Thirty-five years ago — May 14, 1973 — saw the end of an era: The last Saturn V rocket ever to fly was launched from Cape Kennedy. By almost every measure, the Saturn V was the most powerful rocket ever to fly. The Soviet N-1 was a bit wider, and the Soviet Energia was theoretically capable of more thrust, but no rocket has ever been taller, heavier, or pushed more payload into orbit. Perhaps most impressively, every single flight of the Saturn V was a success — no Saturn V ever failed to deliver its payload into orbit. Fairly impressive for a rocket that NASA originally didn’t want to build.

The Saturn V was designed explicitly for the Apollo program, following a mission profile that NASA engineers were initially very reluctant to embrace. There were three competing plans for landing a man on the moon during the early days of Apollo: Direct Ascent, Earth Orbit Rendezvous, and Lunar Orbit Rendezvous.

Direct Ascent involved launching a rocket that would have dwarfed the Saturn V, sending the whole thing to the moon, landing it on the moon, and then dragging some or all of it back to Earth. Earth Orbit Rendezvous suggested sending two craft into orbit separately, joining them above Earth, and then going on to land on the moon. This was deemed more efficient, but technically more complicated, as no one had docked two ships in Earth orbit at the time.

Lunar Orbit Rendezvous sketched out a plan for sending a pair of docked vessels into orbit around the moon, one of which would drop to the surface and then return, rendezvousing with the half still in lunar orbit, which would then return to Earth. This was infinitely more complex than either of the other options, but was by far the most efficient, as it involved dragging the least amount of fuel and equipment into and out of the moon’s gravity well. Thus, reluctantly, the Lunar Orbit Rendezvous plan won out for its energy conservation benefits.

Despite being the efficient choice, Lunar Orbit Rendezvous required construction of the most powerful rocket in human history. Moreover, even though the entire point of the Lunar Orbit Rendezvous plan was that you wouldn’t need to take an actual rocket to the moon, that doesn’t mean no Saturn V has ever touched down on the lunar surface. In fact, several have.

HOW MANY SATURN V ROCKETS HAVE REACHED THE SURFACE OF THE MOON?

Get the answer.

Five is our key number here, as five Saturn V rockets have reached the lunar surface. More specifically, five upper stages of Saturn V rockets have been intentionally crashed into the surface of the moon by NASA, each of them following one of the last five Apollo missions.

The third stage of the Saturn V rocket was called the S-IVB (pronounced S-4-B), and it was the topmost conventional stage of the rocket. The S-IVB was separated from the payload section only by an instrument ring that let the lower two stages know when to cut off and separate. Unlike those lower stages, the S-IVB had a job beyond getting the Apollo capsule into Earth orbit — it was responsible for powering the Apollo craft’s trans-lunar injection. Put simply, the S-IVB was the part of the Saturn V responsible for getting the Apollo module from the Earth to the moon.

The problem with this heady trans-lunar injection role is that it put the S-IVB in a rather precarious proximity to the rest of the Apollo craft, such that it could become a hazard during the rest of the mission. Thus, once the TLI was accomplished and the Apollo module was clear, NASA controllers used the S-IVB’s remaining fuel to boost it out of harm’s way. For the early Apollo missions, this entailed placing the S-IVB into a solar orbit by slingshotting it around the moon. After Apollo 12, NASA scientists realized this was a waste of perfectly good guided ordnance.

Instead of directing the S-IVBs into eternal voyages around the sun, NASA decided to slam them into the moon, where seismometers left behind by Apollo missions could record the impacts. The resulting seismic data was used to map the interior of the moon. Thus, all five S-IVBs from Apollos 13 through 17 met their fate as high-impact moonlanders, fulfilling in some measure the dreams of rocket scientists who originally aspired to land a giant rocket on the lunar surface.

That’s not just some seismically serendipitous space science, but a thematically satisfying slice of Geek Trivia.

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