It's still sounds like science fiction - but one day, space travelers could be flying to Mars. The moon too keeps plenty of researchers and space enthusiasts intrigued. After all, it could one day be used as a base station, from where astronauts could travel to planets farther away. But whatever happens, people will have to spend long periods on space craft or space stations - without regular Earth contact. And it's then that we'd be running into supply problems.

If we ever want to spend months or even years in space - totally independent and self-sufficient - it's not going to be enough for us to ship all the oxygen, food and drink that we'll need from Earth. For a start, we'll have to find a way of getting it up into space, and when you're traveling to space, every kilo counts.

Air out of water

This is where so-called life-sustaining circulatory systems come into play. Already, on the International Space Station (ISS), researchers have started reconstituting all kinds of things.

"Water is extracted from urine, leaving a concentrated residue, which is then shipped back to Earth on the next provisions flight. The water is cleaned up with chemicals and is fed back into the water circulation system," Gerhild Bornemann, a biologist at the German Aerospace Center (DLR), explains.

Water has a particularly important role: it's even used to reconstitute oxygen on the ISS using electrolysis. Electricity is passed through water to split oxygen and hydrogen. The hydrogen is let out into space, while the oxygen provides good breathing air for the cabin.

But the ISS has it easy: it's relatively close to Earth. Several times a year it receives fresh provisions of food and water. But if people start traveling farther away from Earth, they'll have to survive without the luxury of regular, fresh supplies.

And that's why Jens Bretschneider at the Institute for Space Systems in Stuttgart is looking for new solutions. His team thinks the answer lies in biological systems, like micro-algae: "They make it possible to collect exhaled CO2 and create new oxygen, and at the same time build up biomass stocks."

Food, energy and air - all made from algae

Brettschneider is working with a see-through plexiglass tank through which green water runs which bubbles away as exhaled air passes through it. "This tank is a flat panel reactor, with which we can cultivate algae on Earth in an efficient way," Brettschneider says. "The advantage is that the gas is mixing with the algae constantly. That gives us a large contact area. We agitate the algae so that they move towards the light, and then move away from the light again - and that encourages them to grow faster."

There are other types of algae that produce hydrogen instead of oxygen. These algae are anaerobic - which means they live in a reactor without oxygen. If one combines reactors with both types of algae, one can produce both oxygen and hydrogen. And, using fuel cells, one can create energy - with the production of water as a side effect.

"So you get this great circulatory system going, involving energy, food, oxygen and CO2. It's enabling us to close a few gaps in the life support system," Brettschneider says, pointing out the nutritious benefits of the reactor-made algae: "You can make it into a really nice paste, using a microwave or a ball mill, and mix it into your food. We can cover about 20 percent of an astronaut's daily nutritional requirements with micro-algae."

Fresh veggies from space

But imagine eating nothing but algae-paste - astronauts wouldn't stand for it! They want tasty foods too - and biologist Gerhild Bornemann has the answer: Tomatoes and other vegetables can grow in glass tubes which have water piped through them - similar to large glasshouses on Earth.

The glass tubes are filled with lava into which the plants' roots grow and take hold. The lava then acts as a growth substrate - and it helps with composting, according to Bornemann. "The lava acts as a carrier for the micro-organisms that allow the metabolic processes to function … so basically, we're trying to combine the processing of organic waste with the production of the food."

And this is where an astronaut's urine can be useful. "We dilute the urine - the urea in it contains nitrogen, which is needed in fertilizer for plants," Bornemann explains, "bacteria then convert the urea into nitrate - a typical fertilizer."

The system can easily compost other solids, too - such as leaves and stems. "In a system like this, with water, the bits are first shredded, and then passed over a filter. They're then metabolized, just like in a compost-heap, but without soil, and it's quicker, too, because there's a constant flow."

It would be even quicker if people could take fish into space - because fish can keep the system going. "Using fish makes it faster, because they eat the bits in the system, so it's pre-processed," Bornemann explains.

It wouldn't be the first time that fish had been to space - they've been out there for research purposes before, but never as part of a vegetable garden. In any case, the system also works well here on Earth, for example at the DLR offices: "We've had the units set up here in our offices, and we've tried the system with an old bread roll, and it works… and that way we grow these little red tomatoes!"