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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?

Petr Peller
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    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? – NoDataDumpNoContribution Jan 15 '19 at 14:37
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    You mean a disaster like Guam tipping over and capsizing because of all the US Marines on it? – DSKekaha Jan 15 '19 at 20:05
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    There was a very similar question on WorldBuilding not too long ago. – Michael Seifert Jan 15 '19 at 21:16
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    @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. – jpmc26 Jan 16 '19 at 09:34
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    I'd assume the binary mass know as the Earth-Moon system would continue to orbit the sun at its current distance. – Mazura Jan 16 '19 at 10:48
  • You are aware that changing the mass of the moon -- even like, doubling it -- would not change its orbit significantly; as long as it stays small relative to the earth. – Peter - Reinstate Monica Jan 16 '19 at 12:39
  • If you found this question interesting you might like to look at my question here on Earth Science SE. – Adam Bailey Jan 16 '19 at 12:41
  • Ok, so... this question reminded me of the ridiculous premise of a movie I watched a clip of a while back... I think it had "1999" in the title. – Andy Jan 16 '19 at 19:57
  • In truth the Moon is already moving from the Earth (and we have noticed it, just not with the naked eye). Earth's Moon was "born" around 4.5 billion years ago in a titanic collision between our planet and a Mars-sized planetoid. The titanic impact threw debris into orbit around the Earth and from this maelstrom the diffirent debris began to coalesce into the Moon. For the last few billion years the Moon's gravity has been responsible for the tides in the Earth's oceans which the much faster spinning Earth attempts to drag ahead of the much slower orbiting Moon. The result is that the Moon is b – Tom Sol Jan 16 '19 at 10:46
  • eing pushed away from Earth by about 4 centimeters per year and the Earth is actually slowing down it's rotation. – Tom Sol Jan 16 '19 at 10:46
  • @DSKekaha The Wikipedia article on the gentlemen who first claimed that Guam could tip over writes: "Johnson's team quickly went into damage control and released a statement indicating that Johnson was using a metaphor which was widely regarded as a lie." –  Jan 17 '19 at 17:47

2 Answers2

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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.

GdD
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  • What % of Moon mass do you think could have some kind of effect? Let's say if we increased Moon mass by 1%? – Petr Peller Jan 15 '19 at 16:37
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    @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. – Magic Octopus Urn Jan 15 '19 at 16:46
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    Anyone up for an estimate how much mass in the Moon gaining daily from meteorites? – SF. Jan 15 '19 at 17:35
  • @SF. Probably there is, but I don't know it. Somewhere I've read that micrometeorite impacts peeled the top 2 meter of the small-mass asteroids since the stabilization of the solar system before 4.7bln years. – peterh Jan 15 '19 at 18:36
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    @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. – OrangeWombat Jan 15 '19 at 18:39
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    @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. – Eric Lippert Jan 15 '19 at 18:45
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    @SF. According to NASA, about 2800kg of material strikes the moon every day. – OrangeWombat Jan 15 '19 at 18:45
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    @EricLippert Good point! – OrangeWombat Jan 15 '19 at 19:08
  • @OrangeWombat Applying forces is how we get matter from Earth to the moon. – user253751 Jan 15 '19 at 21:36
  • if you are feeling creative, you could replace the commas in 73,500,000,000,000,000,000,000 with non-breaking thin spaces: 73 500 000 000 000 000 000 000 – uhoh Jan 16 '19 at 04:33
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    @Eric Lippert: But tidal recession is something that takes hundreds of millions of years. It's currently around 3-5 cm/year. – jamesqf Jan 16 '19 at 05:25
  • @immibis Not really - when we travel from Earth to the Moon, the forces necessarily balance out to be zero. You first "catch up" to the moon and match it's orbit and rotation (there's a small force from doing gravity manoeuvres depending on your mission profile). To have any force from landing on the Moon, you'd have to hit the ground - e.g. shooting mass from Earth to Moon with a mass-driver, rather than relying on rockets to slow you down. – Luaan Jan 16 '19 at 06:49
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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.

OrangeWombat
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    Note that by "large", we're talking something along the lines of Hygiea or Interamnia. – Mark Jan 15 '19 at 23:00
  • 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. – Peter - Reinstate Monica Jan 16 '19 at 12:37
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    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 = 606024*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. – Peter - Reinstate Monica Jan 16 '19 at 13:29
  • @PeterASchneider, Yes, I should have been more clear, there. – OrangeWombat Jan 16 '19 at 22:39
  • Did you consider that the mass of the earth decreases, when we move stuff to the moon? – Jonas Jan 17 '19 at 07:28
  • @Jonas, Yeah, that's where I was trying to go with the orbital speed calculation, but I wasn't very clear in my description. Sorry about that! – OrangeWombat Jan 17 '19 at 18:06
  • Transferring matter between the Earth & the Moon won't change the period of their orbits around the Earth-Moon barycentre. Please see https://en.wikipedia.org/wiki/Orbital_period#Two_bodies_orbiting_each_other – PM 2Ring Sep 06 '22 at 22:08