If the main life‑support system goes down, the first thing is to stay calm. In zero gravity, panic can turn a small problem into a big one because your body will start to move around and you’ll lose the sense of “up” that helps you orient. Secure yourself to the nearest bulkhead or use a tether to keep from floating into a panel or cable. Next, check the backup systems. Most spacecraft have a secondary oxygen supply and CO₂ scrubbers that can take over for a few hours. You’ll want to divert the main power to the backup, if you can. If you’re in a capsule, there’s usually a small portable oxygen tank on the crew seat; it’s designed to give you a few hours of breathable air until you can re‑connect to the main supply. While you’re dealing with the failure, conserve energy. Keep the lights dimmed, shut down non‑essential equipment, and limit movement. In zero gravity you’ll naturally drift, so staying still helps you use less power to maintain your position and keeps you from jostling the air flow too much, which could disturb the CO₂ scrubbers. If you have a personal survival kit, use it. Most crew members carry a small emergency kit with extra oxygen, a CO₂ absorber, and a compact battery pack for communication. Keep that kit accessible and use it before you try to re‑open the main hatch, which could be a big effort if the pressure is unbalanced. Finally, contact mission control right away. They can guide you through the checklist for a life‑support failure, tell you how long the backup will last, and plan a rescue or patch‑up. A clear head, a steady hand, and a backup system are your best friends when the main life support goes silent.

You’re right, the clock really starts ticking as soon as the main system cuts. In a real emergency the crew’s first instinct is to lock into the nearest tether and immediately shut off any non‑essential loads. That frees power for the CO₂ scrubbers and the backup oxygen. The trick is keeping those systems running until you can patch the primary or get a rescue. If the pressure keeps falling, you have to know the point at which the backup will run out and plan to move to a lower‑pressure module or seal off a habitable area. Always keep the comms open—mission control’s guidance is your lifeline.