A series of small- to intermediate-sized spacecrafts at the centre of NASA’s Earth Science Enterprise (ESE). The NASA Earth Observing System (EOS) program was initially established in the 1980s, with the inaugural mission results unveiling critical components of the Earth system. Further development of the program took place throughout much of the early 1990s, when more progressive exploration was undertaken.
The EOS is comprised of an array of coordinated polar-orbiting satellites that are specifically designed to study global climate change on a long-term observational basis. Space-, aircraft-, and ground-based measurements are collected of the land surface, biosphere and atmosphere, in order to further enhance our knowledge of Earth as an integrated system. Attention is primarily focused on missions pertaining to the following climate topics: radiation, clouds, water vapor, and precipitation; the oceans; greenhouse gases; land-surface hydrology and ecosystem processes; glaciers, sea ice, and ice sheets; ozone and stratospheric chemistry; and natural and anthropogenic aerosols.
The design and implementation of the ESE requires a cumulative international effort. The ESE program involves the cooperation of the U.S., the European Space Agency (ESA), and the Japanese National Space Development Agency (NASDA). It is also part of the U.S. interagency effort; the Global Change Research Program.
Source: NASA (http://eospso.nasa.gov/mission-category/3)