Eventually, the astronauts must return to World Biosphere. One major challenge for the future is for scientists to create a sustainable, artificial biosphere from raw materials. When this is achieved we will be ready to undertake exploration of deep space and with it the possibility of colonising new planets. Ecologists have drawn a strong parallel between our World Biosphere and the biosphere found within a space module. The space capsule has an internal biosphere that quickly becomes degraded.
Resources are finite and the space capsule biosphere becomes incapable of supporting the astronauts.
At a much a larger scale the World Biosphere is also capable of becoming degraded, for example by air pollutants. It is also capable of running out of essential resources - fresh water, timber, fisheries, agricultural land. For this reason, Ehrlich applied the term Spaceship Earth to our planet in an attempt to force its inhabitants to realise that our World Biosphere is not infinite in its supply of life-giving materials. The answer that only one biosphere exists is an oversimplification as within something as complex as the biosphere it is inevitable that there will be many variations. If we take a hypothetical cross section through the biosphere it would look like the diagram shown in Figure 2.