# "Moonfall" physics

#### JackJewell-g

I just read the article "New trailer for 'Moonfall' promises a disaster movie on a scale like no other, but how accurate is the physics?" on the upcoming movie "Moonfall," the interview with Peter Travers and the physics involved.
https://www.space.com/moonfall-new-trailer-vfx-interview
I take issue with the excerpt:
"Even if the moon and the Earth were touching with our current assumptions of the moon, and its density, because the moon is 1/100 of the mass of the Earth. And so the gravitational equation doesn't support that even if the moon and the earth were touching, that you wouldn't, you wouldn't really feel much from the moon."
The article goes on to say that to fix the "problem," they had to "inject a ton of mass." The moon's mass is actually about 1/80 that of earth's, but never mind. With the actual masses and distance of the earth/moon system, tidal effects are already significant. The average moon-earth distance is slightly less than 240,000 miles. If the moon and earth were just touching, the distance between centers would be the sum of their radii, i.e. roughly 4,000+1,000, or about 5,000 miles (only slightly less). The ratio of those distances is then about 48. Suppose that before contact, the surface-to-surface earth-moon distance is a bit less than 1,000 miles, such that the center-to-center distance is 6,000 miles, and the ratio of the present distance to the near-contact distance is 40. Tidal effects vary as the CUBE of the center-to-center separations. Thus the near-contact tidal effects would be 40 to the 3rd power, or 64,000 times as much as the present tides (!). I think that the moon's tidal effect would wreak havoc on earth MUCH before getting that close. Moreover, before it got that close, the moon would be ripped apart by the earth's tidal effects.
This is very basic physics, and widely known. Either one of us is terribly wrong, or the article misleads the reader into thinking an intact moon crashes into earth, or that tidal effects are not included in the physical basis of the movie.
Sincerely,
Jack Jewell

#### Helio

The tidal effects would be huge for the Moon, but not for a person. But tides are more an orbital issue, which holds the Moon away vs. some magic power that puts it next to Earth.

I suspect they simply mean that if you consider the sum of the gravitational effects, there wouldn't be a lot of difference.

If they were toucing, a person on the far side would feel a diminshed gravity being farther from the Earth (~ 40% less gravity), but then you would also add the gravity from the Moon, so the net would be about 0.78g.

If one is standing where they kiss, then Earth's 1 g is reduced by the gravity of the Moon, so a net of about 0.83g.

But this is along the axis of the two. Trying to walk around this Moon would require a powerful jet pack to get half way, at least.

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#### Pogo

‘A ton of mass’! If the Earth scopes up 40 or so tonnes of mass everyday in space dust and micrometeors, the moon must scoop up a fair amount also. OK, so if you could magically add a significant amount of mass to the Moon, it would likely just change the orbit eccentricity a bit and it’s orbital velocity would increase a bit. Even if the mass were to become equal to that of the Earth, the two bodies would likely go into elliptical orbits, speed up somewhat, and the barycenter would be halfway between the two.

#### Helio

The Moon would be inside the Roche limit thus would be torn apart by the tidal forces.
Indeed. The experience on Earth would be huge as land masses and water climbed to great heights. The land stress likely would topple skyscrapers, I would think.

But an individual would not experience anything unusual directly from the Moon, IMO. The person, however, would likely experience the overall ground lifting and breaking. Even worse if near the ocean. The Moon would, if no magic wand is used (somehow), be whirling around in pieces about ever 2 hours with considerable amount of debris falling, but not most of it, perhaps. A ring would form around the Earth.

#### Helio

The impression I have from the interview is that they might be attempting to add mass to cause humans to be lifted off the ground - a stunning graphic for a movie, no doubt. They really would have to add density. Perhaps a super dense core kissing the Earth would lift people below it. That wouldn't be hard to calculate.

Why not?....... If the Moon's core touched the Earth and it was about 4x as dense as the Earth's core (magic wand needed), then the Moon would cause loose things to float towards it. This assumes the use of the Moon's core radius of about 240km. Such a dense core might survive the Roche limit and the overall reduced mass might limit a lot of tidal effects if the goal is to get people floating.

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#### billslugg

An iron core would make it all the way to the Earth intact. Roche limit is for unconsolidated piles of rubble.

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