MIDAS 3 (MIssile Defense Alarm System) was a satellite placed in a circular earth orbit to measure IR background and define IR sources. This was an overall test of a system for detection of missile launchings with satellite-borne IR sensors. The payload was the same as for MIDAS 2, except that an Agena B replaced the Agena A as the upper stage. The satellite carried experiments on cosmic radiation, atmospheric density, radiometric measurement, and micrometeorite detection, and had a plasma probe. The spacecraft weighed 3500 lb (including the second-stage casing), was chemical-battery-powered, and was launched from the pacific missile range. Useful IR radiation data were received for the lifetime of the battery pack.