Astronaut missions to Mars call for voyages remote from Earth on unprecedented scales of time and distance. Mission success will center around the management of certain critical compounds containing the atoms H and O. This generalization is true for both the life support systems and propulsion systems (for early missions at least). Mission lengths of one to three years allow virtually no possibility of timely rescue. Nonetheless, even with one or more major system failures, contingency mode H/O utilization can provide the key to survival on the road back to Earth.