I often find science fiction disappointing. If there is ever an opportunity to be imaginative and dream up intriguing ways the future may pan out, science fiction should be it. But so often the genre simply places characters in space, or on another world, and subjects them to the same problems we face on Earth.
An exploding Moon, fortunately, is one imaginative question about what could happen. Even so, I found the exploration in Seveneves unsatisfying. Crucial questions are never answered (why and how does it explode?), and the aftermath seemed faintly unrealistic. Hence the reason for this article, an exploration of what might really happen if the Moon exploded.
The Moon explodes, shattering into seven large chunks. Who did it, or why, is unknown. Before long, the seven chunks smash into each other, rapidly disintegrating. The debris falls towards the Earth, enters the atmosphere and heats the planet so much that it becomes uninhabitable for five thousand years.
This terrible scenario is the opening premise of Seveneves, a science-fiction novel by American author Neal Stephenson. Seveneves belongs to the hard science-fiction genre, which stresses scientific accuracy and realism. But how scientific is his exploration of an exploding Moon? What would really happen if the Moon exploded tonight?
The idea is not as absurd as it sounds. Moons do sometimes explode. This can happen in two ways — they can be struck by something big, or they can be ripped apart by gravitational disturbances. These two possibilities are not even that rare — the rings of the gas giants in our solar system may have formed from the remains of exploded moons.
Exploding moons and planets were much more common when the Solar System was young. At that time the Solar System was a chaotic place. Unlike today , with eight planets following steady orbits like clockwork, there were many small planets flying all over the place. Sometimes they would collide: the early Earth was almost certainly struck by another planet at one point, and it seems likely something similar happened to Mars.
Those collisions must have resulted in spectacular explosions. In its collision the Earth was likely ripped apart, throwing off a huge cloud of debris. In the years that followed the debris cloud would have settled, reforming the Earth but leaving a large second object in orbit — our Moon.
Today, though, such collisions are rare. Even the largest known asteroid, Ceres, would not destroy the Moon in a collision. This leaves the second possibility — gravitational disturbances. The idea, essentially, is that if two planets or moons get close enough, the force of gravity gets strong enough to start tearing the objects apart.
In theory the Earth could do that to the Moon. If the Moon, for some reason, were to get much, much closer to the Earth — to within ten thousand miles — the Earth would start tearing chunks off its satellite. Fortunately the Moon is over two hundred thousand miles away, so such an event is pretty unlikely for now. But it is not inconceivable.
Billions of years from now the Sun, as it begins to die, will start to expand. The outer layers of the Sun could reach all the way to the Earth. This would slow the Moon down, sending it falling towards the Earth. As it gets closer to the Earth it would start ripping apart, before crashing into the Earth.
In either case, an exploding Moon, by natural means at least, is pretty unlikely for now. That leaves artificial methods — probably some kind of enormous bomb. It is not impossible to blow up the Moon with a bomb, but it would be a massive job (a single nuke isn’t going to cut it, you’d need millions). That you could do that with nobody noticing is, well, laughable. But for the sake of argument, let’s move on.
So what would happen if the Moon exploded? There are two assumptions in the Seveneves scenario. The first is that a large fraction of the destroyed Moon ends up hitting the Earth. The second is that all that debris burning up in the atmosphere heats the Earth enough to leave it uninhabitable.
Let’s take the first assumption first. What would really happen to all the Moon’s debris? The answer depends on the force of the explosion. If it is large enough the Moon will be smashed to smithereens and all the pieces of it will either be blown into deep space or crash, at high speed, into the Earth. That outcome would be bad, for sure. But the damage would be pretty fast, civilization on Earth would probably be wiped out by massive impacts, and anyone unfortunate enough to survive would die of starvation before much longer.
Since that doesn’t happen in the book, we can assume the explosion is less powerful. In fact, the novel states that the Moon splits into seven large fragments which then collide with each other to produce many more fragments. This implies an explosion powerful enough to break up the Moon, but not powerful enough to send bits of it careening across the Solar System.
This brings up a key point — the Moon’s gravity doesn’t disappear when the Moon goes boom. As long as most of the debris of the Moon stays in roughly the same place, all the mass is still there and the Moon’s gravity won’t actually change all that much. The implications of this are important — the debris cloud will keep orbiting the Earth and the Moon will slowly put itself back together.
Because of this gravity, most of the Moon’s debris won’t hit the Earth. Instead the fragments of the Moon will collide with themselves, either breaking up into even smaller fragments, or merging to form larger fragments. Eventually the Moon will reform, carrying on much as before.
There’s a catch though. In some of those collisions a few fragments will get an extra boost, and be thrown clear of the Lunar debris cloud. Think of a large asteroid hitting the Earth. Most of the material stays on the Earth, but tonnes of pulverised rock will be thrown up into the atmosphere or out into space. Some of that debris will escape the Earth completely, entering into orbit around the Sun. The same thing would happen to the remains of the Moon. If the collisions are powerful enough, some of the debris will escape.
What happens to that rock? Most of it will stay in orbit around the Earth, forming a tenuous ring. But some of it will be thrown towards the Earth, and a small portion of it will enter the atmosphere, with most of it burning-up before reaching the surface. This, more or less, is what happens in the novel.
How much debris would be needed to render the Earth unlivable? This, remember, is the key plot driver of the book. As debris rains down on the planet, it burns up in the atmosphere. Gradually the atmosphere heats, until global warming on a massive scale forces humanity to depart for orbital cities.
Debris burning up in the Earth’s atmosphere does indeed slightly heat the planet. But under normal circumstances this effect is almost nothing. Every day around one hundred tonnes of space dust burns up in our atmosphere. Some of it is bright enough to form a shooting star, but for the most part the dust burns up invisibly.
This dust doesn’t add much energy to the atmosphere. Rough calculations suggest the heating effect is around sixty megawatts — roughly equal to the power generated by forty wind turbines. The Sun, by contrast, heats the Earth with one hundred and seventy billion megawatts — massively more than space dust does.
It’s clear we’ll need a lot of dust to heat the Earth enough to make it uninhabitable. What’s more, that dust would all have to arrive over a short time span, otherwise the atmosphere would just cool down again. Would blowing up the Moon achieve that?
According to the book, the falling Moon debris heats the Earth enough to evaporate the oceans. Evaporating the oceans — needless to say — takes a lot of energy. How much? Roughly five hundred septillion Joules (that’s a five followed by 26 zeros). That’s an enormous amount of energy, more than humanity has used since the dawn of civilization.
On an astronomical scale, though, such huge amounts of energy are possible. A large asteroid — big, though much smaller than the Moon — hitting the Earth could theoretically release enough energy to evaporate the oceans. Billions of years ago, during a period known as the Late Heavy Bombardment, thousands of asteroids hit the planet. Some of those asteroids would have left continent sized craters — and perhaps boiled off much of the oceans.
Debris from the Moon could potentially do the same, provided there was enough of it. It would take billions of tonnes of Lunar material hitting the Earth to evaporate the oceans. That might sound like a lot — and it is — but the Moon is also very big. A tiny fraction of Lunar material would be sufficient.
If the Moon does explode tonight, then, we would be in trouble. But if you were really set on doing humanity harm, and you had the firepower to do it, there are probably easier ways than secretly rigging the Moon to blow.