A collision between the Moon and Earth would undoubtedly be catastrophic for life on the planet, it could even pass as a narrative for a science fiction film. But what does science say about it? Can the Earth and the Moon really collide?
According to the plot of the movie called Moonfall, a strangely mysterious and enormous force knocks the Moon out of orbit, causing its inevitable fall towards Earth. The consequence is easier to imagine; an imminent collision between both cosmic objects, but what are the chances of this being possible. Science answers the question posed by the film.
Can the Moon collide with the Earth?
Although our logic based on basic physics pushes us to believe that the Moon could crash into the Earth if we apply a mysterious force towards the planet, the reality is that it does not work as simple as this.
If a force 50 times greater than the Earth’s gravitational field were used to push the Moon toward it, it would not be enough to move the satellite or bring it so close to us that it would fall directly to solid ground. The only thing that would happen would be that the Moon would change its orbit but without any dangerous approach to the planet.
To understand why, you must first know that both bodies do not move independently of each other, but rather they are within a system of orbital mechanics. That is, they are connected through their gravitational fields and affect each other.
In that sense, if the Moon is pushed towards the Earth through the center of mass of the orbital system, the action will not change the angular momentum. This can be described as the measure of rotational movement that intrinsically depends on the mass, speed and position of both bodies.
The Moon’s angular momentum is constant, so as it approached Earth, it would have to accelerate its orbital motion. However, by applying a force towards the Earth the only thing that is being achieved is a lateral movement that will end up pushing the planet to change its orbit with respect to the Sun, although not to a large extent.
In other words, if the Moon is pushed towards the Earth, a collision between the two could not occur, but rather an orbital change of the Moon around the Earth and, consequently, an orbital change of the Earth around the Sun would occur. Therefore, this would not be the way to cause the catastrophic collision, but there is still another way.
A force against the lunar orbital movement
If you use a magnitude the same as in the previous example, about 50 times the Earth’s gravitational field but this time in the opposite direction of the lunar orbital motion, things get a little worrying. Doing this now would affect the Moon’s angular momentum, which means that the overall rotation rate will become much slower, enough for the Moon to hurtle like a rock in free fall towards our planet.
Therefore, the only way the Moon could collide with the Earth would be simply by completely freezing the orbit of the natural satellite. Which is the same as slowing down the speed of the Moon to a complete zero, with respect to the Earth. In this case we should be concerned, since the orbital system would be broken and the Earth’s gravitational field would attract the satellite, increasing its acceleration as time passes, just as gravity acts on a rock in free fall.
The answer to whether it is possible for both bodies to collide, the answer is a yes, although with a big but. For this to happen, there would have to be a mysterious and large force that reduces the lunar rotation speed to zero, although such conditions are highly unlikely to occur. Meanwhile, we can continue enjoying the amazing orbital system made up of our planet and its moon.