Earth observation satellite
Earth observation satellites or Earth remote sensing satellites are satellites specifically designed for Earth observation from orbit, similar to spy satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc. 
The first occurrence of satellite remote sensing can be dated to the launch of the first artificial satellite, Sputnik 1, by the Soviet Union on October 4, 1957. Sputnik 1 sent back radio signals, which scientists used to study the ionosphere.
NASA launched the first American satellite, Explorer 1, in January 31, 1958. The information sent back from its radiation detector led to the discovery of the Earth's Van Allen radiation belts.
The TIROS-1 spacecraft, launched on April 1, 1960 as part of NASA's TIROS (Television Infrared Observation Satellite) Program, sent back the first television footage of weather patterns to be taken from space.
, more than 150 Earth observation satellites were in orbit, recording data with both passive and active sensors and acquiring more than 10 terabits of data daily.
History of spaceflight
The Sputnik 1 became the first artificial Earth satellite on 4 October 1957. The satellite transmitted a radio signal, but had no sensors otherwise. Studying the Sputnik 1 allowed scientists to calculate the drag from the upper atmosphere by measuring position and speed of the satellite. Sputnik 1 broadcast for 21 days until its batteries depleted on 4 October 1957, and the satellite finally fell from orbit on 4 January 1958.