Explorer 12, also called EPE-A or Energetic Particles Explorer-A and as S3), was a NASA satellite built to measure the solar wind, cosmic rays, and the Earth's magnetic field.

[2] Explorer 12 was designed to study space physics, and so had a multitude of instruments including a cosmic-ray detector, a particle trapper, and a magnetometer.

Its instrumentation included 10 particle detection systems for the measurement of protons and electrons and their relation to magnetic fields, a solar cell damage experiment, optical aspect sensor and one transmitter.

One analog channel was subcommutated in a 16-frame-long pattern and was used to telemeter spacecraft temperatures, power system voltages, currents, etc.

[2] The experiment was designed to measure the flux and energy spectrum of charged particles and cosmic rays and to determine their spatial and temporal distribution over the spacecraft orbit.

All directional detectors were mounted so that the axes of their fields of view were perpendicular to the satellite spin axis.

[4] An electrostatic analyzer with a current collector was used to investigate boundary phenomena between the geomagnetic field and the interplanetary plasma and to monitor low-energy proton fluxes at lower altitudes.

[6] This experiment was designed to measure the directional fluxes and spectra of low-energy trapped and auroral protons and electrons.

The energy fluxes of electrons in three ranges were measured separately using scatter geometry, absorbers, and the phototube current.

Except for saturation of some of the proton channels in the heart of the outer belt, the experiment worked properly throughout the life of the spacecraft.

On each orbit, severe degradation began about 2.5 hours before perigee, when the altitude of the satellite was approximately 33,000 km (21,000 mi).

No solar cell degradation was indicated of the banks with 20- and 60-mil shielding when their outputs were compared on 19 September and 3 December 1961, when the incident-Sun angle was the same.

The spacecraft functioned well until 6 December 1961, when it ceased transmitting data apparently as a result of failures in the power system.

[2] A back-up satellite of the Explorer-12 is on display in the Space Science exhibition station at the Steven F. Udvar-Hazy Center in Chantilly, Virginia.