
What If the Moon Orbited at the Same Height as the ISS? - zck
http://www.slate.com/blogs/bad_astronomy/2014/05/19/close_encounter_what_if_the_moon_orbited_much_closer_to_earth.html
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ars
This is not correct:

"Earth’s Roche limit, the distance from the Earth where its tides could break
another object apart"

It's a very common error. This limit is for an object "held together by
gravity alone"! The moon is not held together by gravity alone - the dust on
the surface is, but the rocks in the center are all fused together.

Additionally since they assume the moon will continue to have a synchronized
orbit (the same side always faces us) it won't feel the tides in the same way.
It would feel some constant stretching, but not the severe flexing caused by
rotation.

The closer side would fall "down" and the far side would fall "up". Only the
center would be in free fall. I suspect the moon could survive that - but I'm
not sure.

If not, it would spread out into rings.

There's other errors too: In a situation like this the Earth would also
synchronize it's orbit. A day would speed up and the moon pushed higher until
the two matched. This would stop the flood, burn scenario in the article.

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kimdouglasmason
If the moon was orbiting the earth faster than the Earth's rotation, then the
effects of tides would increase the Earth's rotation and decrease the moon's
orbital velocity. This would result in the moon descending.

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ars
Yes, you are correct. So I wrote this:
[https://news.ycombinator.com/item?id=7769520](https://news.ycombinator.com/item?id=7769520)

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alephnil
The friction of the tidal forces would generate enourmous amonts of heat, and
that energy would be taken from the rotation of the earth and moon, causing
both the moon and earth to be locked into synchronous rotation. At least if
the moon did not fall down on earth as it probbaly would if going this low.
This would cause the moon to stay in a fixed spot in the sky. Since the area
under the moon would be in shadow from it most or all of the time, this would
be an extra region with polar climate, that could bind up large portion of the
earths water as ice, making the earth a lot dryer elsewhere. That is provided
that the water did not boil away in the process of locking the rotation.

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iliis
For a real example see Pluto and its largest moon Charon. And as you probably
already know, our moon IS tidally locked to us ;)

Then, there's the question "What if the earth was a torus?":
[http://www.aleph.se/andart/archives/2014/02/torusearth.html](http://www.aleph.se/andart/archives/2014/02/torusearth.html)

~~~
Crito
The Moon is tidally locked to us, be we are not locked to it. Because the
Earth rotates faster than the Moon orbits, tidal interaction between the two
trades Earth's rate for the Moon's orbital velocity. The Earth is slowing down
(and if this process continued long enough, would become tidally locked to the
Moon) while the Moon is getting further and further away.

If the Moon were in LEO this process would be reversed; the Moon would be
spinning up the Earth while losing orbital velocity.

~~~
ars
> If the Moon were in LEO this process would be reversed; the Moon would be
> spinning up the Earth while losing orbital velocity.

The situation is not reversible like that. Angular momentum is linear, but
energy is to the power of two.

So there is more energy than momentum here. You loose energy from fictional
heating on the tides and energy as the rotation of the earth "pushes" the moon
away.

But if you reverse it than as the moon slows down, it goes lower, but as it
does so it orbits even faster than before, meaning it would have to speed up
the Earth even more.

So rather than having extra energy (i.e. heat) you don't have enough, so
instead of tidally locking to the Earth, the moon just gets faster and faster
and then crashes.

~~~
Retric
Your forgetting about gravitational potential energy. Lower orbits involve
less total energy.

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ars
I'm not forgetting that at all.

The moon can't move up or down without transferring momentum to the earth.

But the amount of momentum transferred and the amount of energy transferred
don't match. The difference is expressed as heat in the tides on Earth.

So in one direction there is extra energy, in the other insufficient energy.

~~~
Retric
Conservation of momentum is why the moons orbit increases as the earth slows
down dispite taking more energy. If your speeding up the earths rotation you
get energy from the lower orbit and angular momentum from the same place.

Though to be really accurate you need to talk about the center of mass of the
earth moon system ect.

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easyname
[https://www.youtube.com/watch?v=w8I25H3bnNw](https://www.youtube.com/watch?v=w8I25H3bnNw)

~~~
deadfall
For people that don't like clicking links without knowing what it is: This is
a video from Vsauce - "What if the Moon was a Disco Ball?"

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VLM
1) Its interesting to contemplate a "death star" the size of the moon but of
immensely lower density. Could be handy! All the cool aesthetics and
applications with none of the planet killing tidal effects.

2) I think the alternative earth dwellers would have some interesting
gravitational slingshot opportunities, given something that heavy and that
low. Perhaps an interesting multigenerational mega-engineering project would
be placing a giant spherical ball of lead in high-ish earth orbit for
interplanetary vehicles to slingshot around, not necessarily moon size but a
serious chunk of metal. Perhaps a large asteroid or two could be refined down
to solid metal (to allow close approach to the center of mass)

3) The ham radio moon-bounce people would be excited to hear it'll take little
more than a HT to bounce a VHF signal off the moon, although the multipath
might make it impossible to do more than CW and the short orbital duration
makes it nearly as annoying to operate as a low orbit satellite. None the less
the effect on the ionosphere would be very interesting to look at / think
about. Multipath in general might be a very serious problem for radio
communication on a planet like this.

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ChuckMcM
I have always wondered about the Moon in geo-synchronous orbit. Over the ocean
it could create a water mountain that didn't move. And geo-synch is nicely
outside the Roche limit :-)

~~~
sitkack
That is a really interesting problem to calculate. What would that water
mountain look like? Could you even tell?

~~~
mrgriscom
Since the position is static the earth itself would deform too eventually. So
I believe the answer is "not much".

~~~
sitkack
So the earth would take the shape of of an egg and the ocean would ride over
it with a constant depth?

I wonder how that would effect rocket launches? Would it be better to launch
on the far side of earth or the moon side?

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chrisBob
I know how to find out for sure.

To:whatif@xkcd.com

Dear Mr. Munroe,...

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gshubert17
Inside the Roche limit[0], the Moon would begin to disintegrate (depending on
its relative strength compared to the disruptive forces) and the Earth would
end up with rings.

[0]
[http://en.wikipedia.org/wiki/Roche_limit](http://en.wikipedia.org/wiki/Roche_limit)

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ufmace
As far as I remember, the ISS orbits low enough that it requires regular
rocket boosts to maintain its orbit against drag from the last bits of the
atmosphere. So if the moon orbited there, I'm guessing it would have fallen
back to the Earth within a few hundred years or so.

~~~
Crito
My guess is that tidal forces (speeding up Earth's rotation, but slowing down
the moon (the opposite of what happens in this reality)) would dominate over
atmospheric drag.

How much atmospheric drag affects a spacecraft in LEO is a complex problem,
being a function of the spacecraft's mass, surface area, orientation, etc.
However the moon is dense enough that it would probably be fairly content to
barrel through the sparse atmosphere for quite some time.

~~~
theandrewbailey
Due to tidal forces, I suspect that the moon would start to steal some of
Earth's atmosphere. I'm not sure if that would slow it down or not.

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gotoblob
I'd argue that the 'fantastic' video is not actually fantastic. Why is the
moon transparent?

~~~
Tloewald
It's not. It's not bright enough to see through backscatter in the atmosphere.
Go look at the fullish moon on a sunny day and you'll see the same effect.

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leeoniya
would the moon be able to orbit at its current angular speed / period as it
does currently if it was closer and still maintain a constant distance?

i would think it would need to orbit much faster because the gravitational
pull would be several fold larger. proportionally if it orbits at 27 days
currently, it would need to orbit at (238,000 / (2160/2 + 260)) = 177.6x
faster...and that's if gravity effects were linear, which they arent...they're
inverse square.

~~~
falcolas
This was addressed in the article, the new orbit period would be 90 minutes.
Scary fast, for the size and distance of the moon.

~~~
leeoniya
ah, i'll read next time :)

it would likely also distort the earth significantly and tides would become
hourly tsunamis.

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lutorm
You really didn't read the article, did you? ;-)

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leeoniya
fml, i meant as in "after work"

