Impact Moon

When Neil Armstrong and Buzz Aldrin went outside the lunar module Eagle for their historic moonwalk, one of their tasks was to place on the surface a seismometer that would study moonquakes after they departed. However, the project to produce this instrument was conceived in a hurry. Its power came from two small panels of solar cells and, unfortunately, the chill of its first lunar night permanently damaged it. It was turned off during the next lunar day.

It fell to the next crew, from Apollo 12, to place on the Moon the first full science station, known as ALSEP, which included a seismometer that drew its power from a self-contained power unit. Subsequently, all missions that reached the Moon's surface, except Apollo 17, emplaced seismometers, creating a network of stations sited across the near side. From Apollo 13 onwards, all S-IVB stages were steered onto trajectories that led to a violent end, each forming a new crater on the Moon's surface.

Flight controllers had two major sources of propulsion with which to control the trajectory of the spent S-IVB. The two APS

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The lunar crater formed by the impact of Apollo 14's S-IVB. Photographed during the Apollo 16 mission.

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The lunar crater formed by the impact of Apollo 14's S-IVB. Photographed during the Apollo 16 mission.

modules had some leftover propellant, and there was still a small quantity of LOX that could be jettisoned through the J-2 engine nozzle under pressure from whatever heat was leaking into its tank. Minor additional thrust came from dumping the remaining hydrogen from the fuel tank and the helium gas from the pressurising system.

Control of the nearly dead stage was seldom very accurate and controllers never brought their rocket stage down on the Moon nearer than 150 kilometres from the planned target. Nevertheless, they were able to track them accurately to their end and the impacts provided lunar geologists with a seismic event of known energy occurring more or less in a known place. With each successful S-IVB impact sending lunar shockwaves to increasing numbers of seismometers, the quality of information that could be mined from each impact improved and, eventually, the network of instruments could provide triangulated readings from any impact, natural as well as those due to the S-IVBs and the discarded ascent stages of the lunar modules. These impacts yielded detailed information about the Moon's interior based on the travel time of the sound wave to the emplaced seismometers.

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