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If we colonized Moon it would mean we are moving matter from Earth to Moon. Since Moon is much smaller than Earth could these changes somehow affect its trajectory/movement to the point we could notice?

How much material would we have to move to the Moon to cause some kind of disaster?

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    $\begingroup$ Huge amounts. Have you maybe looked up what the current mass of the Moon is and how much we can transport to the moon per rocket launch? $\endgroup$
    – NoDataDumpNoContribution
    Jan 15, 2019 at 14:37
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    $\begingroup$ You mean a disaster like Guam tipping over and capsizing because of all the US Marines on it? $\endgroup$
    – DSKekaha
    Jan 15, 2019 at 20:05
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    $\begingroup$ There was a very similar question on WorldBuilding not too long ago. $\endgroup$
    – Michael Seifert
    Jan 15, 2019 at 21:16
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    $\begingroup$ @DSKekaha What's amazing is that this question is actually slightly more plausible. Still outrageous, but the moon's relationship to Earth is actuallly based on mass and gravity, as opposed to islands not floating on the ocean. $\endgroup$
    – jpmc26
    Jan 16, 2019 at 9:34
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    $\begingroup$ I'd assume the binary mass know as the Earth-Moon system would continue to orbit the sun at its current distance. $\endgroup$
    – Mazura
    Jan 16, 2019 at 10:48

2 Answers 2

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No, there would be no measurable effect. But we can consider two things: force and mass.

Let's imagine we planned very poorly, and always landed our ferry craft in the same Earth-moon orientation (so that by landing, the moon was always pushed "away" from its current direction of motion).

The gravitational force between the Earth and moon (the force keeping it in its circular orbit) is roughly 2x10^20 Newtons. The force of a single "ferry" landing on the moon can be estimated... say it is Space Shuttle sized (100,000 kg), and it just lands vertically on the moon. Impact is about 100ms in duration, and change of speed is about 4m/s during that time (these estimates from the lunar missions) by the time it is near the surface. That's a force of 100,000 * 4 / 0.1 = 4 million Newtons.

How many landings would it take to be the equivalent of Earth's gravitational attraction? 50 trillion. Say we get really efficient at space flight, to the point that we can do as many trips to the moon as we have regular flights per day on the Earth... that's about 100,000 flights per day.

To make 50 trillion trips, it would take 1.4 million years.

The issue of mass has already been answered... such a small change in mass would result in an undetectable change in the moon's orbital velocity. But, to answer your second question, how much mass would it take?

Say we want to change the moon's speed by 10%. The equation for orbital speed is $v = \sqrt{\frac{G*M}{r}}$, where G is the Universal gravitational constant, M is the mass of the Earth, and r is the radius of the orbit. This means for v to change by 10%, M must change by 21%. Given the mass of the Earth, this means we'd need to move 1,500,000,000,000,000 billion kg of people and stuff before having to be worried. The average male is 70kg. So, to make a dent in the moon's orbital speed, we'd need to have 220,500,000,000 billion people here. Considering the Earth only has about 7.5 billion people, that may take a while.

The only thing that can realistically alter the moon's orbit is an impact with a large asteroid traveling pretty fast. Nothing man-made could really do it.

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    $\begingroup$ Note that by "large", we're talking something along the lines of Hygiea or Interamnia. $\endgroup$
    – Mark
    Jan 15, 2019 at 23:00
  • $\begingroup$ Changing the mass of the moon (as long as it stays small reltive to Earth's mass) would not change its orbit or speed; somewhere in your question you switched from the mass of the Moon to the mass of the Earth (the capital M in the formula). That's a big change of paradigm and could be made more explicit, partly because it's the main fallacy underlying the OP's question. $\endgroup$
    – Peter - Reinstate Monica
    Jan 16, 2019 at 12:37
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    $\begingroup$ Btw, I made similar calculations for 1% moon mass and a falcon heavy start every second (which is almost 100k starts/day, incidentally). I came up with 7,35E+15 starts needed, in a time of 2.3E8 (or 200 million) years. Details: A generous payload of 100t/start; moon mass 7.3e22 kg; 1% moon mass 7.3e20 kg; missions needed 7.3e15 (at 1e5 kg/mission); number of seconds/year = 60*60*24*365 = 3,15E+07 -> 2,33E+08 years needed. Btw, fuel incl. LOX spent would be 1,13E+22 kg (1/7 of moon mass!) assuming 500t fuel/launch. $\endgroup$
    – Peter - Reinstate Monica
    Jan 16, 2019 at 13:29
  • $\begingroup$ @PeterASchneider, Yes, I should have been more clear, there. $\endgroup$
    – OrangeWombat
    Jan 16, 2019 at 22:39
  • $\begingroup$ Did you consider that the mass of the earth decreases, when we move stuff to the moon? $\endgroup$
    – Jonas
    Jan 17, 2019 at 7:28
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No. The moon isn't that big but it isn't exactly small either.

The moon's mass is 73,500,000,000,000,000,000,000kg, that's 73 sextillion, 500 quintillion kilograms. If we moved the whole of mount Everest from the earth to the moon (162 Trillion kg, which is completely unrealistic for us to do) then that would equate to an increase of 0.0000000022%, which is infinitesimal.

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    $\begingroup$ @petrpeller Adding 735 quintillion kg to one localized area? Seismic activity the likes of which you can't even imagine likely followed by a process to "respherize" (for lack of a better work) the moon, that I don't understand enough to fully explain. Spread out over the surface of the moon? You now have a moon that's 101% the mass of our current moon. $\endgroup$
    – Magic Octopus Urn
    Jan 15, 2019 at 16:46
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    $\begingroup$ Anyone up for an estimate how much mass in the Moon gaining daily from meteorites? $\endgroup$
    – SF.
    Jan 15, 2019 at 17:35
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    $\begingroup$ @PetrPeller Keep in mind that changing the mass of the moon has no effect at all. To change something's motion, you need to apply a force. In the orbital speed equations, the mass of the orbiting object actually cancels out, so even if you doubled the moon's mass right now, nothing would change about it's orbit. $\endgroup$
    – OrangeWombat
    Jan 15, 2019 at 18:39
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    $\begingroup$ @OrangeWombat: That's not quite true. If we doubled the moon's mass, that would have a huge effect on the tides on Earth, and the tides in turn have an effect on the orbit of the Moon because the Earth's motion drags them in front of the Moon, which in effect accelerates the Moon prograde, which puts it in a higher orbit. Whether doubling the mass of the Moon would increase or decrease the rate at which the Moon is moving away, or keep it the same, I do not know; you'd need an expert in tidal forces to compute it. $\endgroup$
    – Eric Lippert
    Jan 15, 2019 at 18:45
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    $\begingroup$ @SF. According to NASA, about 2800kg of material strikes the moon every day. $\endgroup$
    – OrangeWombat
    Jan 15, 2019 at 18:45

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