What Would Happen if We Tried to Pull the Moon Closer to Earth?

[Quadruple Bypass]

if we were to tie a rope around the moon and have the end on earth. what would happen if we were to pull the moon? could we actually pull it closer until it eventually collides with earth?

 

[Danger]

If you have something strong enough to pull it with, yes. I wonder if the mass of the rope alone would be enough to offset the orbital balance without even pulling it.

 

[Jimmy Snyder]

if we were to tie a rope around the moon and have the end on earth. what would happen if we were to pull the moon? could we actually pull it closer until it eventually collides with earth?

That's a pretty big "if". There is no material known with the tensile strength to support it's own weight as it hangs down from the center of gravity to the respective masses of Earth and Moon. Tugging on that rope would add to the stress upon it. If you could come up with such a material and anchored it at both ends, then you could pull on it and set up a force that would tend to bring the two masses together. However, you have other problems to overcome.

Perhaps you are aware that the Earth and Moon are actually receding from each other even as we speak. In order to reverse that motion and have the two approach you would need apply enough force to overcome this effect. Such a force is far beyond the capability of today's technology. If you could come up with such a force, then you could get the Moon and Earth to approach each other. However, you still have
other problems to overcome.

As the Moon and Earth approach, the angular momentum will not decrease, but the moment of inertia will. In other words, the system will start to spin like a figure skater that has pulled in their arms. Long before the two masses approach to near touching, the whole shebang will spin out of control. Perhaps you could overcome this problem by transfering the angular momentum to the Earth's core. However, you still have other problems to overcome.

As the Moon and Earth approach each other, tidal forces will increase dramatically. The anchors which hold the rope will be compromised. Perhaps there are other problems that I haven't considered.

Having said all this, I don't want to overly discourage you. If you intend to pursue this matter, I suggest you start with smaller masses. Perhaps you could start with Pluto and Nix and build from there.

 

[DaveC426913]

Perhaps you are aware that the Earth and Moon are actually receding from each other even as we speak. In order to reverse that motion and have the two approach you would need apply enough force to overcome this effect.

Which, at a mere 4 centimeters per year, is negligible.

In other words, the system will start to spin like a figure skater that has pulled in their arms.

Yeeeessss... The other way to look at it is that, as you pull the Moon toward the Earth, you will be pulling it into a tighter orbit, it will move forward and ultimately outward again (because you will have moved it into an elliptical orbit). The Moon will act to resist your pull.

You rope would need to be strong enough, not just to move the Moon, but also to pull it against its tendency to recede in its elliptical orbit.

Another thing to factor in is that, when you get it to within about 10,000km distance, the Moon will disintegrate (this is the http://en.wikipedia.org/wiki/Roche_limit" - the Moon will be unable to hold itself together against Earth's gravitational tides). Earth will lose a moon but gain a ring, just like Saturn's.

 

[Quaoar]

Libration could foil the rope idea, even if it were strong enough in the first place:

 

[Quadruple Bypass]

wow i never knew about the roche limit, that's cool :P

 

[G01]

Why would you want to chain the moon?

I am reminded of the opening scene of the "Sonic CD" video game where Dr. Robotnic chains the moon to a rock on the planet.

 

[Quadruple Bypass]

well i just really wanted to know if it would be possible

 

[eeka chu]

well i just really wanted to know if it would be possible

Step one in this task will be a visit to your local DIY store for 400 million meters of nanotube based twine, most likely a few tens of meters thick in it's centre - you'll need to use a credit card for that probably.

Next you'll need incredibly substantial anchoring points positioned on the moon and earth, a visit any daytime TV chatshow audience will likely furnish you with these elements.

And besides, think of the complications you'd create with things like evil moon based pads, there'd be no moon to put them on anymore.

"Welcome to my evil ring based lair"... it just doesn't sound right.

 

[Jimmy Snyder]

Which, at a mere 4 centimeters per year, is negligible.

The momentum is not neglible. It must be overcome before the Moon stops receding and starts to approach.

 

[Janus]

I'm surprised that no one has brought up the question of exactly how one would anchor said rope to the Earth, considering that the Earth rotates at a much different rate than the Moon revolves around it.

 

[DaveC426913]

The momentum is not neglible. It must be overcome before the Moon stops receding and starts to approach.

The Moon's elliptical orbit alone causes its distance to change by ~40,000km every two weeks. That is a factor of ~10^13 greater than the Moon's biweekly recession of 1.6mm.

I'm sayin' that's neglible.

 

[DaveC426913]

I'm surprised that no one has brought up the question of exactly how one would anchor said rope to the Earth, considering that the Earth rotates at a much different rate than the Moon revolves around it.

With a slipknot, obviously.

 

[russ_watters]

Too much friction - you don't want to fray your rope. You need a pulley on a bearing.

 

[rbj]

I'm surprised that no one has brought up the question of exactly how one would anchor said rope to the Earth, considering that the Earth rotates at a much different rate than the Moon revolves around it.

With a slipknot, obviously.

naw, tie it to the north pole. let it wrap around. maybe we could pull it in without a winch.

 

[Quadruple Bypass]

OR, you can get all the 6.x billion ppl to jump onto the rope...nvm, just did the math, won't work out :/

well, at least now we know how to get a free ride into outer space

 

[eeka chu]

naw, tie it to the north pole. let it wrap around. maybe we could pull it in without a winch.

Now what would that do to our axial orbit, our resulting gravity and the effect on the moon?

I always thought it was kind of funny watching the waves slosh around on the surface of the sea and thinking, from space this looks flat and this must be what it'd be like to be an electron on the surfaces we consider flat (thinking of that picture of the wave function captured in the ring of atoms) - but probably tamer. Then I wondered about what it'd be like if we had a super dense body orbiting Earth, so dense that it pulled all of the oceans around the Earth with it in one go such that the oceans became a large cone shape extending off the surface of the planet...

"Oh naw, not again! HERE COMES THE SEA?"

 

[Janus]

naw, tie it to the north pole. let it wrap around. maybe we could pull it in without a winch.

Ah, but the moon's orbit doesn't lie on the equator, so it's distance from the North pole would change even if it were in a circular orbit.

 

[rbj]

Ah, but the moon's orbit doesn't lie on the equator, so its distance from the North pole would change even if it were in a circular orbit.

still just let it wrap. it just might not be uniform wrapping around the pole. of course as the moon pulls away the rope would get a little tight.

 

[Integral]

I'm surprised that no one has brought up the question of exactly how one would anchor said rope to the Earth, considering that the Earth rotates at a much different rate than the Moon revolves around it.

Whats the problem? You just anchor it some where on the equator...Yell heads... Let 'er wind up. You would pull it in at a rate of 1 Earth circumference per day. (The heads holler is to let everyone on the path of the rope know that something is coming their way!)

 

[Jimmy Snyder]

The Moon's elliptical orbit alone causes its distance to change by ~40,000km every two weeks.

That momentum need not be stopped, the 4 cm/yr must be.

 

[DaveC426913]

That momentum need not be stopped, the 4 cm/yr must be.

You're clutching at straws.

Regardless of what technique we use to reel in the Moon, it is going to involve forces way greater than that which would create a puny 4cm per year.

1] The recession is due to transfer of energy from the Earth from tidal friction. There won't be any orbit to affect once we attach a rope to it.
2] If we plan on reeling it in within a timeframe that is less than 10 billion years, the force from reeling alone will exceed the force to overcome the recession. (If we want to reel it in within a lifetime, the force will exceed the recession by a factor of 100 million.)
3] The margin of safety on our rope had better be more than 0.00000001%, don't you think?

The force you're concerned with may be large, but it is completely dwarfed by the forces involved in the task at hand.

 

[DaveC426913]

Ah, but the moon's orbit doesn't lie on the equator, so it's distance from the North pole would change even if it were in a circular orbit.

S'only 5 degrees.

 

[Janus]

S'only 5 degrees.

5° to the ecliptic. Which means it is about 17° to the equator.

 

[pervect]

Build lots of pumps, and pump the tidal bulges so that they slow the moon down rather than speed it up :-)

I'll leave calculation of the energy involved to someone else. Offhand I'd guess it's probably within the range of a Karadashev type II civilization,

http://en.wikipedia.org/wiki/Kardashev_scale

but I could be wrong.

 

[Janus]

Whats the problem? You just anchor it some where on the equator...Yell heads... Let 'er wind up. You would pull it in at a rate of 1 Earth circumference per day. (The heads holler is to let everyone on the path of the rope know that something is coming their way!)

That's quite a lot of people you are going to have to warn!

Consider that the moon's orbit is tipped at 17° to the equatoral plane. The rope will tend to wind its way North or South (depending on the orientation of the orbit at the time.) Once it gets to about 17° of the pole,(Actually less, sometime before that, the tension on the rope will be greater than the friciton between the rope and the Earth.) then it will just keep slipping off. Then, as the month progresses, the moon's position will change orientation to the Earth's axial tilt. As it does so, and the Moon gets closer to the equatorial plane, the rope will work its way closer to the pole again. After the Moon crosses the equatorial plane, the rope will start to work its way back toward the equator and then eventually to the other pole. Then the process repeats in the other direction. After a while the Earth will look like a giant ball of string, with just the ice caps exposed.

The above is even a over simplfied scenerio as it ignores the torque placed on the Earth by the off center force of the rope as it works it way toward the poles.

 

[DaveC426913]

That's quite a lot of people you are going to have to warn!

Consider that the moon's orbit is tipped at 17° to the equatoral plane. The rope will tend to wind its way North or South (depending on the orientation of the orbit at the time.) Once it gets to about 17° of the pole,(Actually less, sometime before that, the tension on the rope will be greater than the friciton between the rope and the Earth.) then it will just keep slipping off. Then, as the month progresses, the moon's position will change orientation to the Earth's axial tilt. As it does so, and the Moon gets closer to the equatorial plane, the rope will work its way closer to the pole again. After the Moon crosses the equatorial plane, the rope will start to work its way back toward the equator and then eventually to the other pole. Then the process repeats in the other direction. After a while the Earth will look like a giant ball of string, with just the ice caps exposed.

The above is even a over simplfied scenerio as it ignores the torque placed on the Earth by the off center force of the rope as it works it way toward the poles.

You would take a short section of rope and form a yoke from North to South pole, then attach your main rope to that with a slipknot. Now the Earth can pin and wobble all it wants without slippage.

 

[Janus]

You would take a short section of rope and form a yoke from North to South pole, then attach your main rope to that with a slipknot. Now the Earth can pin and wobble all it wants without slippage.

Okay, but this is different form anchoring to a point on the Equator and letting the Earth's rotatation reel in the Moon, which was what I was responding to.

 

[DaveC426913]

Okay, but this is different form anchoring to a point on the Equator and letting the Earth's rotatation reel in the Moon, which was what I was responding to.

Yewah, as I was writing it, I realized it was drifting from the simplest 'wind em up' arrangement proposed.