officially Earth Resources Technology Satellites
Any of a series of unmanned U.S. scientific satellites.
The first three were launched in 1972, 1975, and 1978. They were designed mainly to collect information about Earth's natural resources. They were also equipped to monitor atmospheric and oceanic conditions and to detect variations in pollution levels and other ecological changes. Three more Landsat satellites were launched successfully, in 1982, 1984, and 1999; radio communication with Landsat 6 was lost immediately after its launch in 1993.