Sending astronauts to Mars is a big challenge. Astronauts will have to take everything they need, including food, air and water. The room inside the spaceship is too limited to hold many things. So packing a spaceship is a huge challenge for a trip to Mars. 

Six astronauts on an eight-month trip to Mars will need at least 18,000 kilograms of water for drinking and washing. They'll also need water to protect themselves from space rays which can pass right through spaceship walls and harm astronauts' living cells. But a layer of water placed around the ship can stop the harm.

That's a lot of water. And sending water into space is too expensive. It costs $33,000 to send one cup of water to Mars. The more you take, the more it costs. We have the technology to get to Mars, but the problem is that it's too expensive.

But Flynn an engineer at NASA, thinks he has a way of solving the problem. Instead of building a heavy spaceship and packing it full of stuff (物料), why not use the stuff as part of the spaceship?

Imagine a spaceship that blows up like a balloon. Inside the walls have pockets like a big honeycomb (蜂巢). These hold lots of plastic bags filled with water, dried food and algae (水藻). All these make hard walls and great radiation (辐射) protection. And stopping radiation doesn't influence the food or water itself—it's safe to eat and drink. And what if the same water could be reused again and again? Flynn and his team call their packed-pocket idea Water Walls.

Making oxygen and taking away CO₂—the waste gas you breathe out—is another problem in space. "Water Walls will take away CO₂ the same way it's done here on our Earth—with living plants," Flynn says. Living plants take in CO₂ to make their food, and give off oxygen. "Plants are hard to grow in a spacecraft, so we use algae," he says.

Algae are tiny plants that live in water. The algae will live in bags in the walls too, fed by human waste and sunlight, eating up CO₂ and making oxygen for the astronauts to breathe. 

Flynn thinks his onboard recycling system has many advantages. He hopes it will help make flying to Mars a reality very soon.