They were a series of solar-orbiting, spin-stabilized, solar cell- and battery-powered satellites designed to obtain measurements on a continuing basis of interplanetary phenomena from widely separated points in space.

They were designed to measure large scale magnetic phenomena and particles and fields in interplanetary space.

Data from the vehicles have been used to better understand stellar processes and the structure and flow of the solar wind.

[5] The experiments studied the positive ions (cations) and electrons in the solar wind, the interplanetary electron density (radio propagation experiment), solar and galactic cosmic rays, and the Interplanetary Magnetic Field.

[5] Instruments differed somewhat between spacecraft, with some being used in all four missions: By ground command, one of five bit rates, one of four data formats, and one of four operating modes could be selected.

In the memory readout mode, data was read out at whatever bit rate was appropriate to the satellite distance from Earth.