If we restrict our conception of the relationship in Figure 2.1 to our planet and
the atmosphere around it, then clearly we do not have a closed system. We derive
most of our energy from the sun, either directly or indirectly. We have also sent
spaceships well beyond the boundaries of our atmosphere. Nonetheless, historically
speaking, for material inputs and outputs (not including energy), this system
can be treated as a closed system because the amount of exports (such as abandoned
space vehicles) and imports (e.g., moon rocks) are negligible. Whether the system
remains closed depends on the degree to which space exploration opens up the rest
of our solar system as a source of raw materials.