The MIDAS Series 2 (Military Defense Alarm System) satellites, MIDAS 3, 4, and 5, were the second development models for the MIDAS early warning system.

MIDAS Series 2 carried a new infrared payload built by Baird-Atomic, one that featured 175 detectors capable of sensing ICBM targets at a maximum slant range of 4200 nm, The payload was designed to scan at a rate of 6 rpm, a rate of rotation three times faster than the Series I payloads.

The Agena-B upper stage of the Atlas-LV3 Agena-B launch vehicle was used as the spacecraft bus and provided power and attitude control to the MIDAS payload. The Agena-B was nearly twice the length of its Agena-A predecessor. The increased tankage and a new dual-burn rocket engine would permit reaching a planned circular polar orbit at an altitude of 3400 km, the orbit then considered most appropriate for an operational constellation of MIDAS satellites. Two deployable solar arrays were mounted on the aft equipment rack of the Agena-B to provide power.

On 21 October 1961, the Air Force launched MIDAS 4. An Atlas roll-control failure shortly after launch propelled the Agena into an improper ascent trajectory. After separating from the Atlas, the Agena used an abnormal amount of attitude control gas during the first and second burns as onboard systems sought to compensate for the trajectory dispersion. Once in orbit, the Agena's attitude continued to fluctuate and all control gas was exhausted by the time it completed its first revolution of the Earth. One of two solar arrays aboard the tumbling Agena failed during the fourth orbit, power depleted, and all electrical equipment was shut down after the 56th orbit.