Watching the Earth

The Stockholm conference has approved a global system for the monitoring of pollution. Next month, as a forerunner of such a system, the U.S. plans to launch an experimental satellite, known as ERTS (for Earth Resources Technology Satellite). A stubby, 1,965-lb. package that resembles an overgrown moth, the satellite will be equipped with three television cameras, a multi-wave-length sensor and a data collection system that can relay environmental information from as many as 1,000 automatic monitoring stations on earth. If the test is successful, ERTS-type orbiters could be used to sound an alarm whenever there is a threat of serious environmental danger: contamination of the seas, climatic changes, even volcanic eruptions.

Fired to an altitude of 492 nautical miles, the satellite is expected to operate for a year in a near-polar orbit that runs almost parallel to the earth's axis of rotation. Sweeping down from high above the Arctic Circle to Antarctica, it will then head back north every 103 minutes. This orbit has an important advantage: it will bring the spacecraft back over the same spot on earth every 18 days at almost exactly the same time of day. Thus, ERTS's photographs, each covering a 100-by-100-mile square, will be taken at each particular site under lighting conditions that remain unchanged except for the gradual seasonal variations in the angle of the sun and different cloud covers. Such consistency gives ERTS a unique ability to spot changes on the earth.

Built by General Electric for NASA'S Goddard Space Flight Center under a $174.6 million program, ERTS cannot see objects smaller than 300 ft. across, but it has very acute color perception. Each of its three RCA TV cameras responds to a distinctly different wave length of light--green, red and near-infra-red. Transmitted to earth, the three separate images can readily be combined into a single, detailed color picture, and ERTS can produce a total of 9,200 pictures a week.

ERTS's infra-red scanners can perform more subtle detective work, since every object, living or inanimate, emits or reflects the various wave lengths of infra-red light with a different intensity. Chlorophyll, for instance, a key chemical involved in the production of oxygen by green plants, has a very distinctive infra-red "fingerprint." Thus, by the color variations in photos, future ERTS satellites could quickly detect any large--and possibly dangerous --change in the chlorophyll content of ocean plankton, a principal source of the world's oxygen supply. By similar "fingerprinting," ERTS and its successors could warn of changes in the health of woodlands, detect harmful acidity in soil, find clues to new oil and mineral deposits, and perhaps even sniff out illegal fields of opium poppies.

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