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A City on Mars: Reality kills space settlement dreams
(arstechnica.com)
This is a most excellent place for technology news and articles.
It's doubtful we'd ship past the initial landing and support phases, which was my point. It's likely we'd send several ships out for any permanent presence, but 18 months is just too long and too much investment between trips.
If you send say 20 people to Mars, let's do the math. An average person requires approximately 2 to 3 lb of food per day. 18 months = 6,500 days x 20 people = 131,000 pounds of food, or about 65 tons. You could probably drop the weight significantly by freeze drying it and recycling the water.
In any case, 65 tons isn't a whole lot - that's about what, half of a starship payload? Zubrin's a case for Mars likewise discussed the need to bring all of your food supplies over with you.
Now over many years you could build up enough buy a waste and build a recycling system to start recycling to buy a waste in a greenhouse, but we don't know how viable like greenhouse on Mars will be for growing food. It's likely going to have to be more of a grow lab/vertical farm setup. Very energy intensive.