Moon Fall

There’s a longstanding philosophical question: if a tree falls in the forest and no one is there to hear it, does it make a sound? The answer touches on the role of the observer in the situation. Sure, the fallen tree makes acoustic waves when it hits the ground but, if the observer is not present, the ears and brain that convert those signals into what we perceive as sound are also not present. So, today let’s modernize this riddle: if a rocket hits the far side of the Moon, does it make a sound? Well, it just so happens that a piece of a rocket is actually going to hit the Moon hard this coming March 4. This rocket was thought to be a derelict SpaceX Falcon 9 rocket left over from the launch of NASA’s Deep Space Climate Observatory in 2015, but recently has been more realistically identified as instead the upper stage of China’s Chang’e 5-T1 mission that was launched in 2014. (I guess you’re off the hook this time, Elon!). At any rate, the rocket stage will hit the Moon at about 2.6 kilometres per second, about 2-10 times the speed of a bullet but of course much, much more massive. This impact, which we won’t be able to see from Earth, is expected to make a new crater about 19 meters in diameter. But will the impact make a sound? For it to make a sound, like the fallen tree in the forest, it has to make acoustic waves that propagate in the air. (The impact will undoubtedly make seismic waves that could have been conceivably picked up by lunar seismographs set up by the Apollo missions to study moonquakes, but those were all shut off in 1977 for budgetary reasons.)

Does the Moon have air? Well, contrary to popular belief the Moon actually does have an atmosphere, one made up primarily of neon, helium, and hydrogen and trace amounts of other odd gases. This “air,” however, is very, very thin, only about 100 particles (atoms or molecules) per cubic centimeter while on Earth at sea level, the atmospheric density is about 100 billion billion particles per cubic centimeter. In fact, the air on the Moon is so thin that its particles aren’t able to collide with the same kind of regularity as Earth air does – the particles just slip by each other. Since such collisions are necessary for the propagation of an acoustic wave, it does indeed look like the upcoming impact won’t make a sound, even if someone was unlucky enough to be standing nearby at the time without a helmet. Relatedly, however, will this impact cause the Moon to move out of its orbit toward the Earth? Well, though the rocket stage will be going really fast when it hits, its estimated 4 metric tons of mass is dwarfed by that of the Moon itself by a factor of 200 billion billion. As a result, we can safely say that that the answer to that question is also no. Furthermore, given recent box office receipts, we can say that the answer to whether anyone wants to see a movie about the Moon falling towards the Earth is also no. Something to think about during today’s full Moon, occurring at precisely 8:59 am PST.