Chapter 5

On Sol 38, Mark Watney plans to eliminate the hydrogen hazard by removing oxygen from the Hab so hydrogen cannot ignite. He lowers the Hab to 1°C to keep soil bacteria alive on minimal oxygen, bags the unsprouted potato plants and moves them to a heated rover, then tricks the atmospheric regulator into over-removing oxygen by feeding it pure O2 at a sampling intake.

Unable to reprogram the regulator, Watney tapes off intakes and uses Johanssen’s suit and a bag to spoof readings. Wearing a medical O2 mask for mobility and using Vogel’s suit sensors to monitor actual levels, he drops the Hab to ~1% O2. He then ignites controlled hydrogen burns using a spark from a stripped flashlight and a trickle of oxygen, producing water as planned—until an explosion blasts him across the Hab and rips off his mask.

Dizzy and oxygen-starved, Watney uses Lewis’s water-filled suit as an emergency O2 source, then reboots the regulator to restore Hab oxygen. He retreats to the rover for the night. On Sol 41, he methodically checks all systems—canvas integrity, oxygenator, regulator, heaters, power, tanks, reclaimer, airlocks, lighting, and computer—and finds no damage. Microscopy shows soil bacteria survived, preserving his potato farm.

Analyzing data, Watney concludes the pressure spike and heat came from hydrogen combusting into water, which then condensed, reducing pressure. He identifies the oxygen source as his own exhalations leaking from the non-airtight medical mask, enriching the Hab and enabling the blast. The water reclaimer captures ~50 L of new water, with ~10 L absorbed by soil; he dubs Lewis’s suit “The Cistern.”

On Sol 42, Watney returns the now-sprouting potatoes to the Hab and resumes hydrazine reduction. To prevent future buildup, he adopts frequent small “hydrogen cleanings” with a tiny flame between production cycles. He settles into a routine of ten-hour CO2 fill times, brief burns, and downtime, cautiously continuing water production.