More than two thousand satellites are currently in orbit. They measure the earth's surface characteristics, ocean currents, clouds and the gaseous content of the atmosphere. One of the oldest programs is Landsat. Run by both the National Aeronautics and Space Administration (NASA) and the United States Geological Survey (USGS), Landsat is still widely used today because it provides its data to researchers free of charge.
The Landsat program—a series of satellites that continuously record information about the earth's surface—has contributed to research on an enormous variety of land-use, ecosystem and development issues.
Landsat data have been used to study mitigation of deforestation and eutrophication; prediction of the effects of climate change; quantification of urban sprawl; and early-warning and response to extreme weather, natural disasters, and vector-borne diseases.
Read more at 'Observing Earth from Space: Landsat Data and Applications'