A dozen or so human-sized, egg-shaped pods may someday dot the surface of Mars, narrow ends pointing into an alien sky. Inside the translucent greenhouses: thickets of sweet potato vines, wheat or rice, food necessary if humans ever decide to explore the Red Planet. A reinforced greenhouse might stand nearby, sheltering strawberries, lettuce and spinach, things nutritious as well as psychologically important to people on the surface of an inhospitable planet about 50 million miles from Earth. This is the vision of eight students in the University of Colorado’s aerospace engineering department, who are designing a Martian greenhouse that could produce food and oxygen for 20 years.