Should vegans go to Mars?

Amateurs talk strategy. Professionals talk logistics. 

Gen. Robert Barrow, US Marine Corps

Beside space, I have great interest in human digestion, becoming an advocate of low-carb diet and intermittent fasting. It brought to me completely different view of the way human beings get and spend their energy intake.

According to this article, it is obvious. Eating vegan is healthy, right? And how we will grow meat in space or Mars? I will argue to the contrary. Not only vegans should not be considered. Actually taking opposite approach could bring huge logistical advantages making initial Mars missions possible.

In the previous post, I estimated that each crew member requires around 5 kg of consumables per day. Most of these consumables can be reclaimed or converted into Raptor propellant. But we still need to carry around 2kg of food for each mission day. In this post I am taking a different view - how can we limit the amount of necessary food without doing a fancy ISRU with extracting carbon and converting it to methane. In this approach, we still need that for (oxygen) generation. But the focus of this article is whether we need to carry that much food initially.

So out of 5 kg of daily consumables, 10% (500 g) will be lost due to feces. 20% can be converted to propellant (assuming we have enough electric power onboard to power the Sabatier reaction, electrolysis of water and liquification). The rest (mostly water) can be recycled. Lets assume additional 10% conversion losses (for example, urine leftovers, misalignment of basic element ratios etc.). That means that only 40% of consumables are required. 20% will be lost and 20% will be converted into the propellant. Which means that for three year mission duration with 100 people, Starship would need just 220 tons of consumables. And 110 tons would end up as propellant anyway. 

Could it be better? Maybe we should consider food packaging? Now we come to the controversial part. Average person needs 2000kcal per day. Or roughly 100W at rest. So a hundred people require roughly 10kW of energy. Something that will be easily matched by solar panels installed on Starship. The hard part is how to convert the solar energy/electric power into edible food. One (obvious) option is through gardening/growing food. Although that is long term goal, it requires lot of infrastructure and R&D. Food growing on Earth is using a large/complex ecosystem which is really hard to downsize reliably. 

Is there an alternative? Carrying consumables seems prohibitively expensive. 5500 kg per person is huge. But we can recycle and reuse 60% of it. It leaves just 2200 kg per person for a three year mission. This seems much better, but still a huge commitment. Can we do better than that? Anyone who has tried to loose weight knows how hard is it. Practice of long term fasting enables most people to avoid food completely for extended period(s) of time, consuming just water and breathable air. How much weight people loose during 40 day water fasting? 1.8x40 (56) kg? No. The lucky ones lose 20kg. Some even less. Eight to twelve kg is typical. Why is that? When starting the fast, a person initially quickly loses weight, but mainly due to reduction of gut bacteria (the poop while cleaning the gut) and water loss (due to reduction of glucose and glycogen reserves which use water). But after that initial kick, a person losses around 200-250 g of weight per day. Why? Because during the fast, our energy comes from burning fat (mostly oleic acid) with oxygen. How much is burned? One gram of fat equals 9 kcal. So our body needs to burn just 2000 kcal/9=220 g of fat per day. So water and oxygen is essentially recycled. Taken to extreeme, a morbidly obese astronaut with weight (or better said, mass) of 300 kg could embark on three year mission to Mars. Without any food. And return after three years with 60 kg of weight. Of course, there are many micronutrients (for example, nitrogen, phosphorous, vitamin C, minerals etc.) that would need to be addressed but I ignore them for the purpose of discussion. 

Such obese person could survive multi-year mission just with the fat stored inside his/her body. Without any complex recycling equipment beside water recycling, electrolysis and Sabatier reaction. Without any additional storage or food packaging. I will ignore need for additional clothing since this is just an thought exercise. Who would send such obese people to Mars? And who would volunteer?

But knowing that is it enough to have just 220 g of fat per day gives us important clue. How can we mimic the same metabolic mechanism for person of normal weight? Instead of 1.8 kg * 365 days * 3 is around 2000 kg of food. What if we could send just 240 kg of food? Essentially we could send just the fat for each person. Is there such a diet? High fat diet? Actually there is. Most popular is ketogenic diet combined with intermittent fasting which mimics the similar cycle. Of course, this diet is usually combined with much lower amount of carbohydrates (up to 5%) and moderate amount of proteins (up to 25%). This is considered controversial topic in dietary circles (with opposing view led mainly by vegetarian/vegan movement). But it offers clear advantages from logistics point of view. 

To conclude, skinny young male vegan is the least favorable person for initial Mars missions. Instead, older female loving fat food would be an ideal candidate due to slower metabolism. No gardens necessary. Who would have thought that?