BIOPSPHERE Ⅱ: EARTH IN MINIATURE

                  A biosphere is a sphere of life--a closed system of living things that is completely self-sufficient and cut off from any other system. So far as is ___51___, the Earth is the only natural biosphere that exists. The Earth supports a ___52___ balanced, delicate network of life, and the only outside help it ___53___ is the energy of the sun.
                  There may be other biospheres in ___54___ space, and people may someday discover them. But the other planets of our solar system, at least, seem barren and ___55___ If people are to ___56___ these planets--and perhaps one day colonize them--they will have to create their own biospheres, just as ___57___ and just as self-sustaining as Earth's system. Can this be done?
                  To find out, a group of researchers has begun an extraordinary __58___: the creation of a separate, enclosed biosphere in the middle of the Arizona desert, near Tucson. Called Biosphere Ⅱ, this huge glass structure will have its own ___59___ of air, water, and nutrients. It will be stocked ___60___ a mixture of plants and animals that, the researchers hope, can live together in perfect ___61___. And in late 1990, after the structure has been completed, eight researchers will live in it for two years, cut off from the rest ___62___ the world. Once the door is closed, only sunlight will enter the structure.