Purdue University scientists are trying to create an enclosed system that relies on four crops to provide oxygen, water and food.

They are working under a $5 million grant from the National Aeronautics and Space Administration, which hopes to have small, self-supporting colonies of astronauts living on the moon by 2010 and on Mars by 2020.Cary Mitchell, a plant physiology professor and director of the research, said the work initially will focus on three plants that will comprise a complete diet for the astronauts: cowpeas, a relative of black-eyed peas; brassica, a mustard-like plant; and two strains of rice.

The hydroponic crops also would give the astronauts oxygen, Mitchell said.

The astronauts shouldn't expect gourmet cuisine, he said.

"The species that were chosen are just the bare minimum that will give a balance of protein and carbohydrates and fat. That doesn't mean the diet will be interesting," he said.

A fourth crop, blue-green algae, will help produce oxygen and consume carbon dioxide.

Unlike the bubble colonies of science fiction, astronauts and plants on the moon would live underground to protect them from the sun. Its damaging rays would be unshielded because the moon has no atmosphere.

"On Mars it's a different story. It would be interesting to use some natural sunlight there," Mitchell said.