(Above) This is a 1971 artist’s concept showing TRW’s small lunar sub-satellite being ejected into lunar orbit from the SIM bay of the Apollo 15 Service Module.
The 80 pound satellite remained in lunar orbit returning data from August 1971 to January 1973. Its main objectives were to study the plasma, particle, and magnetic field environment of the Moon and map the lunar gravity field. Specifically it measured plasma and energetic particle intensities and vector magnetic fields, and facilitated tracking of the satellite velocity to high precision. A basic requirement was that the satellite acquire fields and particle data everywhere on the orbit around the Moon.
A virtually identical sub-satellite was also deployed by Apollo 16