After Andy L saw the movie, he said “One does not simply go back to Mars…”
It was difficult for him not to shout it in the theater, I suspect.
It’s linear, simply the amount of energy radiated by a light source, weighted according to how humans perceive the brightness of different wavelengths of light (which peaks at 555 nm green light). That weighting is actually pretty terrible for grow lights, since chlorophyll has peak absorption and efficiency at the red and blue end of the spectrum. But it’s the weighting that’s used for any lighting intended for humans. If Watney was set up with proper grow lights, I’d WAG that his lighting system would use less than half the power (8 kW).
For conversion to lux I’ve also been assuming perfect distribution of light, where everything is a tight beam perfectly focused on the growing area. In reality that wouldn’t be possible with ordinary interior lighting fixtures, and for any given diffuse light source you’d have to consider that the amount of light that hits a surface scales with the inverse square of distance.
Watney could, however, use a shiny material like mylar or aluminum to turn any light into a reasonable spot light. My WAG is that with improvised light fixtures he’d need double the power I quoted above (32 kW), to account for all the light reflected away or absorbed by fixtures.
That’s what made Rich Purnell a steely-eyed missile man. Duh. 
I think he’s referring to Martinez.
I suspect he was referring to Boromir’s (Sean Bean)'s speech at the Council of Elrond: “One does not simply walk into Mordor…”
Exactly. He does a good imitation, by the way.
And (completely unsurprisingly), here’s an image capturing the joke http://www.socialmemegenerator.com/user-submitted-memes/one-does-not-simply-fly-back-to-mars/
How does the amount of sunlight striking the surface of Mars differ from that of Earth? Mars is farther away and has a different atmosphere that the light goes through. If you had a greenhouse on Mars, would there be any problem in growing plants in it assuming you had the right dirt and atmosphere inside?
Somewhere I saw the comparison that high noon on Mars would be about as bright as a cloudy day on Earth.
Mars mean distance is 1.5 AU (varies from 1.4 to 1.7). So between 51% and 35% of Earth’s intensity based on the time of year. Earth gets about 1 kW per m^2, btw.
Mars averages about 1.5 AU distance (Earth is at 1 AU), so the solar irradiance at orbit is about 45% of what is received at Earth, or around 600 W/m[SUP]2[/SUP]. On the plus side, the low density of atmosphere on Mars means much more sunlight gets through in clear skies; on the downside, a lack of a stratospheric ozone layer means pretty much all of the ultraviolet gets through, which is bad for anything organic, including polymer sheets and seals, and of course plants. Mars is also subject to dust storms that can last from a few weeks to several months. Although not anywhere near as destructive as portrayed in the film (the one substantial deviation from reality in the film as many technically-astute reviewers have noted) it does essentially block out all sunlight.
As the Hab was not set up for growing plants on a large scale I would assume that Watney rigged or modified some kind of light source with sufficient UV output for plant growth. (Note that the fluorescent bulbs give off an unhealthy amount of UV that is mediated by coatings inside the bulb between the glass and the phosphor layer.) Since Watney was a botanist one would assume that one of his mission objectives was to see how well things grow on Mars under various conditions so he probably had some kind of equipment for this purpose but it seems unlikely he had enough to run his entire “farm”. Like everything else, he must have found a way to jury rig suitable illumination powered off of the Hab’s electrical systems.
Stranger
I’ve asked twice, but how many potatoes come from 200 plants?
Is there some reason why people have spoken of “sols” but no one has bothered to define it?
I thought this thread was “Open Spoilers”. But just in case it’s some kind of problem, I will put the definition in a spoiler box:
[spoiler]
Timekeeping on Mars - Wikipedia, the free encyclopedia
The term sol is used by planetary astronomers to refer to the duration of a solar day on Mars. A mean Martian solar day, or “sol”, is 24 hours, 39 minutes, and 35.244 seconds. When a spacecraft lander begins operations on Mars, the passing Martian days (sols) are tracked using a simple numerical count.
[/spoiler]
Probably enough such that you would never, ever, ever want to see another potato for the rest of your life.
But seriously, the answer would likely be too varied to ever lend itself to an exact number.
This page says that each plant yields 3-5 pounds of potatoes, but of course that’s under Earth conditions. I wouldn’t count on getting that many.
Using the Thanksgiving potatoes was a great plot point, but I would expect that they would have been regularly doing experiments on growing food while on Mars. If NASA was really doing those missions to Mars, what plants would they be trying to grow in that environment? Would potatoes be likely candidates? Or would they be trying to grow things like algae and bacteria as food?
Potatoes are a hardy crop that is easy to grow, provided you keep phytophththora infestans away from it, but as someone noted, it is fairly low in proteins. Mushrooms are an obvious choice, as are soybeans and peanuts, but personally, I’d take a good look at quinoa and amaranth as being high in proteins and especially the amino acid lysine. However, unless you really have a massive capacity for farming space and a crew really into veganism, you’re probably going to want some animal sources of proteins and fats, for which chicken, crayfish, or some other high protein value, fast growing creature can provide. “Algae and bacteria” may seem to be an ideal space colony crop because some types can be easily grown in lab conditions (“in a vat” so to speak)’ while many types of algae have been used in various cuisines and can provide important micronutrients it has never been a staple crop of any society and generally have poorer throughput efficiency in terms of nutritional content per mass or volume than staple foods and animal proteins.
Stranger
Thank you, gentlemen. 
Yes - very interesting. Thanks.
Best username/post combo this thread.