Probably about the same amount of energy that it would take to stop it, if it were already rotating. If I understand Newton’s law of motion correctly.
No, because it is already rotating (one rotation in 27.3 days).
You’d just need enough force to speed up its rotation noticeably out of its tidal lock.
A characteristic of Earth’s Moon that no one has yet mentioned is that the mantle of the Moon has a higher density and more mass on the side facing the Earth, which is a result of the gradient of Earth gravity during the early development of the Moon and is responsible for the stability of the tidal locking in the Earth-Moon system. Because of this, the Moon does rock back and forth a bit compared to an observer at the L1 libration point (or any of the libration points, but most obvious from directly viewing the Earth-facing side). To get the Moon rotating you’d have to apply sufficient torque to overcome this stabilizing moment. There is also a wall part of the Moon’s core that is partially liquid iron that would provide a damping torque to a change in rotation speed.
From a practical standpoint, applying a torque to cause the Moon to rotate would have to be sufficiently distributed such that the point loads at the footings of whatever thrust structure you are using don’t just tear away or liquify the crust. Realistically, providing enough energy to overcome the stabilizing moment and get the Moon moving would likely fracture the crust and separate it from the mantle even if the thrust was distributed evenly around the equator. To get the Moon to rotate without basically tearing it apart you would need to have some kind of technomagical tractor beam that applies the load more uniformly through the mantle of the Moon, or else somehow rigidize the Moon to resist the internal stresses.
The problem with using a solar sail, aside from the massive scale of the structure required to get any appreciable fraction of the required force, is that Moon is constantly rotating so the sail is only oriented for positive torque only less than half the time, and is opposing the desired rotation the other half. Solar wind can only push outward, whereas applying a torque requires the application of a couple, i.e. two forces applied in opposite directions. This is true even when directly applying torque via rotation; at some point, there are two or more points which connect to “ground” for which the vector sum is equivalent to two forces in equal magnitude and opposite directions applied at the same distance away from the application of torque.
Stranger
ETA: **Stranger **wasn’t there when I started.
With a little (well a lot) of help from Wolfram Alpha [We do live in truly awesome times] …
The total rotational kinetic energy of the Moon is 3E23 Joules.
If we wanted to spin the moon up to double its current rotation the visual result would be full moons alternate between the face we’re used to and the other one; the current far side. So that sounds to me like a nice round goal.
All we need to do is create 3E23 J and couple that losslessly to the Moon. The Moon’s mass is 7E22 Kg. So we need to add about 0.5E1 J/Kg. At least we won’t melt the whole Moon doing it.
But the Moon’s surface area is 4E13 m[sup]2[/sup]. So if we try to attach this energy to the surface we’re applying 3E23/4E13 = 1E10 J / m[sup]2[/sup] . Can you say “molten incandescent lava surface?”
XKCD has more on the downsides of a molten incandescent Moon here: Laser Pointer
IMO you really can’t make a statement like that with quantifying it with some numbers, parameters, time frames, and calculations.
Not that there is much chance of me bothering to do them at the moment.
For that matter, the moon didn’t rip apart as it slowed down to be tidally locked in the first place. Or the Earth that helped with the locking and crust/geology wise is an egg compared to the moon.
Miles Davis Kind of Blue released.
SD Member Leo Bloom released.
Nitpick: To double the Moon’s rotation rate, you’d need to quadruple its energy, not just double it.
The moon didn’t “rip apart” as it slowed down because it was still congealing and the gravity gradient from the Earth was relatively shallow, even much closer in where it started.
As for calculations, I did this very quick off the top of my head while waiting in line with crappy reception so you’ll have to excuse any errors and approximations, but I think the rough order magnitude should be about right. For the purposes of calculation I’ve assumed that the moon is a perfectly distributed sphere with no offset torque. The moon has a radius of about 1740 km and a mass of 7.35 x 10[SUP]22[/SUP] kg. It is rotating once every 27.3 days which gives it an angular velocity of about 2.66 x 10[SUP]-6[/SUP] rad/s. The rotational inertia should be about 8.90 x 10[SUP]34[/SUP] kg-m[SUP]2[/SUP], and the resulting angular momentum is about 2.37 x 10[SUP]29[/SUP] J-s. If we want to slow down or speed up the rotation by 10% of its angular speed over the period of a year the required torque will be 7.50 x 10[SUP]20[/SUP] N-m, and a concentrated force at the surface would be 4.31 x 10[SUP]14[/SUP] N. Distributed evenly across the entire circumference of the moon’s equator, that is 39.5 x 10[SUP]6[/SUP] N/m, or 226,000 lbf per linear inch.
So, basically if you could figure out some way to affix your torque structure to the Moon’s surface continuously with the strength of approximately a thousand times the strength of a 1 inch double fillet weld in steel…you’d still probably end up shearing this ring away from the adjoining regolith because the shear strength of soils or even solid rock just isn’t that high. You’d have to embed this ring kilometers down and outward (and likely down to the upper mantle) in order to have any chance of being able to apply this torque without tearing away from the crust.
Stranger
Well, you could stop its rotation altogether for the same effect. Now we just need to find a way to harvest the Moon’s rotational energy.
Good catch. Thanks. Not that it much alters the big picture. :eek:
I had this in mind, but couldn’t quite put it into words as you have when I posted this:
So, is it close to saying the extra mass on the near side puts the Moon in a sort of “cradle” you’d have to tip it out of, before you can begin accelerating its rotation?
Yes, exactly. Of course, any kind of constant thrust will eventually overcome this, but again, you have a problem apply thrust to “the moon” in a sufficiently distributed fashion that you could effect a change in angular momentum without just breaking it apart, or at least fracturing and liquifying the crust.
However, if you could just negate the inertia somehow, you could rotate it at any arbitrary speed with almost no thrust. So, go figure out how to violate one the basic laws of mechanics and you got yourself a stew going.
Stranger
…Did someone say “stew”?
You could cover the lunar surface in big vertical mirrors, reflective on one side and black on the other, so solar light pressure would exert torque on the moon.
It would take a really long time, you don’t need it done soon do you? Actually, I doubt it would even overcome the earth’s tidal locking. Would make the moon look cool though, like a white/black domino mask.
So it sounds like we’ll have to dig a set of trenches into the surface of the moon, ramping downward until the far is a kilometer or more below the surface. We start tunneling in there horizontally for our rocket engines. And those are just linear accelerators like a rail gun to eject pieces of moon opposite the it’s direction of rotation. We have a lot of loose moon around after digging those trenches so we won’t have to dig up much more fuel, at least for a while. It doesn’t have to have that 48 hour period, it could take years to perform one rotation relative to earth, but for maximum viewing entertainment I’d suggest a period of no more than one year. I can’t believe we haven’t started on this project already. Here we are, planning to go to Mars, and we haven’t even altered the rotation of the moon yet. Kids today, I’m tellin’ ya.
That’s gravity, and AFAIK whether the Moon is rotating or still doesn’t matter in this regards.
More precisely, that’s harvesting the energy of the Moon’s orbiting around the Earth, not that of the Moon rotating about its axis.
So as you say, whether the Moon is rotating fast, slow, is tidally locked to Earth, or is non-rotating vs. the background stars doesn’t matter for tidal power extraction out to quite a few decimal places.
Yes, thank you for clarifying what I meant but didn’t say unambiguously :o.
My mistake.
But it’s not totally unrelated (I think). The moon is tidal locked because that’s the most stable state given its density distribution and orbit. If we were to add or remove rotational energy from it, it would slowly return to tidal lock, and we could extract energy from that process via tidal generators.