Astronauts who traveled into space reported the following to ground control: "Looking down on the Earth from space, half of the world is on the right and the other half is on the left. We can see the Earth in its entirety. The Earth is very small." Both Russian and American astronauts reported that what impressed them most while on space flight is that in the unfathomable universe, the Earth really looked like a very small, uninteresting trifle.

However, we who live on Earth hold completely different feelings. The Earth is not small at all. It looks very big and it is also the place where continuous and dynamic change takes place. Its unique characteristics cannot be explained with one answer.

Only part of the answer can be found in the distance between the sun and the Earth. The Earth receives sunlight or heat from the sun that is not hot enough to make water boil or cold enough to make it freeze, just the right temperature so that water forms into seas. The other part of the answer is the existence of living creatures. Oxygen exists in the air because trees emit it into the air. And animals keep on living their lives through metabolism, that is, breathing in oxygen.

We can still find a part of the answer in the clash of sedimentary strata of the Earth and its fossil contents. This exposes fresh rocks that contain nutritive elements necessary for sustaining living beings. Yet there still is a different factor that is not yet confirmed. The astronauts' remarks about the unique peculiarities of the Earth also convey a significant message. The Earth is a closed planetary body with interaction in the solar system. This closed planetary body exchanges only energy in the magnetosphere, not material. The Earth receives energy from the sun and it emits long-wave energy back into outer space. Except for meteorites that fall to the Earth, the volume of material exchanged between the Earth and the cosmos amounts to nil.

The fact that the Earth is a closed planetary body conveys a significant message to us. Although such elements like water, calcium and carbon circulate through a varied and complicated process of interaction in the atmosphere, sea water and biosphere, the total volume of such elements is constantly fixed. Therefore, unless mankind embarks on an exploratory voyage toward outer space in search of new resources, the total volume of natural resources available on Earth is limited. This fact teaches us the grave lesson that mankind has to learn how to live within our own means.