The Environmental Research Satellite (ERS, alternatively Earth Resources Satellite) program was a series of small satellites initially operated by the United States Air Force Office of Aerospace Research. Designed to be launched "piggyback" to other satellites during launch, detaching once in orbit, they were the smallest satellites launched to date—what would today be classified as microsatellites. 33 ERS satellites in six different series were launched between 1962 and 1971,[1] conducting scientific research and serving as test beds to investigate the reliability of new spacecraft components.[2]