On August 14, 1959, the Explorer 6 satellite opened its electronic eye and showed us the Earth in a way it had never been seen before. Even with limited resolution, early satellite views of our planet were a revelation. "When the first images appeared, people would talk about the folds in the Appalachian Mountains," says James Irons, a veteran scientist at the Earth Sciences Division at NASA's Goddard Space Flight Center. "There had been textbooks written that described the processes that lead to those formations. For the first time it was possible to observe from great height what people had been talking about for hundreds of years." In the half century since, researchers have gained another shift of perspective-not just of space but of time, as satellite sensors document changes to Earth during their repeated orbits around the world. Some of the visible alterations result from geologic events such as volcanic eruptions. Others originate from ecological disturbances created by our species. Many times, the interaction between natural and human action is of interest. The global maps on the opposite page, produced by the Atmospheric Infrared Sounder (AIRS) instrument aboard NASA's Aqua satellite, capture a particularly important interaction.
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