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.
Vanguard TV0
Due to later problems with Vanguard it was not the first rocket to put into orbit an unmanned satellite. The first small-lift launch vehicle was the Sputnik rocket, it put into orbit an unmanned orbital carrier rocket designed by Sergey Korolyov in the Soviet Union, derived from the R-7 Semyorka ICBM. On 4 October 1957, the Sputnik rocket was used to perform the world's first satellite launch, placing Sputnik 1 satellite into a low Earth orbit.
The USA responded by launching the Vanguard rocket, that was intended to be the first launch vehicle the United States would use to place a satellite into orbit. Instead, the Sputnik crisis caused by the surprise launch of Sputnik 1 led the U.S., after the failure of Vanguard TV3, to quickly orbit the Explorer 1 satellite using a Juno I rocket launched on January 31, 1958. Thus Vanguard I was the second successful U.S. orbital launch. Thus started the space race, that gave the drive to put men on the moon with the USA's Apollo program.