To Get to Mars, Use Wheat
Sometime in the future, in a spacecraft en route to Mars, an astronaut may reach into a container and grab a handful of wheat straw. She'll be holding the key to a sustainable mission, the substance that will convert her incinerated waste into fertilizer for the plants she eats and nitrogen for the air she breathes during her three-year round trip.
Not bad for straw, the inedible portion of wheat used as bedding in horse stables on Earth. Its journey from the farm to Mars comes by way of a team of Lawrence Berkeley National Laboratory (Berkeley Lab) and National Aeronautics and Space Administration (NASA) scientists whose work brings our closest planetary neighbor even closer.
"To get to Mars, we need to develop a fully regenerative life-support system," says Ted Chang, a senior scientist in Berkeley Lab's Environmental Energy Technologies Division (EETD) who led the research.
Here's the problem and the opportunity: it's impossible to pack into a tiny spacecraft enough provisions for a whole three-year round trip to Mars, so the crew members will have to grow their own food along the way. This means they'll need enough fertilizer to sustain several harvests.
At the same time, the process of growing and eating food produces waste. The astronauts will have to dispose of not only their own waste but also unused plant fiber. Incinerating these waste products yields reusable compounds like carbon dioxide, water, and minerals, along with noxious pollutants like nitrogen oxides and sulfur dioxide.
This is the making of a short-lived cycle: a dwindling supply of plant fertilizer and a growing pool of pollutants.
But it could also be the making of a sustainable system. Locked in the pollutants are nutrients that can help grow new crops (see Figure 1). Nitrogen oxides can be converted into the fertilizers ammonia and nitrate. They can also be converted into nitrogen to replenish the nitrogen in the spacecraft's air supply. And sulfur dioxide can be converted into sulfate, another fertilizer. Food to waste to nutrients, then back to food; it's a textbook sustainable system.
The trick is stripping nutrients from the pollutants. This process is routine on Earth; one common method uses catalysts with limited life spans, and another relies on spraying an alkaline solution through incinerated waste. However, these methods will not work in space. Short-lived materials will not survive a multi-year mission, and sprays misbehave in a low-gravity environment. "You can't use expendable materials, gravity-dependent processes, or dangerous gases," says Chang. "So we focused on material that is available and can be continuously regenerated."
Chang didn't have to look far. As long as astronauts grow wheat, they'll have a steady supply of straw. Straw that is converted to activated carbon could facilitate a cyclical flow of food, waste, and nutrients.
To test the idea, Chang's team first shreds straw into tiny bits and heats it in an oxygen-free chamber to 600 degrees Celsius. This process converts the cellulose into char, a hydrocarbon product formed during the incomplete burning of organic material. Next, the char is activated by heating in the presence of carbon dioxide or water, which breaks the char's carbon-carbon bonds and increases its surface area and porosity. The broken carbon bonds also create unpaired electrons that are ready to bind with new compounds.
The activated carbon is placed inside a steel tube and exposed to a gaseous stream of incinerated waste and its pollutants. With the aid of oxygen, nitrogen oxides are grabbed by the carbon column's unpaired electrons and adsorbed onto its pores. A final step, in which the column is heated, regenerates the activated carbon and converts the adsorbed pollutants into nitrogen gas. Alternative steps yield other useful compounds: exposing the column to water produces nitrate, and, if the column has adsorbed sulfur dioxide as well as nitrogen oxides, exposing the column to water produces ammonia. To replenish the small portion of activated carbon lost in the final heating step, the astronauts can simply harvest more wheat straw.
But is this process sustainable month after month? Early calculations are optimistic. A six-person crew would eat 1.5 kilograms of wheat per day, which could yield 203 kilograms of wheat-straw-derived activated carbon each year-enough to supply the crew's needs.
"Waste has nutrients that shouldn't be thrown away, and in fact could help sustain a mission for its entire duration," says Chang. "Our method could allow astronauts to reuse valuable resources."
To further evaluate the Berkeley Lab system, scientists at the NASA Ames Research Center at Moffet Field, California, are currently conducting large-scale tests. The research appears in the September/October 2003 issue of Energy & Fuels.
For more information contact:
- Ted Chang
- (510) 486-5125; fax (510) 486-7303
This research was sponsored by the National Aeronautics and Space Administration's Ames Research Center.