In a cavernous arena outside of Peoria, Illinois, two industrial robots worked against the clock last weekend to finish their tasks. Each had been converted into a towering 3-D printer and programmed to build one-third-scale models of extraterrestrial habitats. For 30 hours over three days, generators chugged and hydraulics hissed as robotic arms moved in patterns, stacking long beads of thick “ink” into layers. Gradually, familiar forms began to emerge from the facility’s dirt floor: a gray, igloo-like dwelling and a tall, maroon egg. The machines belonged to two teams, one from Penn State and the other from a New York-based design agency called AI SpaceFactory, that were competing in the final phase of NASA’s 3D-Printed Habitat Challenge. Each team had to develop an autonomous printer that operated with as little human intervention as possible, used materials or recyclables found on Mars or the moon, and passed the scrutiny of judges as well as rigorous structural testing. The stakes were high. The winner would take home US $500,000. Lessons the teams learned would inform not only how humans might one day survive on Mars, but also how they might live more sustainably on Earth....