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The colour photograph of Earthrise, taken by Apollo 8 astronaut William A. Anders on December 24, 1968, is a powerful and eloquent image. One leading environmentalist has rightfully described it as "the most influential environmental photograph ever taken". Inspired by the photograph, economist Kenneth E. Boulding summed up the finite nature of our planet as viewed in the context of the vast expanse of space in his metaphor "Spaceship Earth". What had been perceived throughout human history as a limitless frontier had suddenly become a tiny sphere: limited in its resources, crowded by an ever-expanding human population, and threatened by our use of the atmosphere and the oceans as repositories for our consumptive wastes.