In the beginning of the eassay lifeboat ethics what do environmnetalists compare to a spaceship?

In the beginning of the essay "Lifeboat Ethics," environmentalists compare Earth to a spaceship, emphasizing the idea that our planet is a closed system with limited resources. Just as a spaceship must manage its supplies carefully to sustain its crew, so too must humanity recognize the constraints of our ecological and resource limits. This analogy serves to highlight the need for responsible stewardship and the ethical implications of resource distribution among nations and populations.