The Passive Seismic Experiment Package (PSEP) was a scientific experiment deployed on the lunar surface by the astronauts of Apollo 11 as part of the Early Apollo Surface Experiments Package (EASEP). The experiment's goal was to determine the structure, tectonic activity, physical nature, and composition of the Moon. PSEP was the first seismometer to be deployed on a planetary body other than Earth.
Background
The California Institute of Technology had been part of multiple prior attempts to measure the Moon's seismic activity, providing instruments on Ranger 3, Ranger 4 and Ranger 5. All three landers on these missions failed to reach their destination successfully. Separately, a research group at the Lamont–Doherty Geological Observatory had developed a seismometer to fly on landers that were part of the Surveyor program. Due to reduced payload capacities and NASA's decision to prioritize experiments that would provide transferable knowledge related to crewed landings on the Moon, the probes did not carry seismometers.
Instrument
Power to this experiment was provided by EASEP via solar cell panels and operations of the experiment were limited to the lunar day. On later Apollo missions as part of Apollo Lunar Surface Experiments Package, power was provided to those packages' seismic experiments via a radioisotope thermoelectric generator.
The experiment's sensor system was made of two parts:
three low-frequency seismometers in an orthogonal triaxial arrangement to capture horizontal and vertical movement. one single-axis high-frequency seismometer that only measured vertical motion.
Deployment and operation
The PSEP was placed from the Apollo 11 Lunar Module at Tranquility Base on July 21, 1969. A set of 15 different commands could be sent to the experiment package from mission control on Earth to direct the instrument's levelling and calibration motors.
The instrument temperature reached a maximum of , greater than the expected maximum of . This higher than expected temperature resulted in minor effects such as intermittent transient signals on the low frequency seismometers, but did not significantly affect instrument performance. The low frequency seismometers drifted out of their operating range on the last two days and could not be recalibrated because the package could not receive commands.
The seismometers were particularly affected by the amount of seismic noise that emanated from the Lunar Module. This seismic noise was the result of venting, fluid circulation, and the Lunar Module's response to changes in temperature. Future missions would seek to deploy seismometers at the farthest possible distance from the Lunar Module.
Measurements and science
The background seismic levels on the Moon were found to be very low, far lower than that measured by any known instrument on Earth at the time. While there are large thermal variations due to the diurnal cycle, this does not result in a high background noise. A different hypothesis suggested that the Moon would have a continuous level of meteoroid impacts that would result in a high background noise, but this was similarly disproven. The most significant hypothesis these observations led to was the knowledge that the Moon was absent of any of the significant tectonic processes observed in the Earth's crust.
Seven types of signals were identified by the instrument package. L-events were the only signal classification with a hypothesized lunar surface origin, and with their main characteristics being described as having a long duration (<1 hour) and an emergent initiation. These events would produce a signal that would build up slowly to its peak and then slowly fade to the background level. There was great variability in their character and occurrence, being detected by both the long-period and short-period seismometers. This type of signal was detected 83 times during the Apollo 11 missions at an average of 4 per day. The origins of these signals were suggested to be either meteoroid impacts or shallow moonquakes and the frequency matched predictions previously made. I-events and X-events were suggested to be the result of either the relief of stresses in the vehicle due to heat expansion or the result of micro-meteoroids impacting either the Lunar Module or the experiment package.