-generation spaceship. Such a craft has some obvious advantages, mainly in terms of what type of propulsion system is needed, as with very long travel times allowed, there are a lot of different options. But what about the problems such a craft would have? How do we build one? Just how big would it have to be?
A short list of minimum requirements:
1. A method to completely recycle all elements within the craft. While most of the requirements for keeping people alive are common (mainly hydrogen, carbon, oxygen, nitrogen, calcium, and phosphorus) and are fairly easy to handle, there is a long list of trace elements that human body requires, some of which are not easy to handle (such as fluorine) and even harder to get into forms that the human body can assimilate. Having a hydroponic garden is not going to be enough, unless the plant variety can be so wide that all the trace elements are available in their output – and a width this wide would probably need a fully functioning closed ecology, something we don’t know how to put together yet. This implies at least some form of chemical processing plant, or a pretty large stock of non-perishable vitamin and mineral supplements. This also means that everything has to be recycled, including people (and no, this is not a Soylent Green scenario, it happens on Earth – just most people don’t think about it). As a completely sealed environment is almost impossible to achieve, some allowance in terms of foodstuffs and other raw materials must be made and appropriately stocked. Also, some very close attention needs to be paid to air recycling. Very small contaminants that build up in the atmosphere would be deadly, so we need both some very good scrubbers and the effects of ‘natural’ plant recycling.
2. A certain minimum number of people of child-bearing age. Studies have shown that if your starting population is too small with too little genetic variation, the species will die out due to excessive recessive reinforcement (not counting the possibilities of disasters, which on a starship we hopefully don’t have!). There are some estimates that indicate that at least 100 females would be needed, other estimates indicate you might need 10,000. For our purposes here, let’s assume the lower number, with an equal number of males (not totally required, genetically, but it will probably ease the social problems of this group – sorry about that, guys, but we just aren’t all that indispensable).
3. Living habitat. People don’t respond very well if they have to live cooped up in a small, never-changing environment (think prison). We’ll need bedrooms, parks, entertainment areas, exercise rooms, places to work (more on this later), schools, hospitals, and places of worship, and at least some of this ‘cityscape’ needs to be changeable in reasonable time frames at reasonable effort levels. And there needs to be some provision for expansion, as the second and later generations may be quite a bit larger than the starting one.
4. Something for everyone to do that can’t be seen as just ‘makework’. Needed are cooks, teachers, doctors, janitors, astrophysicists, police (even a society this small probably needs someone to enforce decorum), farmers, drive and power plant maintenance men, judges, preachers (?!), ecologists, electronic repairmen, artists and musicians, homemakers, and probably several other professions. Not needed are stock and bond dealers, salesmen of any stripe, soldiers (at least not until they get to their destination), construction workers, check-out clerks, accountants, race-car drivers, weathermen (again, not till arrival point), etc, etc. Matching up people’s talents and desires to the required jobs is not a trivial exercise, and it would get worse at second generation and beyond, which implies some sort of forced assignment to particular positions whether you liked it or not – I don’t think this society can function as a completely democratic one, but rather must be a autocracy.
5. A method of maintaining a reasonable gravitational field (or its equivalent in terms of acceleration) throughout the trip, along with some very good shielding from interstellar radiation.
6. Absolute fail-safes on all critical ship operational equipment – anything from the space drive to light and heat controls. NASA has shown how to handle this one, but it means multiple redundant backups for everything.
So what does the above imply about the size of the craft needed?
Minimum personal living space per person would have to be at least 10,000 sq ft. This may sound like a lot, but the stir-crazy factor becomes a big issue when you have to live your entire life there. Communal space is even larger; add another 20,000 sq ft per person. Working space (offices, hospitals, schools, etc) adds another 10,000 sq ft/person. But the really big space factor is space for farms and hydroponics – under Earth conditions we’re looking at 1.2 acres (about 52,000 sq ft) to support one person. Even assuming large use of hydroponics and plant types that don’t waste a lot of space, we still need to allocate something like an acre/person. For round number purposes, let’s call it 40,000 sq ft/person. So far, we have 80,000 sq/person, and we’d have to plan for at least a maximum population of 400, for a total of 32,000,000 sq. ft.
Then there are the storage space requirements for water, other supplies, and replacement parts: about 2,000,000 sq ft (most of this is water storage, figuring on total use of water, including agriculture, at 300 gallons/day, with a 100 day free supply before recycling). Let’s add another 2,000,000 sq ft for power plant, sewage piping, recycling plants, machine shops, etc. This gives us a total of 36,000,000 sq ft.
For our space-ship design, let’s model it as a torus, which will allow for easy generation of pseudo-gravity by a simple spin. If we make the ‘height’ of the torus 1000 ft, this gives a radius of just about 6000 ft to achieve our 36,000,000 sq ft habitat. Assuming that we use 2” thick aluminum or other light metal walls, our craft would weigh about one billion pounds, or about 500 million kg. While this may seem large, it’s probably buildable even with today’s technologies – just take a look at some of the aircraft carriers we’ve built. Though it would certainly have to be assembled in space, with all its component parts ferried up, as we certainly have nothing that could lift something of that size into space all at once (not even counting the incredible stresses such a launch would place on an object of this size and topology). But it’s doable.
For propulsion, we could use a light sail augmented by an ion engine. This isn’t going to get us there very fast, but a light sail has a great advantage in not needing reaction mass, and while the amount of acceleration derivable from one even near the sun is not great, it adds up over time. How long will it take to get to the nearest star?
Assuming we have a light sail of 10000 meters on a side (something this large is not buildable with today’s technologies), we will get an acceleration at Earth’s distance from the sun of about 400 nanogravities (that’s a decimal point followed by 6 zeroes and a 4). After 100 days of sailing, we’ll be traveling at about 68 miles/hour, and it would take about 560 years to reach the nearest star. We’re a pretty slow tortoise, and even for a multi-generational starship this is probably too long. So we might think about augmenting the solar flux with some large laser arrays to point at our sail, and achieve an acceleration rate of perhaps 10-20 times what we can get from the sun alone. This would reduce our trip down to about 140 years, short enough that people might actually consider doing this.
But as this little exercise shows, traveling to the stars with enough of a population to guarantee the continued existence of the human species even if some major disaster strikes Earth is not impossible, just difficult, expensive, and very time-consuming. Perhaps this plan should at least be put somewhere on the priority list of things the human race should be doing.