During Apollo, NASA fired grenades and crashed abandoned spacecraft into the Moon, measuring the tremors to investigate its hidden interior

It was from late 1969 through the early-to-mid 1970s that NASA purposefully went around causing explosions on the surface of the Moon. Not because of fun and games, but to get information. As reported in NASA's own Johnson Space Center podcast, lunar seismologist Dr. Ceri Nunn says that the moment the Apollo 11 seismometer began functioning, it started registering vibrations which were completely unknown to the Earth and which would take up to an hour to settle down.

At that point, the scientists thought that the device must be faulty. However, it was only a few months later, when NASA deliberately made the empty ascent stage of Apollo 12's lunar module crash land on the surface in November 1969, that engineers got a matching signal and confirmed the Moon itself was producing this effect. That matching signal was picked up by the Passive Seismic Experiment Package (PSEP), a separate stationary instrument that astronauts Pete Conrad and Alan Bean had set up on the surface hours before liftoff. The ascent stage itself carried no instruments of its own; it was simply the object being watched from a distance.

An unexplained anomaly from Apollo 11 turned into a deliberately tested and confirmed phenomenon, rather than a contradiction. That combination of an accidental first clue and a purposeful follow-up experiment became the beginning of one of NASA's strangest scientific endeavors.


The Moon’s unending echo

Earthquake signals die away quickly since the Earth is humid, stratified, and composed of materials that dissipate seismic waves. In the case of the Moon, however, it seems to do just the opposite. When the Apollo 12 astronauts' discarded lunar module ascent stage was sent crashing into the lunar surface in November 1969, the PSEP they had deployed nearby picked up a signal that grew for several minutes and went on and on and on.

According to the 1976 report titled "Apollo 14 and 16 Active Seismic Experiments and Apollo 17 Lunar Seismic Profiling," authored by Kovach, Watkins, and produced by Stanford University's Department of Geophysics under contract to NASA, such a signal indicated the presence of an almost non-seismically absorptive layer right under the lunar surface. In essence, the surface of the Moon is fractured and fragmented enough from years of bombardments that any shock waves generated just bounce around within its crust, never dissipating.

While NASA emphasized the "rang like a bell" part in the press release of the discovery, and while it is technically defensible, it is also a bit of an exaggeration for a very technical but intriguing discovery about the physics of porous rock. This wasn't a one-off either: used-up Apollo mission Saturn V third stages (S-IVB boosters) from later Apollo missions were also deliberately flown into the Moon at high speed to observe the shock waves.

Enter the grenades

However, crashing a spacecraft only gives an idea about the deeper structure of the celestial body. Since NASA wanted to find out what was under their feet, a more controlled explosion was required, and so the space agency created one. From NASA’s Active Seismic Experiment mission archive, it is known that the equipment carried to the Moon on Apollo 14 and Apollo 16 included two separate devices: a handheld "thumper," which an astronaut fired directly into the lunar soil to set off 22 small explosive charges one at a time, and a completely separate mortar box assembly, staked into the ground some distance away, that was designed to rocket-launch up to four grenades several hundred meters further out. A line of geophones laid out between the two measured the resulting shock waves.

Needless to say, grenades were not shot by the astronauts standing right next to them; they were launched by radio control from Earth after the mission was finished. In Apollo 16's case, three out of four grenades were detonated successfully at distances up to 900 meters, but the fourth grenade could not be exploded due to a technical malfunction. Apollo 14's mortar, by contrast, was not due to any in-mission malfunction, but because engineers determined beforehand that launching it in its assigned position risked damaging nearby instruments with dust and back-blast, a conclusion also confirmed by a 2020 peer-reviewed review, "Lunar Seismology: A Data and Instrumentation Review," in Space Science Reviews.

Why this isn't just a fun fact

One might assume that this belongs to the category of "wild stuff that was part of the space race and then forgotten." However, this does not belong entirely to ancient history. The questions Kovach and his team were trying to answer remain relevant to NASA's current lunar ambitions. Apollo 14 carried an unfired mortar as part of its 1971 mission, while the grenades that were actually launched and detonated were used during Apollo 16 in April 1972. NASA still needs to determine how deep the lunar regolith is, how stable the ground is, and where lava tubes might offer safe shelter.

NASA's Artemis program, after a February 2026 replan, is now aiming for its next crewed lunar landing no earlier than 2028, on the Artemis IV mission (Artemis III itself has been reworked into an uncrewed-landing-adjacent test in Earth orbit). Modern instruments of NASA, such as the Farside Seismic Suite and the proposed Lunar Geophysical Network, are the sophisticated heirs of the same grenade-and-geophone approach. Only a quieter and more sensitive version.