They are images of Earth taken at an altitude of 567 miles (912 kilometers) by an orbiting Landsat satellite, or ERTS (Earth Resources Technology Satellite). The Landsats were originally launched in the 1970s. Rather than cameras, the Landsats use multispectral scanners, which detect visible green and blue wavelengths, and four infrared and near-infrared wavelengths. These scanners can detect differences between soil, rock, water, and vegetation; types of vegetation; states of vegetation (e.g., healthy/unhealthy or underwatered/well-watered); and mineral content. The differences are especially accurate when multiple wavelengths are compared using multispectral scanners. Even visible light images have proved useful—some of the earliest Landsat images showed that some small Pacific islands were up to 10 miles (16 kilometers) away from their charted positions.

The results are displayed in “false-color” maps, where the scanner data is represented in shades of easily distinguishable colors—usually, infrared is shown as red, red as green, and green as blue. The maps are used by farmers, oil companies, geologists, foresters, foreign governments, and others interested in land management. Each image covers an area approximately 115 square miles (185 square kilometers). Maps are offered for sale by the United States Geological Survey.

Other systems that produce similar images include the French SPOT satellites, Russian Salyut and Mir manned space stations, and NASA’s Airborne Imaging Spectrometer, which senses 128 infrared bands. NASA’s Jet Propulsion Laboratories are developing instruments that will sense 224 bands in infrared, which will be able to detect specific minerals absorbed by plants.