In October 1982, I was close to achieving intellectual escape velocity. I had just been selected to receive a grant from NASA to study how to grow food to sustain astronauts on long-term space missions. This is not a trivial task. Thirty-three years later, my students and I are still tinkering with the inputs to the space farm.
We have long been interested in the possibility of sustaining life away from Earth. Like Andy Weir's character, Mark Whatney, from The Martian, we calculate the mass and energy balances needed to maintain space colonies.
The photo below shows radish and lettuce plants growing under light emitting diodes (LEDs) in one of our research chambers. These plants are experiencing the "orbital photoperiod" of the International Space Station, which cycles every 90 minutes: 60 minutes of bright light followed by 30 minutes darkness. The crops are grown in soil-less media and watered with a hydroponic solution by drip irrigation. In preliminary studies, these crops have tolerated this with only a small reduction in growth compared to control plants in a 16 hour day/8 hour night Earth day.
There are many challenges and many benefits of growing food on Mars. For a long-term mission, it isn't cost effective to haul food to Mars if we can grow it there. Eat local.
Eating local isn't the only benefit. Crops do more than provide food. If we grow 100 percent of our food in a closed system, the photosynthesis of the crop plants keeps the oxygen and carbon dioxide in perfect balance. But these critical gasses are not in balance every minute of every day. Our plants do not automatically grow faster to provide extra oxygen just because we go for a run on the treadmill. We need buffers to stabilize their concentrations.
Optimizing the mass of these buffers is a huge challenge. They must be big enough to sustain life through times of instability, and yet small enough to be economical. We can calculate the effects of potential perturbations, but in life support systems, small and stable are oxymorons. For centuries, our massive oceans have been our buffer. Unfortunately, there are no oceans on Mars.
An adequate supply of fresh water is a second challenge with growing food on Mars. Plants use at least 200 liters of water to produce a kilogram of food. The good news is that plants recycle and filter water for free. We can put gray water on their roots and the water vapor that they "exhale" from the pores in their leaves (stomates) is more pure than the best bottled water. As long as we grow our food in a closed system, we will have ample clean water -- no high-tech filtering systems are necessary. And with the reports of Martian salt water this week, we can start a biological life support system by filtering the salt out of the water that is already there. Reverse osmosis filtration of sea water is already being used in water-limited cities on Earth. We can use this technology on Mars.
A third major challenge is getting enough light for photosynthesis. Unlike houseplants, crop plants cannot survive without bright light. They live in the photosynthetic fast lane. A well-lit office has less than 1 percent of outside sunlight and less than 3 percent of the minimum light needed to grow potatoes or other crops. To further complicate things, Mars is 1.5 times farther from the sun than the Earth and, although its thin atmosphere minimally filters solar radiation, it has only 60 percent of our light intensity at the surface. This means reduced electric power and reduced photosynthesis. Both processes follow the Stark-Einstein Law: One photon excites one electron. There are no magic shortcuts.
Except, perhaps, in science fiction.
In The Martian, Mark Watney magically grows potatoes with office lighting inside a habitat designed to block all electromagnetic radiation from the sun.
Designing a Mars greenhouse presents formidable challenges. It requires an exceptionally rugged, transparent membrane that can withstand meteorite bombardment without any leaks. It also needs to filter all of the cosmic radiation without filtering photosynthetic radiation. For crop plants to thrive there, we would likely use a cutting-edge technology from 2015: parabolic, concentrating reflectors on the roof and fiber optic transmission of sunlight. With concentrated sunlight and optimal environmental conditions, our calculations indicate that one person could grow all of their food in an area the size of a large living room (25 square meters).
Another point of science fiction is that Mark Watney survived on protein bars, vitamin pills and the carbohydrates in his potatoes for almost two years. We don't yet know the long-term complications of such a limited diet. We normally eat the products of hundreds of plants a week. Can we reduce this to 50 types of plants? Ten types of plants? Possibly, but we need long-term studies with people in closed systems on Earth to determine the effects of reduced diet diversity. To keep the area small, an efficient Mars diet will be strictly vegan and will not include fruits or nuts from trees.
Our early studies indicate an enormous psychological value of plants. Mark Watney reminisced about living with his potato plants. He missed them after they were gone. When hardened astronauts return to Earth, they repeatedly tell us of the bond they developed with the plants they were growing. Ten years ago, after almost a year in space, a cosmonaut summarized his psychological experience at a press conference: "Long-term space travel without plants is impossible."
The Earth is a closed system hurtling through space. Several of the best minds in the world are now focused on understanding the implications of one seemingly tiny change: The carbon dioxide in our atmosphere has increased from 0.03 to 0.04 percent over the past 100 years. We are only just beginning to study the impacts of this subtle change.
There is much we can learn from small-scale biological life support systems where changes occur rapidly. Perhaps Mark Watney's adventures will inspire the youth of the nation to continue our work.
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