Pocket Book - Used
I'm stranded on Mars.
I have no way to communicate with Earth.
I'm in a Habitat designed to last 31 days.
If the Oxygenator breaks down, I'll suffocate. If the Water Reclaimer breaks down, I'll die of thirst. If the Hab breaches, I'll just kind of explode. If none of those things happen, I'll eventually run out of food and starve to death.
So yeah. I'm screwed.