ERS (Environmental Research Satellite) are spacecraft designed for piggyback-launching from large primary mission vehicles. With masses of 0.7–45 kilograms and payloads of 1–14 experiments, the ERS hitchhikers provided a low-cost, flexible way of making scientific and engineering measurements in space. One of their major roles was to act as a test-bed to determine the reliability of improved components and subsystems destined for use in later generations of spacecraft. A unique feature of the system was its ability to function without a battery by having solar cells fastened to all exterior surfaces. At least 12 satellites were launched from 1962 to 1969 for a variety of missions and sponsors.