The lunar penetrometer was a spherical electronic tool that served to measure the load-bearing characteristics of the Moon in preparation for spacecraft landings.
[7] At the time, immense pressures from the ongoing Space Race caused NASA to shift its focus from conducting purely scientific lunar expeditions to landing a man on the Moon before the Russians.
While radio and optical technology situated on Earth at the time could make out large-scale characteristics such as the size and distribution of mountains and craters, there wasn't an Earth-based method of measuring small-scale features, such as the lunar surface texture and topographical details, with adequate resolution.
[8][9] In 1961, NASA's chief engineer Abe Silverstein proposed to the U.S. Congress that Project Ranger would help provide important data on the Moon's surface topography to facilitate the Apollo lunar landing.
Once funding was provided to the Ranger program, Silverstein directed NASA laboratories to investigate potential instruments that could return information on the hardness of the lunar surface.
Knowledge of the complete impact acceleration time history would have also made it possible for NASA researchers to ascertain the physical composition of the soil and whether it was granular, powdery, or brittle.
[10] In order to function properly, the lunar penetrometer was designed to sense the accelerations encountered by the projectile body during the impact process and telemeter the collected information to a nearby receiving station.
[12] The lunar penetrometer's spherical instrumentation compartment had an omnidirectional acceleration sensor located at the center surrounded by concentrically placed batteries and electronic modules.
[11] The researchers at Harry Diamond Laboratories originally employed a hollow piezoelectric sphere but later transitioned to modifying a conventional triaxial accelerometer.
[11] Due to limitations in available power, antenna efficiency, and other factors, the impact acceleration information from the lunar penetrometers could not be transmitted for extensive distances.
When located within moderate range of a receiving station like a parent spacecraft, the relay craft served to simply amplify and redirect the lunar penetrometer signals.
Above the lunar surface, the spacecraft would release the payload, which would spin for axis attitude stability and use the main retrorocket motor to reduce the descent velocity.
The device was suggested to assist astronauts in on-the-spot decision making regarding whether a safe landing of the lunar module could be made.