When it comes to human spaceflight, what goes up does indeed need to come down, typically at significant expense on both legs of the journey.
A team of NASA engineers and scientists thinks there is a better way -- turning all that trash into rocket gas and other useful items.
"The goal is to make sure what you ship up in space you actually use," said chemist Paul Hintze, who oversees the Trash to Supply Gas project at the Kennedy Space Center in Florida.
Hintze and colleagues are researching several different technologies for turning trash and other waste materials, including human waste, into products of value.
Initially, the project focused on producing methane, which could be used for rocket fuel. The scope has since expanded to include oxygen and water, which can be used for life support, carbon dioxide, which could support plant growth, fuels for power systems, and other materials.
"There's also a whole series of chemistries where you can take trash and convert it into something completely different," Hintze said.
The space shuttles used to return home from the International Space Station loaded with trash from the orbital outpost. Now, most of the station's garbage is packed into cargo ships that are released and burn up in the atmosphere.
"We want to make something useful out of that trash," Hintze said.
The initiative is similar to efforts on Earth to recycle trash, but adapting the technologies for spaceflight adds complexities. For example, because it is expensive to fly anything to orbit, equipment needs to be miniaturized. The process also needs to be highly automated to minimize crew time.
"We don't want to produce anything hazardous and we don't want to produce anything that smells really bad," Hintze said. "Cleaning and purification of these gases is going to be a challenge."
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Combustion alone doesn't convert everything in trash into gas. Aluminum in the food packaging and salts in uneaten foods, for example, produce ash, which has to be dealt with. Other materials and processes leave behind waxes and tars.
The system should be able to accommodate human waste as well.
"Because we're doing a full oxidation of the product, we won't have any microbes coming out the back end," Hintze said.
The project began last fall with development of prototypes. The team is currently analyzing which designs are most efficient and plan to home in on the best technology for possible future test flight.
Although a prototype may one day fly on the space station, the real goal is to develop a system for astronauts traveling beyond the station's orbit, where a continuous line of re-supply ships will not be possible.
"By converting waste materials that would be otherwise useless to propellants or fuels, the need to launch fuel to locations beyond Earth orbit is reduced," Hintz wrote in a paper presented at the American Institute of Aeronautics and Astronautics conference in Pasadena, Calif., earlier this month.
Recycling also would resolve the problem of what to do with the approximately 11 pounds of waste generated daily by a four-person crew.
"There is tremendous value to be gained by conversion of waste to fuels, as well as protecting the habitable environment," Hintz wrote.