<p>Your question doesn't work because it's based on a false premise - that you can have a stable 2nd moon in orbit. You can't. If you accept the fact that the 2nd moon won't be stable, the size it becomes dangerous is the size that you wouldn't want to fall to Earth - if it's garbage and loosely held together, 100-200 feet in diameter would probably be safe enough and make a big flash and a lot of noise if it hits the Earth, but not cause too much damage.</p>

<p>Space garbage disposal is pretty easy - because there's so much empty space. In densely packed regions like low Earth orbit with lots of satellites, you'd want to avoid dumping garbage, but in the vastness of the Earth's Hill sphere, letting garbage just float in empty space wouldn't be all that dangerous, unless there was a lot of it. If there's enough manufacturing that you can't let garbage float, or if there's toxic minerals like lead or plutonium that you don't want to let fall to Earth, then creating a garbage dump in space makes sense, but it'd be way smarter to just use the Moon as a garbage dump than to build a 2nd one.</p>

<p>If the Earth-Moon system could hold an additional moon in a stable orbit, then your question on how massive could be answered, but there's no practical reason and no benefit towards creating a garbage moon, it would have an unstable orbit and require periodic re-adjusting or it would be dangerous. Just make a garbage dump on the moon.  It's way smarter.  </p>

<p><strong>Would the L4 and L5 Lagrange points be unstable?</strong></p>

<p>Doing a bit more research, it seems that Earth-Moon L4 and L5 are stable. NASA did some research into it. Perhaps that makes my answer not correct to this question.  An L5 garbage moon would be possible. - See <a href="https://space.stackexchange.com/questions/8658/which-lagrange-earth-moon-point-is-better-for-a-space-settlement">@Andy256 answer here.</a></p>

<p>As noted in the comments, a moon-sized object is a lot of material. The Moon is 20 times the mass of the asteroid belt. The number of comets and asteroids that would need to be captured, mined and discarded into the L5 orbit to create a 2nd moon is huge. We're not likely to create anything but a tiny moon out of captured objects because the Moon is comparatively enormous. I rather like the idea of using L5 as a place to capture, place and mine asteroids.</p>

<p>There's still a size limit though. If it grows to a few times bigger than the Moon, it's no longer the Moon's trojan, the Moon becomes its trojan and there's a mass limit for a stable trojan orbit, perhaps about 2 lunar masses, after which, the Moon's orbit would become unstable - and could be a world-ending scenario.   </p>

<p>(old update below)</p>

<p><em>That's a tough call. The Earth-Moon system is far from ideal for stable L4/L5 Lagrange points because the Moon's orbit wobbles due to solar tugging. The mass ratio of 81 to 1 is large enough, with about 26 to 1 being the minimum required for stable L4/L5 saddle points, but the eccentricity and <a href="https://en.wikipedia.org/wiki/Lunar_precession#Apsidal_precession" rel="nofollow noreferrer">lunar apsidal precession</a> could be problematic for long term stability.<br>
I'm not smart enough to say for certain whether they would be stable or not.  Perhaps another question, or it also fits here if someone else wants to answer.</em></p>

CC BY-SA 3.0
2018-02-14T02:29:43.710Z
Would the L4 and L5 Lagrange points be unstable?
Keith Thompson
CC BY-SA 3.0
2018-02-14T10:39:11.630Z
@KeithThompson  updated.  You're correct, they would be stable, I was wrong in thinking they wouldn't.
userLTK
CC BY-SA 4.0
2018-02-08T01:14:56.030Z
2018-05-05T12:14:28.723Z
2018-05-05T12:14:28.723Z
userLTK
<p>Question in the title. Let's say that I have a lot of garbage- mined-out asteroids, spent nuclear fuel, satellite debris, and just general trash. Also safe ways to get these into Earth orbit. Instead of sending this into the Sun, Jupiter, or out of the Solar System's plane, what if I took some random quasi-satellite, put it a bit behind the Moon on its orbital path, and started piling garbage on it? </p>

<p>How large could it get before bad things start happen to Earth? For example, tides drowning the coasts. I'm not going to consider collisions, because of (insert megacorp here) Moon Tethers™.</p>

CC BY-SA 3.0
2018-01-17T12:19:31.993Z
"... *before bad things start happen to Earth? For example, tides drowning the coasts*." I think the answer to that would require an understanding of Earth Science, *but not* Astronomy.
uhoh
CC BY-SA 3.0
2018-01-17T13:39:23.947Z
INB4  That's No Moon!
Carl Witthoft
CC BY-SA 3.0
2018-01-17T15:23:20.443Z
@CarlWitthoft What is no moon?
gerrit
CC BY-SA 3.0
2018-01-17T19:04:15.533Z
@gerrit  Google Star Wars quotes
Carl Witthoft
CC BY-SA 3.0
2018-01-17T23:09:51.497Z
Such a garbage moon would presumably only interact with the Earth through gravity, so the answer would be "pretty damn large indeed" - at least on the order of the mass of an asteroid.
Allure
CC BY-SA 3.0
2018-01-23T05:06:32.547Z
I wonder if it would be as simple as: As big as you want as long as the Hill Sphere of the Garbage Moon doesn't contain the Natural Moon or vice versa. I would choose a 1:2 orbital resonance for stability (ie. a period of about 2 weeks for the garbage moon. Of course, as the garbage moon gets bigger there would be tidal effects (imagine when the two moons lined up). If this is right, the math wouldn't be hard.
Jack R. Woods
CC BY-SA 4.0
2018-05-05T15:38:20.293Z
Possible duplicate of [Is there a ceiling for stable L4 or L5 masses?](https://astronomy.stackexchange.com/questions/3595/is-there-a-ceiling-for-stable-l4-or-l5-masses)
rob
CC BY-SA 3.0
2018-02-04T06:35:37.133Z
You're never going to create enough trash to make a satellite large enough to create problematic, nor probably even noticeable effects from its gravity. But what a pain it would be to launch it all.
Nathanael Vetters
CC BY-SA 3.0
2018-02-14T10:42:51.807Z
I've edited my answer.  A "garbage moon" at L4 or L5 is theoretically possible.  See this question here:   https://space.stackexchange.com/questions/8658/which-lagrange-earth-moon-point-is-better-for-a-space-settlement   The size limit would be maybe (ballpark ) twice the mass of our moon before the stability breaks down.   As @NathanaelVetters points out - that's a whole lot of trash, and I'm not sure it's a good idea, but that's the answer.  The limit would be somewhat less than 1/26th the mass of the Earth for a stable L4/L5 point.
userLTK
<the-moon><earth><satellite>
How large could a second moon of Earth be?
GoingFTL
# How large could a second moon of Earth be?

(Asked by: username_0 on 2018-01-17)

Question in the title. Let's say that I have a lot of garbage- mined-out asteroids, spent nuclear fuel, satellite debris, and just general trash. Also safe ways to get these into Earth orbit. Instead of sending this into the Sun, Jupiter, or out of the Solar System's plane, what if I took some random quasi-satellite, put it a bit behind the Moon on its orbital path, and started piling garbage on it?

How large could it get before bad things start happen to Earth? For example, tides drowning the coasts. I'm not going to consider collisions, because of (insert megacorp here) Moon Tethers™.

**Tags:** the-moon, earth, satellite
> **Comments:**
> **username_1**: "... *before bad things start happen to Earth? For example, tides drowning the coasts*." I think the answer to that would require an understanding of Earth Science, *but not* Astronomy.  
> **username_2**: INB4  That's No Moon!  
> **username_3**: @CarlWitthoft What is no moon?  
> **username_2**: @gerrit  Google Star Wars quotes  
> **username_4**: Such a garbage moon would presumably only interact with the Earth through gravity, so the answer would be "pretty damn large indeed" - at least on the order of the mass of an asteroid.  
> **username_5**: I wonder if it would be as simple as: As big as you want as long as the Hill Sphere of the Garbage Moon doesn't contain the Natural Moon or vice versa. I would choose a 1:2 orbital resonance for stability (ie. a period of about 2 weeks for the garbage moon. Of course, as the garbage moon gets bigger there would be tidal effects (imagine when the two moons lined up). If this is right, the math wouldn't be hard.  
> **username_7**: You're never going to create enough trash to make a satellite large enough to create problematic, nor probably even noticeable effects from its gravity. But what a pain it would be to launch it all.  
> **username_8**: I've edited my answer.  A "garbage moon" at L4 or L5 is theoretically possible.  See this question here:   https://space.stackexchange.com/questions/8658/which-lagrange-earth-moon-point-is-better-for-a-space-settlement   The size limit would be maybe (ballpark ) twice the mass of our moon before the stability breaks down.   As @NathanaelVetters points out - that's a whole lot of trash, and I'm not sure it's a good idea, but that's the answer.  The limit would be somewhat less than 1/26th the mass of the Earth for a stable L4/L5 point.  
> **username_6**: Possible duplicate of [Is there a ceiling for stable L4 or L5 masses?](https://astronomy.stackexchange.com/questions/3595/is-there-a-ceiling-for-stable-l4-or-l5-masses)  

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## 1 answers

### Answer by username_8 on 2018-02-08. Score: 1

Your question doesn't work because it's based on a false premise - that you can have a stable 2nd moon in orbit. You can't. If you accept the fact that the 2nd moon won't be stable, the size it becomes dangerous is the size that you wouldn't want to fall to Earth - if it's garbage and loosely held together, 100-200 feet in diameter would probably be safe enough and make a big flash and a lot of noise if it hits the Earth, but not cause too much damage.

Space garbage disposal is pretty easy - because there's so much empty space. In densely packed regions like low Earth orbit with lots of satellites, you'd want to avoid dumping garbage, but in the vastness of the Earth's Hill sphere, letting garbage just float in empty space wouldn't be all that dangerous, unless there was a lot of it. If there's enough manufacturing that you can't let garbage float, or if there's toxic minerals like lead or plutonium that you don't want to let fall to Earth, then creating a garbage dump in space makes sense, but it'd be way smarter to just use the Moon as a garbage dump than to build a 2nd one.

If the Earth-Moon system could hold an additional moon in a stable orbit, then your question on how massive could be answered, but there's no practical reason and no benefit towards creating a garbage moon, it would have an unstable orbit and require periodic re-adjusting or it would be dangerous. Just make a garbage dump on the moon. It's way smarter.

**Would the L4 and L5 Lagrange points be unstable?**

Doing a bit more research, it seems that Earth-Moon L4 and L5 are stable. NASA did some research into it. Perhaps that makes my answer not correct to this question. An L5 garbage moon would be possible. - See [@Andy256 answer here.](https://space.stackexchange.com/questions/8658/which-lagrange-earth-moon-point-is-better-for-a-space-settlement)

As noted in the comments, a moon-sized object is a lot of material. The Moon is 20 times the mass of the asteroid belt. The number of comets and asteroids that would need to be captured, mined and discarded into the L5 orbit to create a 2nd moon is huge. We're not likely to create anything but a tiny moon out of captured objects because the Moon is comparatively enormous. I rather like the idea of using L5 as a place to capture, place and mine asteroids.

There's still a size limit though. If it grows to a few times bigger than the Moon, it's no longer the Moon's trojan, the Moon becomes its trojan and there's a mass limit for a stable trojan orbit, perhaps about 2 lunar masses, after which, the Moon's orbit would become unstable - and could be a world-ending scenario.

(old update below)

*That's a tough call. The Earth-Moon system is far from ideal for stable L4/L5 Lagrange points because the Moon's orbit wobbles due to solar tugging. The mass ratio of 81 to 1 is large enough, with about 26 to 1 being the minimum required for stable L4/L5 saddle points, but the eccentricity and [lunar apsidal precession](https://en.wikipedia.org/wiki/Lunar_precession#Apsidal_precession) could be problematic for long term stability.  
I'm not smart enough to say for certain whether they would be stable or not. Perhaps another question, or it also fits here if someone else wants to answer.*

> **Comments:**
> **username_9**: Would the L4 and L5 Lagrange points be unstable?  
> **username_8**: @KeithThompson  updated.  You're correct, they would be stable, I was wrong in thinking they wouldn't.  

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