Could humans alter the moon's orbit significantly with current technology? (2016) | Hacker News Reader

Could humans alter the moon's orbit significantly with current technology? (2016)

worldbuilding.stackexchange.com

28 pointsdownboots31 comments

If someone asks if you can move a big boulder, and the answer is that it is too heavy, it really matters if it is in a location where it can roll down, like on the side of a mountain. In that case, heavy may not be such a problem.

It so happens that for every elliptical orbit, there is a circular orbit with the same angular momentum but a lower energy. The amount of energy excess available from this orbital change is 0.5mv^2*(eccentricity)^2. The moon's orbit has an eccentricity of 0.055, so 0.055^2 or 0.3% of the moons orbital energy could be extracted when changing its orbit from elliptical to circular. That is about 1.1e26 J, more than 1000 times the energy from burning all fossil fuel reserves.

The orbital energy could be gathered by sending a swarm of satellites connected by long cables into a circular orbit around the Earth, matching the period and phase of the Moon. The satellites would be spun around a common axis, like a Klemperer rosette, but using cables instead of gravity to keep them in place. The moon would then pass through the middle of this ring twice each month. Each time it did, the moon's gravity would pull the satellites closer to the center, and the cables would be reeled in. When the moon passed by, the cables would be reeled out again to their original position, allowing for generators to gather the energy. Gathered energy could be used to put more or heavier satellites into place using mass from the moon, making the process rate increase exponentially.

It may not alter the moon's orbit 'significantly', but it is doable with our current state of technology.

Fun thought experiment. If you haven't read Seveneves[1], the book details what happens when the moon inexplicably explodes (spoiler alert, it's not pretty). It's a crazy Neal Stephenson novel, and I highly recommend it. Allegedly they'll make it into a movie, but I don't see how. It's too out there.

[1] https://en.wikipedia.org/wiki/Seveneves

> what happens when the moon inexplicably explodes (spoiler alert, it's not pretty).

Ah. Section 31 at it again, I see.

#StarsDontSuddenlyGoNova

#MoonsDontJustExplode

#JusticeForPraxisAndRomulus

I watched an awful movie the other day where the Moon's orbit changes and they want to move it back, and I wondered the same thing. I guess the answer is no.

https://en.wikipedia.org/wiki/Moonfall_(film)

(don't watch this film, it's not even so bad it's good)

Cloud Atlas (released 6 years after Moonfall was released as a book) also has a story with the earth being destroyed by mans attempt to harness the moon for energy (and also has a bad film adoption).

Moonfall is the first book I know of that used the concept, but it seems like humans have been fascinated with man or god(s) manipulating the moon for a very very long time, the oldest case I can think of is from the Quran (https://en.wikipedia.org/wiki/Splitting_of_the_Moon), so we've been fixated on the idea for a long time.

Maybe there are more cases and I will read about them in the comments here.

Weirdly, Cloud Atlas also stars Halle Berry. I really like the movie of Cloud Atlas, myself. I've read the book at least three times, but I don't remember anything about the Moon?

Oh wow that's because there is not a Moon breakup scene in Cloud Atlas!

It's in the movie adaption of The Time Machine. In the book though he just arrives in the future and the moon is already gone (Chapter XIV The Further Vision https://www.gutenberg.org/cache/epub/35/pg35-images.html).

I think I read cloud atlas and the time machine, and watched the time machine movie around the same time, and got everything mixed up.

That was a fun afternoon reading through bits of books until I could figure out what I was remembering.

Haha.. no worries.. you had me thinking I hadn't been paying attention when reading Cloud Atlas too. I was about to read it a fourth time! ;)

The movie is awful but the book is a lot of fun and features last months Eclipse in the opening scene.

I hate to see Jack McDevitt’s work trashed.

Damn, that's a shame. I'll look up the book, thank you.

One of the things that always confused me when watching The Time Machine was the moon falling. It was cool, but it was complete movie magic. Nuclear excavation would in no way send the moon careening into the Earth.

That’s hilarious because the first thing I thought when reading the article title is basically the premise of that movie.

There exists good explanation. Moon kinetic energy is ~10^28 J. As stated in wikipedia, Sun Luminosity ~10^26 W.

So to stop Moon, humanity should convert all Luminosity of Sun to force with 100% conversion rate for just 100 seconds, or just 1% for 3 hours (~10000 seconds).

For comparison, whole humanity annual power consumption is about 580 000 000 * 10^10 J or 5.8*10^18 J, or about 10^13 W, so to achieve 1% of Sun luminosity per second, need about 10^6 seconds, or about 10 years.

So in other words, if humanity will spend all consumed energy to stop Moon, will need about 10^10 seconds, or 10 000 years.

Sure, alter orbit about 1% seem significant enough, this will be just 100 years at full power.

And yes, it is now hard to believe, that all humanity consumed power could be used in such project, but if use extraterrestrial resources, could cover all Moon with Solar panels and combine their power into few huge ion thrusters, and this will give comparable effect.

Yes, this is not easy, but with current achievements with technologies (especially AI), looks like will become achievable to start such project in nearest decades.

If you like absurd questions like this with real scientific responses, you might enjoy the “What If?” books [1] by Randall Munroe (creator of xkcd).

They are full of questions like this, often with a humorous spin. Entertaining and educational.

Some sample questions from the Amazon description:

- What if you tried to hit a baseball pitched at 90 percent the speed of light?

- How fast can you hit a speed bump while driving and live?

- If there was a robot apocalypse, how long would humanity last?

[1] https://en.m.wikipedia.org/wiki/What_If%3F_(book)

> What if you tried to hit a baseball pitched at 90 percent the speed of light?

https://www.youtube.com/watch?v=3EI08o-IGYk

The most relevant one should be https://what-if.xkcd.com/13/ when is tried to change moon brightness with laser pointers. In the end, with current technology, we change the moon orbit, with light.

Kinetic energy != momentum.

KE = ½ m v^2. Moon 3.8e28 J

Tsar Bomba 4e17 J

Only somewhat related and much less serious, but hopefully humorous: https://www.youtube.com/watch?v=GTJ3LIA5LmA

tl;dr: No, unless you have 200 billion nuclear bombs.

Start by altering the orbit of a minor asteroid using a normal gravitational tug/gravity tractor.

Use that to work your way up to larger and larger asteroids, until you slam Ceres into the Moon.

This reminds me of some of the "cold war scares" that I've read about:

> "If MAD happens (ie USA/RU nuclear bombing each other), then the axis of the earth would permanently be altered and every human on the planet would not be able to survive"

I guess the modern era can look at that with some scrutiny; but 40, 50 years ago, that could've been a real fear?

To be fair, the axis of the earth shifting wasn't a high priority in the list of fears if "MAD happens"

Edward Teller wasn't sure the atomic bomb they were building wouldn't light the entire atmosphere on fire for a while there.

Not really, the answer you are referring to is for completely stopping the moon by delivering enough energy to cancel out its current kinetic energy, of the entire moon!

We don't need to stop the moon to change its orbit. With careful and deliberate adjustments via bombing and letting Earth gravity do its job, we could alter its orbit by adding to its force vector instead of trying to negate it.

Applying millions of nuclear bombs to either slow down or speed up the Moon by the same amount will change its orbit by a similar magnitude in terms of the sheer change in energy.

The energy required to achieve a specific Δv is the same whether you are accelerating or decelerating. This is because energy change associated with velocity (ΔE) in a conservative system is dependent on the square of the velocity change (Δv^2), regardless of direction.

Potential energy is twice the kinetic energy in a stable orbit. Changing the orbit by epsilon/r would require roughly U*epsilon/r energy. Which means that a tsar bomb would only move the moon distance to earth by 10^-10

All of those answers are talking about the energy required to stop the moon dead in it's tracks.

What if we just wanted to slow it down 0.0000000000000000000000000000000000001% ? (which will alter it's orbit).

That seems totally doable.

We are already doing that. A orbital mechanics maneuver used in deep space spaceflight called a gravity assist or slinghshot trades a cosmic object's kinetic energy to change a spacecraft's flight path. Since that takes a large amount of energy at the speeds spacecraft go, an equal amount of energy must get subtracted from that object. Of course, the same amount of energy that's very significant for a spacecraft is meaninglessly small for an object such as a natural satellite like the Moon or a planet as Jupiter. So if you slingshot around Jupiter you are altering Jupiter's orbit around the sun an almost infinitesimally small amount, but you are altering it.

But of course, the question was asking for a significantly altered orbit, which this most definitely isn't.

Solar-powered railgun accelerating rocks to lunar escape velocity in directions chosen to make the desired delta-v. Speed the moon up or slow it down or change whatever other orbital elements you please.

This would work better to change the orbit of a small asteroid. For the moon it would take (a lot) longer.

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