This article from NASA takes a look at the Landsat project. It is interesting that computing power was the limiting factor at the time it was launched.
"Each Landsat image is several hundred Megabytes. Until recently, it would have taken a super computer to sort, join, and geolocate (map the latitude and longitude of) the thousands of images needed to make a global picture. “For decades, the computing challenge was huge,” says Masek."