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.
Explorers Program
The Explorers Program was the United States's first successful attempt to launch an artificial satellite. It began as a U.S. Army proposal (Project Orbiter) to place a, "civilian", scientific satellite into orbit during the International Geophysical Year; however, that proposal was rejected in favor of the U.S. Navy's Project Vanguard, which included the first sub-orbital flight Vanguard TV0. in December 1956. The Explorers Program was later reestablished to catch up with the Soviet Union its launch of Sputnik 1 on October 4, 1957 sparked the Sputnik crisis. Explorer 1 was launched on January 31, 1958, becoming the first U.S. satellite, as well as discovering the Van Allen radiation belt.
After NASA was established in 1958, the Explorers Program was transferred from the US Army. NASA continued to use the name for an ongoing series of relatively small space missions, typically an artificial satellite with a specific science focus. Explorer 6 in 1959 was the first scientific satellite under the project direction of NASA's Goddard Space Flight Center. Over the following two decades, NASA has launched over 50 explorer missions, some in conjunction to military programs, usually of an exploratory or survey nature or had specific objectives not requiring the capabilities of a major observatory. Explorers satellites have made many important discoveries on: Earth's magnetosphere and the shape of its gravity field; the solar wind; properties of micrometeoroids raining down on the Earth; ultraviolet, cosmic, and X-rays from the Solar System and universe beyond; ionospheric physics; Solar plasma; solar energetic particles; and atmospheric physics. These missions have also investigated air density, radio astronomy, geodesy, and gamma ray astronomy.