What would happen if we were to detonate a very very powerful nuclear weapon on the moon?
Would the explosion knock its orbit towards earth, causing tidal waves and misery? Could
the moon be destroyed, showering the earth in a rain of meteoric death?
During the cold war the moon was a major target for space exploration and you know, military
bases. So the US Air Force commissioned a serious study into the effects of a nuclear
detonation on the surface of the moon. But just quoting stuff is boring, so let’s conduct
a very important scientific experiment with an imaginary 100 megaton thermonuclear warhead,
about twice as powerful as the most powerful bomb ever detonated. We’ll also place a
number of curious astronauts around the moon as observers.
Let’s push the button and slow down time. For the first few milliseconds nothing much
happens outside our weapon. Meanwhile inside, high explosives send a shockwave to a radioactive
metal core, compressing it so much that it reaches criticality and starts a nuclear fission
chain reaction. The 100 million degree plasma created in this first stage sets off the second
stage, with atomic nuclei fusing like they do in the core of a star. Very briefly, our
weapon contains one of the hottest places in the universe. And only now, barely ten
milliseconds later, does the rest of the universe find out that anything has happened, as suddenly
the bomb dissolves and a flaming star of nuclear death is born.
So far so good. But everything that happens now is very different from what we’re used
to on earth because of one major difference: There’s no atmosphere.
As the fireball shines it releases a flash of X-rays and thermal photons, a wave of silent
heat which rushes outwards in all directions. On earth, this heat would char and burn everything
within a 50 kilometer radius at least. But on the moon, without an atmosphere and oxygen-rich
air, there’s no burning at all. Also there are no things to burn. The crunchy topsoil
of the moon is made from silicate rock and metals chewed to dust by eons of meteorite
impacts, mixed with tiny traces of water. When heated by the explosion, X-rays from
the fireball vaporize a thin cloud of rock from the lunar surface, while the unlucky
dust that’s inside the fireball melts into glass. Any astronauts watching the show within
about 50 km can expect to be fried.
And now, we begin to see one of the biggest differences between explosions in space and
on earth. On earth the atmosphere fights back against the plasma bubble: Its expansion is
violently stopped within moments by the pressure of the atmosphere. But this is not good news.
As the fireball rams the atmosphere it produces the most destructive part of a nuclear explosion
on earth: the shockwave. Compressed air around the explosion rushes out faster than the speed
of sound, shattering buildings and roaring so loud it ruptures organs. But on the moon
there is no shockwave. No atmosphere means nothing to impede the expanding explosion
in space. On the moon, the fireball just grows in eerie silence as there is no atmosphere
to stop it or to give it a voice.
This would be an amazing thing to watch from a safe distance. Unfortunately there’s hardly
any safe viewing distance for a nuclear explosion on the moon. Without an atmosphere weakening
the deadly ionizing radiation that can scramble DNA, anyone close enough to get a good look
will be exposed to fatal amounts of radiation. But of course, that’s not all.
While all of this happens the explosion hammers against the moon, transferring about a tenth
of the explosion energy into seismic waves, powering an intense moonquake. The moon is
much smaller than the earth, and our astronauts will feel an inescapable violent shaking no
matter where they’re standing. Compared to an earthquake of 7 on the Richter scale,
this shaking could seriously damage or even level infrastructure we might have built anywhere
on the moon. Those who hid on the far side of the moon would have no idea it was an explosion,
the quaking would feel like an asteroid the size of the Great Pyramid had struck.
And it’s not over yet: Where our bomb explodes, the ground splatters like water when a rock
strikes a pond. As the explosion pushes against the surface it may excavate as much as a hundred
million cubic meters of dust and rock, forming a crater a kilometer across while bedrock
is pulverized to rubble. Debris is shot into the sky in every direction.
Again without an atmosphere there’s no drag to slow any of it down. Much of the debris
scattered never returns to the moon, flying off faster than escape velocity. A flurry
of micrometeorites have been cast off to explore the solar system, many of which will rain
down on the earth, though few will be larger than pebbles. Any satellite, astronaut or
space station in the way though will have a really bad time.
Micrometeorites are launched at many speeds and angles, allowing them to spread all over
the surface of the moon. Like bullets they’ll punch through our curious astronauts no matter
where they stand.
Finally, our explosion comes to an end. On earth, the fireball rises like a hot air balloon
forming a sort of stalk; as it reaches up cooler air is drawn in around it, rounding
the top into a mushroom cloud. But on the moon, well, you know by now: no atmosphere,
no mushroom. The larger the plasma gets, the cooler it becomes, and the less energy it
has to make interesting or terrifying things happen. Within seconds of pulling the trigger,
the bubble reddens and fades from view. It would be visible from the earth, like a star
flickering to life only to fade out right away. A spark, and then nothing. As the cloud
of tiny debris reaches far above the surface of the moon, it’s illuminated by the sun
for a few minutes giving it an eerie beauty for anyone left to observe the spectacle.
What about the moon’s orbit? It is basically unchanged. Trying to move the moon with a
nuke is like trying to move a truck by blowing on it. Nuclear explosions may be big, but
space is bigger. Our mighty explosion just leaves another crater. One among millions.
Still, anyone on the moon will continue to not enjoy themselves. The material that ends
up raining back to the moon is radioactive, and without any natural processes to wash
it away or bury it, the surface of the moon will remain contaminated. Although fortunately,
the worst of the radiation will have decayed to a level comparable to natural levels from
cosmic rays in about a year.
In conclusion we can say with confidence that while the moon itself does not care about
being nuked and will barely notice, using the moon as a nuclear test ground kind of
ruins it for everyone trying to spend some time there or to build something useful. So
maybe we should just not do that.