OP Tipping wrote:... slightly handy botanist ...
In the book the character mentions that he has a degree in mechanical engineering. If I recall correctly, he trained as a botanist for the mission, because someone had to.
OP Tipping wrote: ...He grew his potatoes under the internal lights of the Hab but used agriculture statistics based on outdoor earth growing.
Sometimes you work with what you've got. It wouldn't be surprising to find that the lights in the hab are full spectrum lights though. I'm not sure if he mentions it in the book.
Also note that Andy Weir (the author) worked out a lot of the math behind the situations in the book, so they aren't too removed from reality.
It's not about the spectrum. I grow tons of plants in my apartment under artificial light (no, not *those* kinds of plants!
). The simple problem is that you're trying to replace an incredibly
powerful light source - the sun - with something that just simply isn't (indoor lighting). The difference between indoors and outdoors doesn't seem like that much to our eyes, but that's due to a quirk of human perception - the light intensities are orders of magnitude different.
The sun puts down about 1000 watts per meter squared, primarily in the visible spectrum. Now, picture the brightest CFLs you can buy - maybe 40 watts power draw? They'll put out maybe 15% of their energy as light (note: this is different than luminous efficiency, lumens are a terrible scale for plants as they're optimized around the perception of the human eye). So to match the output of the sun at noon on a clear day, you'd have to have 167 of them in every square meter
, and that square meter would consume 6.7 kilowatts. That square meter, in normal industrial farming practices in an optimal potato-growing area, will yield about 7 kilograms of potatoes over the course of one growing season (maybe 8 months). That's about 16000 calories, aka 66 calories per square meter per day. He'd need 38 square meters for a normal diet, meaning about 250 kilowatts of light drawn from the equivalent of 6325 super-bright CFLs.
See the problem?
Now, there's some things that naturally push the odds more toward your favor, and things you can do beyond this.
1. The natural sun isn't always high noon on a clear day. There are clouds. There is night. The sun goes to off angles. Etc. Working against you, however, indoors is trying to make sure all of your light that you create actually hits your plants. Otherwise you're lighting up even more
square meters than you mean to. But overall this cuts the challenge by half an order of magnitude or more.
2. Outdoors is often a crappy environment for a wide range of reasons. Pests. Wind. Cold. Excessive heat. Drought. Flood. Etc. Indoors you can yield a nice peaceful controlled growth environment. Well, sort of... because there's all kinds of gotchas indoors that can devastate your crops in a matter of days (I once lost tons of my most prized plants by simply starting shutting the door to the room for about a week... the temperature got too hot and fried them). And spider mites are devastating in any greenhouse environment, and they always find you sooner or later - I wouldn't be surprised if even on a mars ship a couple snuck aboard, they're so utterly miniscule. But maybe he'd be lucky in that regard and there's literally zero. Anyway, IF you can manage to not screw up and you do everything perfectly, you can get another leg up versus the sun, cutting off maybe another quarter of an order of magnitude of the sun's advantage over you.
3. By using LEDs instead of fluorescent or HID, you can get about the same amount of growth for about half the power. So another quarter order of magnitude advantage. Note however that this is for optimal grow LEDs, mainly a mix of red and blue, ideally with a bit of UV. White LEDs are just blue LEDs with a phosphor that wastes part of their energy, so they don't give you as much advantage.
4. You can reduce the number of lights you need by not giving the plants a day/night cycle and having the lights on at max intensity 24/7. Not all plants need a day/night cycle; potatoes don't. This of course doesn't change your required power consumption.
So basically, if he did everything perfectly and lucked out, he might - maybe
be able to squeeze by on maybe 25 kilowatts of power and red-and-blue LED lamps of an output intensity of around 500-1000 super-bright-CFLs over maybe 25 square meters (a square 5 meters / 16 feet per side) of grow area. This is the sort of thing that could be possible to set up on Mars if that was a mission goal. But scrounging that together on a mission not designed for it? I hate to have to say it, but that's just not going to happen.
I've not read the book. But if they had him growing potatoes indoors using a couple office lights, I'd be hitting my head and groaning. On the other hand, if they had him macguyvering a solution to use natural light
using several hundred square meters of inflatable clear plastic... well, maybe. Where he'd get the plastic from, I don't know**. He'd have a ridiculously hard time stopping leaks and he'd be in a constant battle to clean off the dust, esp. on a plastic not designed to repel dust - it's a perfect material to build up a static charge. I hope his suit can take such protracted daily EVAs. And I don't know how well potatoes would deal with the radiation. He'll have a huge battle with thermal management, with the whole thing becoming warm during the day and frigid at night, transitioning rapidly between the two - to stand a chance, the base would have to have a nuclear generator and he'd have to shunt the reactor's waste heat in every night and stop it during the day, ideally either in an automated fashion or with a temperature alarm - otherwise he'll surely screw up at some point and lose his entire crop. His hab surely has this sort of temperature regulation on its own but also certainly doesn't have the capability to shunt in the sort of massive heat he'd be losing at night with all of the new uninsulated surface area, so he'd have to give the HVAC system a serious upgrade. The plants would grow slowly due to the reduced light conditions even if he can keep the dust off the plastic - he'd need significantly more acreage than usual. BUT, all of that said... it might be achievable.
Indoors or outdoors, a couple gotchas:
* He's going to need nitrogen as fertilizer, martian regolith will not contain nitrates. Maybe there's some sort of supply of nitric acid or ammonia onboard for some kind of industrial process or fuel that he can use as a feedstock. His own waste and plant composting won't suffice, some nitrogen will be lost.
* If he wants to use regolith as his growth medium (note that one can use almost anything as a growth medium - even no growth medium if you keep the roots constantly misted), he's going to have to neutralize the perchlorates first (such as by baking it). And hope that the regolith in his area isn't too salt rich or manage to wash out the salts. I hope his water supplies are plentiful and/or renewable.
** - To any chemical engineers here - would there be any way you can conceive of that someone could take bulk acrylic panels and make thin acrylic sheeting out of it? This is out of my realm of expertise, apart from knowing that acrylic is a thermoplastic and thus I would expect it to be possible to reform into different shapes via the application of heat. It's quite possible that a building, rover, or somesuch would have large thick acrylic panels as windows (exterior or interior) or internal walls, flooring, etc; there might even be whole spare acrylic panels onhand in storage, isn't inconcievable. But is there any realistic process that one could use on Mars to turn a couple thick panels into thin sheeting with 2 orders of magnitude more surface area?