Perhaps one of the most important and controversial uses of satellites today is that of monitoring the Earth's environment. Many satellites study features on the ground, the behaviour of the oceans, or the characteristics of the atmosphere. Satellites that observe the Earth to collect scientific data are usually referred to as “Earth observation satellites.” Sometimes the interpretation of their data has been controversial because the interpretation is difficult and people have used the data to call for substantial changes in human behaviour.

The first satellite to be used for Earth observation purposes was Explorer VII, launched in October 1959. This satellite was equipped with an infrared sensor designed to measure the amount of heat reflected by the Earth. This measurement, referred to as the “radiation budget,” is a key to understanding global environmental trends, for it represents the difference between the amount of incoming energy from the sun and the outgoing thermal and reflected energy from the Earth. But it was not until the launch of the Earth Radiation Budget Satellite (ERBS) in 1984 by the National Aeronautics and Space Administration (NASA) that more authoritative readings of this important figure were obtained. Many Earth observation satellites like ERBS use specialized sensors that operate in non-visible wavelengths like the infrared, allowing them to gather data on many different types of atmospheric and ground phenomena.

The most important early Earth observation satellites were members of the Nimbus series. NASA launched eight Nimbus satellites between 1964 and 1978, with only one failing to reach orbit. Although they started out as part of the weather satellite program, the Nimbus satellites were not weather satellites, but carried a number of instruments for measuring the temperature and humidity of the atmosphere. This was a major advance, for earlier weather satellites like Tiros (Television Infrared Observation Satellite) had only been capable of taking visible light photographs of clouds and could not provide the kinds of traditional weather measurements that meteorologists normally used. Eventually many of the instruments demonstrated on Nimbus, named “sounders,” were incorporated into later weather satellites. Atmospheric sounders are now common on many meteorological satellites, as well as on scientific satellites and even planetary space probes.

 
The Tracking and Data Relay Satellite System (TDRSS) is a communication signal relay system that provides tracking and data acquisition services between low earth orbiting spacecraft and the facilities on the ground.

In July 1972, NASA launched the Earth Resources Technology Satellite (ERTS-1) into orbit. ERTS-1 used advanced instruments to view the Earth's surface in several infrared wavelengths. These sensors enabled scientists to assess vegetation growth, monitor the spread of cities, and make many other measurements of how the Earth's surface was changing. ERTS was so successful that it was followed by two more satellites named Landsat. By the early 1980s, with the launch of Landsat 4, the satellites became an “operational” system rather than an experimental one, and their data was heavily used around the world by farmers, urban planners, geologists and environmentalists. Landsat and similar satellites are often referred to as “remote sensing satellites,” a term that is usually used to refer to satellites that focus on the ground rather than the oceans or atmosphere.

In the mid 1970s NASA also conducted numerous observation experiments aboard the Skylab space station. Skylab was equipped with handheld as well as fixed cameras using special film. It also had an array of other instruments. Data the crews obtained during their three visits to Skylab was used to refine the instruments on other satellites, such as Landsat. Skylab also demonstrated the value of other observations, such as tracking icebergs and the breakup of sea ice.

In 1978 NASA launched SeaSat, an ocean observation satellite with a synthetic aperture radar, or SAR. SAR works by taking several radar images from different positions and combining them to produce a more detailed single image. SeaSat's radar produced detailed images of the surface of the ocean, providing valuable data on waves and the interaction of the ocean's surface with the winds. Although SeaSat's mission ended prematurely due to a malfunction, it demonstrated the immense value of space-based SARs.

 
The Earthïs radiation budget as seen by the Earth Radiation Budget Satellite.

Around the same time the United States was experimenting with SeaSat, the Soviet Union launched a similar series of satellites known as Okean. Later, during the late 1980s, the Soviet Union orbited several large radar satellites. These spacecraft, launched aboard Proton rockets, produced radar maps of the Earth's surface and were also used to measure waves on the oceans' surface. In 1991 the Soviet Union launched Almaz-1, which was another of this series of satellites but the first that the Soviet government openly acknowledged. Although they announced that this was a civilian Earth observation satellite and sought international customers, many experts speculated about the military uses of these satellites and their role in searching for objects such as submarines, which can create waves on the ocean surface when traveling at high speed at shallow depths. Because such data has military uses, SAR technology has always been sensitive. Although the Soviets attracted the attention of western military officials, they found no commercial customers for their satellite.

During the 1980s, and 1990s NASA, along with German and Italian participants, conducted several Space Shuttle missions carrying a large SAR in the Shuttle's payload bay. This radar, called SIR (for Shuttle Imaging Radar) produced topographical maps of much of the Earth's surface. The radar equipment was modified several times to collect more accurate data during the latter missions. In February 2000, NASA flew another mission called SRTM, for Shuttle Radar Topography Mission (SRTM), with Italian and German participation. This time NASA used a modified version of the radar capable of obtaining much more precise altitude data. Three-dimensional electronic maps produced from the SRTM data are highly accurate and can be used in aviation to guide aircraft and missiles, even over rough terrain like mountain ranges. In 1991 and again in 1995, the European Space Agency launched the ERS-1 and ERS-2 (European Remote Sensing) satellites. Both were equipped with SARs and were highly successful.

 
UARS collected data on the Antarctic ozone hole. This photo shows the massive ozone depletion in the Antarctic lower stratosphere caused by the high concentration of the radical chlorine monoxide. The data was collected September 18, 1992.

In September 1991, NASA launched the Upper Atmosphere Research Satellite, or UARS, from the Space Shuttle. UARS, as its name implies, was designed to study the upper regions of the atmosphere where sounding balloons and airplanes cannot reach. Other satellites are planned to make more detailed observations of this poorly understood region of the atmosphere. Many of the American Earth observation satellites operate low-altitude, high-inclinatio orbits and relay their data through the NASA Tracking and Date Relay Satellite System (TDRSS), a system of communication satellites flying in geosynchronous orbit.

In 1988 astronaut Dr. Sally Ride led a committee to evaluate America's future in space. One of her suggestions was that NASA focus more attention on environmental monitoring in response to increasing scientific discussion of global climate change, a program the agency called Mission to Planet Earth. As a result, NASA started the Earth Observing System, or EOS. At the turn of the century, a number of EOS satellites were launched, most importantly Terra and Aqua, to be followed by Aqua's sister-satellite Aura. Terra, as its name implies, is focused upon monitoring the Earth's surface. It is equipped with instruments like MOPITT, the Measurements of Pollution in the Troposphere, and MISR, the Multi-Angle Imaging Spectroradiometer. Aqua has instruments such as microwave, infrared, and humidity sounders. These provide information on clouds, precipitation, snow, sea ice, and sea surface temperature.

 
The flagship in NASA's Earth Observing System (EOS), Terra launched on December 18, 1999, and began collecting science data on February 24, 2000. The photo shows the ground near one of the long-dormant Three Sisters volcanoes in the Cascade Mountains of west-central Oregon has risen approximately 10 centimetres in a 10-by-20-km parcel since 1996, meaning that magma or underground lava is slowly flowing into the area, according to a research team from the U.S. Geological Survey.

In 1992, an Ariane 42P rocket launched a spacecraft named Topex/Poseidon. A joint French space agency (CNES-Centre National d'Etudes Spatiales) and NASA spacecraft, it was equipped with a radar altimeter to allow it to measure ocean topography, or surface features. Data gathered from Topex/Poseidon over years of operation have allowed scientists to accurately map ocean circulation, a key factor in understanding both global weather and climate change. In particular, Topex/Poseidon has been able to track the phenomenon known as El Niño, a warming of the ocean surface off the western coast of South America that occurs every four to twelve years. El Niño affects weather patterns in various parts of the world as well as fish and plankton populations. Another spacecraft, called SeaStar and carrying the Sea-viewing Wide Field-of-view Sensor, or SeaWiFs, was launched in 1997 to study biological organisms in the oceans such as algae and phytoplankton (microscopic marine plants).