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[origamiscienceguy]

All the molecules that you had at the start are still in the ship. You can drink your own pee to get almost everything back.

ALMOST everything. The rest is waste. That mass that is waste needs to be replenished by something on the ship. Meaning no indefinitability.

[Reelya]

Just have a microbe tank to reprocess the waste. A photosynthetic human would produce much less waste than normal, so a much smaller reprocessing set up would be needed. Human waste is hardly nuclear waste that never breaks down.

[origamiscienceguy]

Just have a microbe tank to reprocess the waste. A photosynthetic human would produce much less waste than normal, so a much smaller reprocessing set up would be needed. Human waste is hardly nuclear waste that never breaks down.

But Fiber is incredibly hard to break down. Even so, the laws of thermodynamics say that something is always going to eventually break, so some spare materials would be needed to fix what broke. Be it part of a cell, or part of the whole ship.

[Arx]

Even so, the laws of thermodynamics say that something is always going to eventually break

I suspect that in this case the laws of thermodynamics are shown by the necessity of purifying the waste, which requires energy. If you have solar panels though, there's a lot of basically free energy available.

[MetalSlimeHunt]

I think you'd die a lot sooner than you'd think with even the best photosynthesis system. Humans require a variety of materials, not just calories. Better than nothing, but not a replacement for food.

[Reelya]

Just have a microbe tank to reprocess the waste. A photosynthetic human would produce much less waste than normal, so a much smaller reprocessing set up would be needed. Human waste is hardly nuclear waste that never breaks down.

But Fiber is incredibly hard to break down. Even so, the laws of thermodynamics say that something is always going to eventually break, so some spare materials would be needed to fix what broke. Be it part of a cell, or part of the whole ship.

Humans don't actually produce fiber as a waste product, you know.

If you're living on sugars from photosynthesis in your own cells, there's no fiber produced nor are you consuming any with the food you're (not) eating.

[Sheb]

I seriously doubt you'd get enough photosynthesis out of a human to feed a human. Why not simply have a tank of microbes recycling your waste and producing some kind of soylent-like product?

[RedKing]

You guys make the future sound so appealing...

"Hey, welcome to space! For dinner, we've got your own recycled feces with a big tall glass of urine!"

Where do I sign up?! _{to stay on Earth}

[Gentlefish]

...I feel like, since we're talking about human photosynthesis being a possibility here, why can't we make microbes to make the nutrients we can't produce on our own?

As for the waste matter, you only have waste if you expend energy. Who says we won't be spending flights in extended periods of medically-induced comas, or other low-energy states?

[Starver]

That's an interesting couple of pages.

Anyway... just like (normal) humans can't live without the resources ultimately provided to us by photosynthesising plants (aggregated high-energy molecules and the oxygen to breath with which we ultimately 'burn' the molecules back to ground-state), or an artificial equivalent (the less efficient artificial air-scrubbers and oxygen regenerators that we currently have access to, which need topping up regularly with supplies from home), photosynthesising humans would probably have problems when they find themselves running out of carbon dioxide in their closed-system and perhaps a little too much oxygen enrichment for the Health & Safety Department's liking...

(It's probably thanks to the development of us animals that the plant population survives at all...  Or the equivalents, way back when in the unicellular days of evolution...  It may now be a personal battle between a thing that eats and things that eat them (every which-way!), but the overall conflict isn't actually a war, but actually more harmonious.  So long as there's no upstart species that decides that the whole planet is theirs and causes severe imbalance.  Yes triffids humans, I'm looking at you!)

Unless you're going for a 'twofor' organism (like plants actually are, already, with their night-time respiration cycle; but much more so).  Selective self-cannibalism?  As long as there's enough energy coming in to make up for the inefficiencies, and no absolute dead-ends insofar as 'indigestible sludge' end-products that we haven't put in some biological mechanism to process and 'recharge' with solar energy (however many steps removed) to make it useful.  But good luck getting that to work 'perfectly enough'!

(Also, obXKCD...)

[fakeedit: No I don't want to review my post...  Gentlefish has made different important points..]

[jaked122]

I still think that uploaded humans are a better option for this sort of thing than anything we're likely to think up.
Unless of course we can get some sort of frame-dragging drive working so we don't need fuel, it's unlikely that any advancements are going to justify the mass that we can't remove as biological beings.

Get some generalized robotic shells that can be piloted, build some fabricators, and then reincarnate back into fleshy bodies for the insystem travel, if that is really required.

Or you could remain as a ghost in a machine, a machine with sufficient sensory information to not drive you insane, some of them just made up to augment the reality and reduce claustrophobia and sensory-deprivation. Ultimately, unless major advances are made, given that ramscoops don't appear feasible currently(according to more recent estimates of interstellar medium densities), we don't have much choice. We'll have to go digital and move out into the universe and then when we get there improvise something.

I may have been reading some Orion's Arm.

[PTTG??]

You guys make the future sound so appealing...

"Hey, welcome to space! For dinner, we've got your own recycled feces with a big tall glass of urine!"

Where do I sign up?! _{to stay on Earth}

Ah, Earth, where all the food comes from sanitary plastic packages in stores, and not from inside animals or from dirt with animal crap smeared all over it or anything.

[i2amroy]

I seriously doubt you'd get enough photosynthesis out of a human to feed a human.

A similar problem was pointed out for photosynthesizing cows in this xkcd what-if.

Basically animals have way to small of a surface area to extract anything close to the energy that they need to survive. In the linked what if, the calculations came out to green cows that stood optimally were only able to photosynthesize around 4% of their normal energy intake, which is miniscule. As creatures that are designed to be more predatory (and thus leaner, but still with high energy requirements), I'd expect humans to probably have an even worse surface area to energy required ratio, and thus be able to get even less of their total required energy from photosynthesis than cows' tiny 4%. There's a reason why we eat things instead of making our own food from the sun.

That said probably the best approach is simply going to be the space-biodome one. Plants and the environment are already adept at taking waste processes and "recycling" them, so to speak. You basically cover the entire outside of your spaceship in a transparent material and run farms in that outer shell. Any waste products are recycled back into your farms to work as fertilizer and are filtered in the process to provide clean water (you'd still probably need to run the water through a few other filters to get the required level of thickness to your earth filtration system). Over time your plants grow up, and then you eat the plants. The only thing you'd need to watch out for would be feces-born parasites, since that's the one major drawback of using night soil instead of fertilizer from other animals.

The plants also serve another purpose, that of fixing CO₂ from your ship and turning it back into O₂ for you to breathe. It's water filtration, waste recycling, air recycling, and food generation all in one.

[origamiscienceguy]

unfortunately, the ship would have to be extremely large to accomidate enough plants for one human so the idea of an indefinite escape pod is out of the question.

[i2amroy]

Of course it's going to be huge, that's the price you pay for having something that lasts forever. Sunlight isn't exactly the best energy source, after all, and humans need a lot of energy to keep running.

You could just have big huge solar panel wings that stretch out into space, convert the energy into electricity, and then convert it back to light later to stack your farms into a single centralized location instead (which could hypothetically save you some space). Of course the price you are going to pay is the solar energy conversion factor, meaning that even with literally perfect solar panels (of which we are still a long ways away from making), you'd need about 20% extra surface area with solar panels than if the plants were just exposed directly.

Alternatively you might look into scooping up hydrogen from space, fusing it to generate electricity, and then using that electricity to generate lights for plants rather than grabbing sunlight directly, though then you run into the problem of needing a gigantic scoop on the front of your vehicle instead of huge solar panels (and the problem of being unable to sit still for too long, you have to keep moving or you will run out of fuel).

In short, though, if you want some sort of tiny escape pod option you're either going to need to do something like brain uploading, or (more realistically) cryonic systems where the pod basically just kills you and doesn't revive you until you reach your destination. Active humans simply use way too much energy to be able to sustainably gather that energy from space over anything smaller than a huge area.