The Red Planet has been a subject of fascination for humanity for thousands of years, becoming part of our folklore and popular culture. The most Earthlike of the planets in our solar system, Mars may have harbored some form of life in the past and may still possess an ecosystem in some underground refuge. The mysteries of this fourth planet from our Sun make it of central importance to NASA and its science goals for the twenty-first century.

In the wake of the very public failures of the Mars Polar Lander and the Mars Climate Orbiter in 1999, NASA embarked on a...