How Hard Would It Be to Keep the Moon from Floating Away? | Hacker News Reader

How Hard Would It Be to Keep the Moon from Floating Away?

askamathematician.com

41 pointsIntronExon18 comments

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.

Lunar tides would stop, but wouldn't Earth still have solar tides, causing bulges on the area of Earth closes to the Sun and on the opposite side?

The Moon would still be passing over those bulges twice a lunar month. Wouldn't that continue to slowly modify its orbit?

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.

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.

That'd be cool, but as it returned closer to Earth, wouldn't the tides resume and start the cycle all over again? (Assuming the oceans are still present and liquid, of course.)

No, the duration of a solar day would decrease with the duration of a lunar month.

It might be marginally more feasible to build an enormous space tether (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?)

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.

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

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.

> Sci-fi has covered pretty much "everything"

Then there's also that book, in which it turns out that the Moon is a giant spaceship.

https://en.wikipedia.org/wiki/Mutineers%27_Moon, the first book of Dahak trilogy.

In Fritz Leiber's A Pail of Air, the Earth has been ejected from the solar system by a passing "dark star". Maybe black hole? Anyway, the atmosphere has frozen out. Survivors collect oxygen snow.

I challenge you to sit through the Space:1999 "Dragons Domain" episode and not come away impressed!

And then, afterwards, calm down with a little retro fun:

http://www.defence-force.org/index.php?page=games&game=space...

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.

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!

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

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

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

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