What comes out of the human body and goes into a toilet is often thought of as an end product. But for future residents of Mars, human waste will need to take on a more vital role.

Like becoming fertilizer for growing food.

Because it costs thousands of dollars to lift every pound of stuff into space, and even more to get it to Mars, mission planners don't want to waste a thing. Human excrement, being rich in nutrients, can be a composter's dream. So a group of European researchers is taking on the task of figuring out just how to safely manage a high-tech Martian outhouse.

Because somebody, of course, has to do it.

The European Space Agency is building an "artificial ecosystem" near Barcelona, Spain, that will use three rats to test an idea for processing rodent waste with microorganisms. The rats will be kept under close veterinary supervision throughout the test, according to ESA officials.

To the possible chagrin of countless women around the world of a certain name, officials have given the project the imperfect acronym MELISSA, for Micro-Ecological Life Support Alternative. Scientists say MELISSA will build upon a system on the International Space Station that purifies water and recycles exhaled carbon dioxide (but does not recycle organic waste).

In case you're wondering: Yes, the scientists involved in MELISSA have considered that this is not the most glamorous space-research project ever undertaken.

More to the point: "Psychological issues and microbial security issues have to be very, very seriously studied, and we are working on these issues," said Christophe Lasseur, a scientist from the Netherlands who is a spokesman for the project.

Lasseur told SPACE.com that the goal is "to build and perfectly control an artificial ecosystem which produces food, water and oxygen for the crew with a minimum level of inputs and outputs to and from the loop."

The loop is a series of five interconnected chambers. In three of them, waste will be progressively broken down by different fermentation processes. In the fourth, algae or plants will be grown to produce the stuff intended to be eaten, drunk and breathed.

The rats (or "consumers" as the scientists call them) live in the fifth chamber, presumably a hut too small for humans but representative of an environment colonists might one day inhabit on Red Planet.

The idea of making compost out of human waste is not new. Asian farmers were doing it more than 3,000 years ago. More recently, a man named Joseph C. Jenkins wrote a book, "The Humanure Handbook: A Guide to Composting Human Manure," about it.

Compared to the ancient ways, Jenkins said the European approach is more scientific, a "conscious use of controlled biological processes to do such recycling." Technically, it isn't really composting at all.

But is it safe?

"Of course," Jenkins said. "A simple microbial biological process will render human manure hygienically safe for human food production."

In fact, your morning orange juice might be a beneficiary of such a process.

Americans produce about 20,000 tons of sludge every day, according to the Environmental Protection Agency (EPA). Sludge is the solid leftovers after a sewage treatment plant removes some or most of the liquid from everything that's flushed down toilets or otherwise flows into drains. Federal law says sludge can no longer be dumped in the ocean. So the name was changed to something a little more hip -- biosolids -- by the EPA.

Dried and made into pellets, sludge has a reputation in some quarters as being a darn good fertilizer. Or even a fuel.

Almost all states in the U.S. allow biosolids to be used to foster plant growth, said Ellen Harrison, director of the Cornell Waste Management Institute, which studies the science of this sensitive topic and advises local, state and federal agencies. And most states, under pressure to find less expensive alternatives to landfills and incineration, promote the concept.

Golf courses and cemeteries are among the big markets for this fertilizer. Even some commercially available garden compost products contain nutrients leftover from things you may have once eaten. State and federal laws require treatment for potentially dangerous pathogens first.

Harrison said most of the large-scale "land application," as it is called, involves range or pasture land, cotton crops, corn grown for animals, and similar indirect food uses. But sludge has been used to grow food for human consumption -- in cases that have raised the ire of consumer groups. Harrison said sludge pellets are used in Florida for citrus crops, for example.