|To Get to Mars, Use Wheat|
|Contact: Dan Krotz, firstname.lastname@example.org|
Sometime in the future, on a spacecraft en route to Mars, an astronaut may reach into a container and grab a handful of wheat straw. She'll hold the key to a sustainable mission, something that converts incinerated waste into fertilizer for the plants she eats and nitrogen for the air she breathes.
Not bad for straw, the inedible portion of wheat used to fill horse stables on Earthand not much else. Its journey from the farm to Mars comes by way of a team of Lawrence Berkeley National Laboratory and 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 who led the research.
Here's the problem, and the opportunity: a roundtrip mission to Mars, a distant goal of the space program, will take about three years. It's impossible to pack several years of provisions into a tiny spacecraft, so the crew will have to grow food such as wheat along the way. This means they'll also have to somehow acquire enough fertilizer to sustain several harvests.
But with food comes waste. The astronauts must incinerate unused plant fiber and their own waste, a process that yields reusable compounds like carbon dioxide, water, and minerals, as well as noxious pollutants like nitrogen oxides and sulfur dioxide.
It's the making of a short-lived cycle: a dwindling supply of plant fertilizer and a growing pool of pollutants. But it also has the making of a sustainable system. Locked in the pollutants are nutrients that can help grow the next batch of plants. Nitrogen oxides can be converted into the fertilizers ammonia and nitrate. They can also be converted into nitrogen, which can replenish 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 fooda textbook sustainable system.
The trick is stripping nutrients from the pollutants, a routine technology on Earth. One method uses catalysts with limited life spans. Another relies on spraying an alkaline solution through incinerated waste. Although the methods work down here, space travel presents complications. Short-lived materials are an instant deal-breaker on a multiyear mission, and sprays misbehave in a low-gravity environment. Constraints pile up quickly in a tiny capsule hurling toward Mars.
"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 look far. As long as astronauts grow wheat they'll have a steady supply of straw. And if straw is converted into activated carbon, it could facilitate a cyclical flow of food, waste, and nutrients. To determine if the system works, Chang's team shredded straw into tiny bits and heated it in an oxygen-free chamber to 600 degrees Celsius. This converts the cellulose into char, a hydrocarbon product formed during the incomplete burning of organic material. Next, the char is activated by heating it in the presence of carbon dioxide or water, which breaks the char's carbon-carbon bonds. This increases the substance's surface area and porosity. The broken carbon bonds also create unpaired electrons that are ready to bind with new compounds.
This activated carbon is placed inside a steel tube and exposed to a gaseous stream of incinerated waste, and its pollutants. Nitrogen oxides are grabbed, with the aid of oxygen, 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 it 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 it sustainable month after month? Early calculations are optimistic. A six-person crew would eat 1.5 kilograms of wheat per day, a pace that could yield 203 kilograms of wheat-straw-derived activated carbon each yearenough 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 in Moffet Field, California, are currently conducting larger scale tests.