# Is there a chance for moon to collide with earth?

I have learned that attraction between the bodies increases when it's mass increases ,if the population increases day by day and hence it can increase the mass of earth which causes attraction between the earth and moon more intense, so moon should ultimately collide with earth ?

• Um... If I eat a huge meal, the mass of the earth doesn't increase significantly, though mine does. The law of conservation of mass is perfect for such scales; it's a big deal in chemistry. Is this a joke question? – user191954 Aug 29 '18 at 15:01
• @Chair Come on I am just a beginner – user204640 Aug 29 '18 at 15:11
• It's perfectly genuine to ask if increasing the mass of the earth 'somehow' will cause the moon's orbit radius to decrease to zero, and it's also interesting to try to find out why gravitational force depends on the mass of the bodies. But the proposition that the mass of the earth increases when animals reproduce is pretty surprising, to put it mildly. – user191954 Aug 29 '18 at 15:14
• According to this answer by David Hammen, the mass of the Earth is most likely decreasing, primarily through loss of hydrogen, but it's hard to get a good estimate on how much mass we pick up from space dust. – PM 2Ring Aug 29 '18 at 16:05

1. Conservation of mass: As a whole, mass (atoms) on Earth is roughly conserved (in fact, the Earth loses a lot of hydrogen gas every year, but it can be considered negligible). Manufacturing new objects requires natural resources to be extracted from somewhere else.
2. The Moon will not collide with Earth because it is travelling in approximately a circular orbit around Earth, which requires a centripetal force that is provided by gravitational force. In fact, due to tidal effects, the Moon is drifting further from Earth at approximately 3.8 cm per year.

The population is made out of atoms, like carbon and oxygen, which have to be extracted from somewhere else on Earth, so the total mass of Earth does not change as population grows.

• But Earth's mass does increase from all the micro- and not so micro-meteorites we are bombarded with every day, as is the moon. And at the same time we turn plenty of resources into energy, and since E=mC^2, I suppose we are losing some mass that way. – CrossRoads Aug 29 '18 at 15:18
• @CrossRoads We're losing some mass, but primarily because of material escaping from the atmosphere. Please see: scitechdaily.com/earth-loses-50000-tonnes-of-mass-every-year and phys.org/news/… – Dhruv Saxena Aug 29 '18 at 16:10
• I like the final conclusion "The net loss is about 0.000000000000001% every year". So it'll definitely be around for a while. – CrossRoads Aug 29 '18 at 16:13
• And then the 2nd link talking about our atmosphere being jetted out - why couldn't our excess CO2 be one of the subject gasses? That would have been convenient. Instead, we lose hydrogen & helium. – CrossRoads Aug 29 '18 at 17:35

The mass of the Earth does not change as the population changes, as V.F. pointed out. Of course, the masses of the Earth and Moon are not constants of nature so both are subject to change. However, the actual changes in the masses of the Earth are Moon (due to ordinary processes such as meteor impacts) are far too small to have any effect on the dynamics of the Moon's orbit.

A much larger and more interesting effect is the tidal interaction between the Earth and Moon, which causes the Earth's rotational velocity to slowly decrease (the day has increased by 1.7 milliseconds over the last century) and the Moon to slowly recede from the Earth (about 4 centimeters per year-- much less than the escape velocity at the Moon's current distance, so the Moon will not escape from the Earth).