
How Hard Would It Be to Keep the Moon from Floating Away? - IntronExon
http://www.askamathematician.com/2018/02/q-how-hard-would-it-be-to-keep-the-moon-from-drifting-away/
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sliken
Heh, not particularly accurate. The moon won't “eventually escape entirely”.
It's orbit will keep increasing till the solar day = lunar month. At which
points the tides stop and there's no more energy to increase the moon's orbit.

The fun part is because vacuum is not perfect and the incredibly small effects
of gravitational waves the orbit will start decreasing.... till it hits the
Roche limit and becomes a ring!

All that ignores the sun expanding and consuming the inner planets.

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mcstafford
Which object's tidal forces would cause the moon's disintegration? I'm
guessing either the Earth's, or the Sun's. I've only become aware of Roche
limits from your comment, and glanced at the Wikipedia page, but don't really
understand them.

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gizmo686
Tidal forces result from the difference in the strength of gravity at
different sides of a body. Because we are so far away from the Sun, it does
not exert any (significant) tidal force on us or the Moon. Moreover, the
amount of tidal force exerting by the Sun is not affected by the distance the
Moon orbits at [0]. In contrast, the moon is close enough to the Earth that
those tidal forces are significant, and would drive the Roche disintegration.
(They are also what drive the tides, the moons current orbital changes, and
why many moons are tidally locked).

[0] Technically, a smaller orbit will make the tidal force exerted by the Sun
slightly more constant.

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chrisamiller
It might be marginally more feasible to build an enormous space tether
([https://www-istp.gsfc.nasa.gov/Education/wtether.html](https://www-
istp.gsfc.nasa.gov/Education/wtether.html)). By dragging it through Earth's
magnetic field (which extends out past the moon on the "downstream" side of
the solar wind), you could: 1) generate large quantities of electricity, which
would be extracted by orbital energy 2) use that electricity in some useful
way to further slow momentum (like powering some of the ion engines they talk
about in the article).

Anyone with a stronger physics background want to do the math and tell me why
this won't work (or at least be marginally better than the proposal in the
article?)

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ars
The opposite would happen. Because the Earth rotates once a day, and the moon
once a month, the tether would have the effect of _speeding up_ the moon!

Which will cause its orbit to move farther away.

That's what the tides are: They consume the energy difference between the two
speeds, and they are what is causing the moon to move.

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PhantomGremlin
Sci-fi has covered pretty much "everything", so it's not a surprise that there
was a TV series in the 1970's that had this exact premise:

 _In the opening episode, set in the year 1999, nuclear waste stored on the
Moon 's far side explodes, knocking the Moon out of orbit and sending it, as
well as the 311 inhabitants of Moonbase Alpha, hurtling uncontrollably into
space._

I never could bring myself to watch it, because the premise was just too
absurd.

[https://en.wikipedia.org/wiki/Space:_1999](https://en.wikipedia.org/wiki/Space:_1999)

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Jaruzel
Space:1999 is Great! It's a perfect piece of 1970s _silly_ Sci-Fi. None of the
stories hold up in a science/physics sort of way, but they ARE fun to watch.

If you can't bring yourself to watch it properly, at least hunt down a good
copy of the opening episode (It's been released on Blu-ray I think) and watch
that. I promise you'll fall in love with the 'Eagle' spaceships.

Also, it's a Gerry Anderson production, so if you liked any of his puppetry
shows (Thunderbirds, Fireball XL5, Captain Scarlet etc.) you'll probably enjoy
Space:1999.

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ars
Until I saw this title, this wasn't really something I was worried about.

But now I'm worried.

On a more serious note the moon will stop drifting away once it's orbital
period matches the length of an earth day.

It's like a kind of "friction" where the daily rotation of the Earth tries to
speed up the monthly rotation of the moon.

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logicallee
can someone explain:

>They’re also the most efficient type of space propulsion, maxing out around
80% efficiency. The most powerful use about 100 kW of power and are capable of
generating about 5N of force (about 1 pound).

How can 80% efficiency get you just 1 pound of force if you put in 100 kW of
power? This seems many orders of magnitude off!

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taneq
Depends how fast you're going. 100kW input power at 80% efficiency = 80kW
output power.

P = F * V

80,000W = 5N * V m/s

V = 16,000m/s

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logicallee
What is 16,000 m/s in this example? (For what we're discussing.) I don't
recognize that constant...

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opwieurposiu
16,000 m/s is the speed of the argon as it exits the ion thruster.

