In The Martian, why did NASA design the HAB to last only 31 days?

Question

At 17:11, Watney says that NASA designed the habitat to last for only 31 days on the surface of Mars.

Why did they not design something to last longer?

Answer 1

This is explained slightly later in the novel. The mission was only intended to last that long.

The surface mission was supposed to be thirty-one days. For redundancy, the supply probes had enough food to last the whole crew fifty-six days. That way if one or two probes had problems, we’d still have enough food to complete the mission.

_{^{The Martian: Andy Weir}}

Assuming the NASA in the books operates in the same way that our own NASA do, they will have used Reliability Engineering Analysis to make sure that there is a near-zero-percent chance of a critical equipment failure (mission-affecting or life-affecting) occurring within those 31 days.

NASA’s Reliability and Maintainability (R&M) program ensures that the systems within NASA’s spaceflight programs and projects perform as required throughout their life cycles to satisfy mission objectives. Mission objectives include safety, mission success and sustainability criteria.

_{^{NASA - Reliability and Maintainability Program}}

Typically the best way to ensure that a piece of equipment has the lowest possible chance of breaking during its projected mission is to massively over-engineer it, hence why Mark has so much redundancy in his equipment and why most of his kit lasts years instead of weeks. Note, for example that he has multiple additional valves. They aren't likely to be needed, but scrubbing a multi-billion dollar mission for the want of a $2 valve isn't something that NASA would countenance.

Getting the tubing through the balloon canvas wasn’t too hard. I have several spare valve patches. Basically they’re ten-by-ten-centimeter patches of Hab canvas with a valve in the middle. Why do I have these? Consider what would happen on a normal mission if the regulator valve broke. They’d have to scrub the whole mission. Easier to send spares.

_{^{The Martian: Andy Weir}}