Active Seismic Experiment

Lore & Background

The ASE consisted of three major components: a set of three geophones laid out in a line by an astronaut, a mortar package designed to lob four grenades varying distances away, and an astronaut-activated 'Thumper' used to detonate one of 22 charges. The explosive fill in the mortar ammunition canisters was hexanitrostilbene, chosen for its insensitivity but high explosive properties. On Apollo 14, thirteen of the twenty-two thumper charges were fired successfully, but none of the four mortar explosives were fired due to deployment concerns and later failure after dormancy. On Apollo 16, a new mortar base was used to improve the experiment. Three grenades were fired: the first at approximately 30 m, the second at about 150 m, and the third reaching up to 900 m. After the third, the pitch sensor went off scale, and the fourth explosive was not fired. Nineteen of the Thumper charges were successfully detonated.

Reader's Guide

The Active Seismic Experiment, along with the Lunar Seismic Profiling experiment, enabled researchers to derive a compressional velocity profile of the lunar subsurface at the landing sites of Apollo 14, Apollo 16, and Apollo 17. Seismic velocities of 108 m/s and 114 m/s were inferred for the lunar subsurface at the Apollo 14 Fra Mauro site and the Apollo 16 Descartes site, respectively. These velocities suggested brecciated and highly porous material, likely the result of fragmentation and comminution caused by meteorite impacts. The experiment provided critical data on the structure and properties of the lunar regolith, contributing to the understanding of the Moon's geological history and impact processes.

Did You Know?

• The ASE used a thumper device and a mortar with explosive charges to explore subsurface lunar structure.
• Hexanitrostilbene was the main explosive fill in the mortar ammunition canisters.
• On Apollo 14, none of the four mortar explosives were fired; they remain unfired after loss of uplink capability in 1975.
• On Apollo 16, three grenades were fired up to a distance of 900 m before the pitch sensor went off scale.

Engineering Design & Component Architecture

The ASE was built around three distinct hardware elements that worked together to probe beneath the Moon's surface. A linear array of three geophones, deployed by an astronaut from the Central Station, served as the detection system. A mortar package was engineered to launch four explosive grenades at varying distances from the ALSEP, with ranging calculated by assuming ideal ballistic trajectories. The third element was an astronaut-operated Thumper device capable of detonating one of twenty-two small charges to generate a localized shock. The entire package weighed 11.2 kilograms, consumed 9.75 watts of power, and streamed data at roughly 10,000 bits per second. The mortar ammunition relied on hexanitrostilbene as its primary explosive fill, with individual grenades carrying between 45 and 450 grams of the compound. This particular explosive was selected for its combination of high detonation force and relative insensitivity, making it safer to handle in the demanding lunar environment.

Mission Execution & Operational Setbacks

The two missions that carried ASE revealed very different operational stories. On Apollo 14, the thumper managed thirteen successful detonations out of twenty-two charges, but misfired five times out of eighteen attempts, a problem traced to dirt contaminating the firing switch actuator bearing surface. The mortar, however, never fired at all during the mission due to deployment concerns. A belated attempt to launch the grenades near the end of ALSEP's operational window also failed, the charges having gone dormant too long. The experiment's fate was ultimately sealed on March 5, 1975, when uplink capability to the Apollo 14 central station was lost, rendering the remaining grenades permanently unfired. Apollo 16 benefited from a redesigned mortar base. Three grenades were successfully launched, reaching distances up to 900 meters, but a pitch sensor went off scale after the third shot—possibly caused by a range wire exerting a downward force post-launch. The fourth explosive was never fired. The thumper fared better, with nineteen of its charges detonating successfully.

Subsurface Findings & Seismic Velocities

The data gathered by ASE, combined with the follow-on Lunar Seismic Profiling experiment on Apollo 17, allowed researchers to derive compressional velocity profiles of the lunar subsurface at three separate landing sites. Although the regolith depth varied from site to site, the characteristics revealed by the seismic data proved remarkably similar across the Fra Mauro site (Apollo 14), the Descartes site (Apollo 16), and the Apollo 17 landing site. Inferred seismic velocities of 108 meters per second at Fra Mauro and 114 meters per second at Descartes pointed to a subsurface composed of brecciated, highly porous material. Researchers interpreted these properties as the signature of fragmentation and comminution caused by meteorite impacts. The consistency of these findings across three different landing sites reinforced the conclusion that the upper lunar subsurface shares a common impact-processing heritage, regardless of the specific surface regolith depth above it.

Leadership, Context & Scientific Legacy

ASE was conceived and led by Robert Kovach of Stanford University, who served as the experiment's principal investigator. It was carried on Apollo 14 and Apollo 16 as part of the Apollo Lunar Surface Experiments Package, the instrument suite deployed at the landing sites to conduct extended scientific measurements. The experiment's purpose was to use active seismic sources—a mortar with explosive charges and an astronaut-operated thumper—to explore subsurface lunar structure and elastic properties. On Apollo 17, ASE was succeeded by the Lunar Seismic Profiling experiment, which carried forward the active-source seismology approach on the Moon. The ASE data, combined with the Apollo 17 results, enabled researchers to derive compressional velocity profiles at three distinct landing sites. The experiment's methodology draws direct parallels to terrestrial reflection seismology and seismic refraction techniques familiar from exploration geophysics, making it a distinctive chapter in the broader field of lunar seismology.

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