Let's face it: humans are destined for the stars. But it's an inhospitable world up there. On Earth, we have air, food and water, and it all gets recycled naturally and automatically. In space, we have to bring everything with us. It's sort of like a big camping trip. A very expensive one. Every pound we transport from Earth to Mars costs about $140,000! And for permanent missions-think lunar colony-bringing everything isn't even an option.

So what are we to do? Recycle -- Mother-Earth-style. NASA already recycles air and water on the International Space Station using machines. But machines use lots of energy and need a constant supply of filters and parts and other stuff. An alternative is to use biological methods like plants, algae and bacteria, all renewable resources if handled with care. In reality, we would use a combination of biological methods and machines to build what's called a regenerative life support system. For a crew of four on a mission lasting several years, here's how one might work:

What Will They Eat?

The crew will stick to veggies. Eating plants is much more efficient than eating animals that eat plants. Natural light or solar panels and LEDs (light-emitting diodes, like the lights in traffic signals) will shine on them. It takes an area of approximately 500 square feet to keep one person fed continuously, so our cosmic garden will be 2000 square feet, or almost half the area of a high school basketball court. The crops will be mostly potatoes, wheat, lettuce and soybeans, with other plants and herbs for variety. In the mood for sweet potato waffles? How about a tofu burger or a veggie sandwich?

How Will They Breathe?
The crops will take care of the air, too! They'll use all of the CO2 the crew exhales, and produce all the oxygen they need. Toxic gasses tend to build up from plants, humans and machines, so the air will be continuously moved through the plants' soil beds, where microbes will digest the bad stuff.

What about Waste?
Dishwater and bathwater will pass through filters made of microbes that eat the impurities. Urine will be frozen so the pure water crystals can be used again. Feces and inedible leftovers will go to a tank called a bioreactor. This tank full of hungry microbes will convert the waste to 97 percent water, 3 percent solids, and some gasses. For four people, the tank will need to hold 11 to 13 gallons. The system will run continuously. Once per day, a tenth of it will be drained off, and then more refuse will be pumped into the tank. The solid waste will become fertilizer, and the liquid will go to the plants too. The plants will absorb the water through their roots and draw it to their leaves. There it will evaporate, and the purified water will collect in a dehumidifier above. Drink up!

The idea of a planet in a box, where plants and dirt replace machines, sounds simple. But a lot of things can go wrong. Biological systems can be unpredictable. In this system, plants will compete for light and nutrients, and may suffer from fungal growth or microbial pests. Their rates of food and oxygen production change over time, and they might pass on toxic elements they collect. The bacteria population in a bioreactor might change drastically too. If any living thing in the life support system dies, the people could die. That's why, for the foreseeable future anyway, there will be machine backups to all parts of the system.

Expect years of research before NASA launches a full habitat like this. In the meantime, they'll keep building experimental systems on the ground, like BIO-Plex, where escape is just a hatchway away for the crew. Flight experiments don't go much beyond growing some bacteria or a few plants in space, for research or an occasional astronaut salad. But if humans expect to go much farther out there, expect that we'll be taking a piece of Earth with us.