In 1970, NASA deliberately crashed Apollo 13’s rocket into the Moon; the impact made the lunar interior “ring” for over three hours

Apollo 13 will always be remembered as the mission that failed spectacularly. The oxygen tank exploded two days into the journey; there was no moon landing; and the astronauts who made it back to Earth had to figure out how to keep breathing for the rest of the voyage. The one thing that many people are unaware of is the fact that NASA managed to squeeze in a real scientific experiment even after all the complications, and it included purposely crashing an entire rocket stage into the moon, a step that had been part of the mission plan long before the oxygen tank ever exploded.

As can be seen in the Apollo 13 S-IVB impact video released by NASA via the Goddard Space Flight Center, the upper stage of Apollo 13's Saturn V rocket was intentionally crashed onto the surface of the moon, providing invaluable insight into what was happening inside it. Apparently, commander Jim Lovell jokingly claimed: “Well, at least something worked on this flight.”

A plan that survived a crisis


Here’s what happens in reality but tends to get brushed over: the idea for crashing the rocket into the moon had been conceived prior to any of the problems occurring. The Apollo missions, beginning with Apollo 11, and followed by 12,14, 15, and 16 left a seismometer on the surface of the moon. By the time of Apollo 13, only one seismometer was actually on the Moon to detect it, the one left five months earlier by Apollo 12. Apollo 11 had also deployed a seismometer, but it had already stopped transmitting after about three weeks. The missions Apollo 14, 15, 16, and finally Apollo 17, that would go on to expand this seismic network, which added its own advanced seismic profiling experiment, had not yet flown in April 1970; they followed over the next two and a half years. The mission planners knew that the rocket could be used as something of a controlled blow against the surface, allowing geologists to study the effects of the shock waves in the ground.

The S-IVB stage of Apollo 13, the third empty stage of the Saturn V rocket, was targeted for impact at a particular area near the Mare Cognitum, and crashed onto the Moon's surface on April 14, 1970, about 137 kilometres from where Apollo 12 had placed their seismometer five months earlier. NASA didn't abandon this plan just because the mission was in crisis; the impact had already been locked into the flight trajectory before the explosion happened, so there was nothing left to decide once things went wrong. The remarkable part is simply that the experiment went ahead and worked exactly as intended, even as the crew was fighting to survive.

The moon that would not stop shaking

On Earth, such an impact would shake the ground for a couple of seconds and stop. But this one did not stop. Based on NASA’s own mission records, the signal from the booster impact lasted three hours and twenty minutes, and it was so strong that the ground controllers were forced to reduce the sensitivity of the seismometer during the recording process to prevent it from exceeding the scale of the readout. That is not a misprint. Three hours of shaking after one impact on an airless planet, without any oceans, and supposedly without any geologic activity.

A similar result had been observed five months earlier, when Apollo 12’s empty lunar module was deliberately crashed onto the Moon and rang for close to an hour, longer than expected. Seismologist Gary Latham described the behavior at the time as the moon ringing “like a bell," and the expression stuck. This time, the crash of the heavier and faster-moving Apollo 13 booster resulted in a bigger and longer resonance.

As reported by Gary Latham in his 1971 USGS publication that summarized the results of seismic activity on the Moon due to the Apollo program, the ringing was not an indicator of any hollow artificial structure within the Moon despite certain claims made amongst internet communities. On the contrary, the Moon's crust is completely dry and broken up into chunks as a result of being bombarded by meteorites for billions of years. Hence, there is nothing to absorb the energy from the seismic activities.

Why this still matters

Based on NASA's Gravity Assist podcast episode with planetary scientist Walter Kiefer, the impact measurements from these Apollo missions are still used today, decades after the impact, simply because very few have done such experiments since then. Each seismometer NASA plans to deploy under the Artemis program will essentially continue the work from where that 1970 booster impacted.

This is the part that should stick with us. The classic image of Apollo 13 is that of a cramped and freezing craft that had to be splashed down in the ocean. Less well-known is the forgotten piece of space debris, thrown at its target 240,000 miles away with no margin for error, which managed to score a scientific victory amidst a crisis. NASA could do nothing to stop the Apollo 13 mission from failing. But the plan it had already put in place to make something useful of the empty booster stage didn't have to fail with it, and NASA let that plan run its course, turning a small piece of a very bad week into something genuinely productive.