{I annoed this once but I think it deserves its own posting}

I read somewhere I can't find anymore that, other than at the bottom of some deep craters, a human would have to perpetually move in a straight line at four kilometers an hour to stay ahead of the solar terminator line when temperatures go from -400 degrees to hot enough to melt lead.

This would make setting up a mining colony problematic and likely to fail, but what if we could change that hot/cold scenario?
On the Colonize Mercury idea [link] it was proposed to slow Mercury’s' spin so that one side always faces the Sun but it would take far less energy to use harmonic resonant waves to tilt Mercury and reorient one of its poles to point towards the Sun. This could be accomplished by nuclear bombardment, which might have the added advantage of pre-fracking the areas to be mined and do a fair amount of smelting in the process.
This would allow for the entire opposite hemisphere to be mined in the shade while the temperature differential at the new terminator line is used to provide the power requirements of a colony.
-- 2 fries shy of a happy meal, Mar 24 2013

Colonize Mercury Colonize_20Mercury
[2 fries shy of a happy meal, Mar 24 2013]

Composition of Mercury. http://www.universe...osition-of-mercury/
Estimated over seventy percent metals and thirty precent silicas. [2 fries shy of a happy meal, Mar 24 2013]

Mercury 2.0 Mercury_202_2e0_20(the_20planet)
So much. [bungston, Mar 24 2013]

Meteorite from Mercury (maybe) http://news.yahoo.c...irst-145455980.html
[not_morrison_rm, Mar 30 2013]

So nothing like Mein Kampf?

Am I the only person to be disappointed that Gemeinschaft actually has nothing to do with mine shafts? Funny language, German.
-- not_morrison_rm, Mar 24 2013


How many nukes would it take to stop the rotation? How much would it cost to deliver them there (lots)? How much benefit would we gain from mining Mercury and how would we deliver the ore back here?

If you're looking for a planet to mine, one of the reasons I think we should be focusing on Jupiter instead of Mars is that it is a cornucopia of resources. You could setup a station on a moon of Jupiter and use the planet itself to refuel spacecraft to deliver tons of fusion fuel and even antimatter back to Earth. If we ever leave this solar system Jupiter will have to play a key role.
-- DIYMatt, Mar 24 2013


What kind of "harmonic resonant waves" did you have in mind, and how will they produce a tilting effect?
-- Wrongfellow, Mar 24 2013


Nope, nothing like mein kampf.

//How many nukes would it take to stop the rotation? How much would it cost to deliver them there (lots)? How much benefit would we gain from mining Mercury and how would we deliver the ore back here?//

I don't want to stop its rotation at all, and I don't know how many nukes it would take to tilt its axis. All of them preferably...
If the nukes were thought of as mining equipment then the cost of delivery would have to be weighed against the potential profit.
Refined materials could be returned to near Earth orbit by magnetic rail-gun launch from Mercury to slingshot around the sun for some added oomph.

//What kind of "harmonic resonant waves" did you have in mind, and how will they produce a tilting effect?//

Well... think of Mercury as just one big gyroscope.
From within any gyroscope a wave in-line with its spin, if amplified, will maintain a constant rate of rotation. By incrementally increasing the tempo of the wave the gyroscope can be made to increase in rotational rate.
By introducing a lateral wave perpendicular to the direction of spin, in place of the initial standing wave, the gyroscope can be made to tilt on its axis with a minimal effort.
That same effect should scale up even with a gyroscope the size of a planet. As Mercury is already lifeless and bathed in radiation I figured that nukes would be about the size of wave needed and we could put the damned things to decent use in a planetary effort to share in the wealth of our solar system instead of mutually assured destruction.

A shit-to-shinola scenario. Crazy I know. It'd work though.

Hey, that rhymed.
-- 2 fries shy of a happy meal, Mar 25 2013


Can we mine Mercury for Uranium to replace all the nukes?
-- DIYMatt, Mar 25 2013


Well, if you have all those nukes, then why not spin Mercury a lot faster, say, 10% slower than escape velocity at the equator?

Then when the miner has found a lump of something useful they can just throw it up into the sky where it gets caught and processed in the orbital processors.

//Can we mine Mercury for Uranium to replace all the nukes?

Nahh, that's all on Uranus...<wonders if this is true>
-- not_morrison_rm, Mar 25 2013


I like the idea of spinning it faster although the closer to one Earth gravity the better for colonists.
Of course Uranium would be mined if there is any... we'd have to re-flip the planet once one hemisphere is depleted.
-- 2 fries shy of a happy meal, Mar 26 2013


I gave it a bun to equal out the rotation!
-- xandram, Mar 26 2013


//I like the idea of spinning it faster although the closer to one Earth gravity the better for colonists.

That's where the much, much faster comes into its own. It would be spinning so fast that it'd have 1g gravity, but going up as it were, due to the centri-whatever effect.

This shouldn't make so much of a difference in the mines, just all the miners will be standing on the ceiling of the mine, and clunking away at what should be the floor with pick-axes. Getting the ore out is easy, as you just drop it into the mine shaft and it'll fall away from Mercury's centre etc
-- not_morrison_rm, Mar 26 2013


//it was proposed to slow Mercury’s' spin so that one side always faces the Sun but it would take far less energy to use harmonic resonant waves to tilt Mercury and reorient one of its poles to point towards the Sun. // It doesn’t take less energy to change the axis of rotating body by about 90 degrees than to stop its rotation. It takes about 2 times the energy. The change in momentum of needed to change a rotating body’s axis is the same as the change its momentum around the old axis to zero, plus that to change its momentum around the new axis to the desired non-zero quantity. It the two axes are roughly orthogonal (at right angles), the scalars of this vector sum is roughly equal to the sum of its two scalars.

To cause Mercury to keep the same face toward the Sun, you wouldn’t want to do anything but change its rotation to match its orbit. Since its rotational velocity is almost exactly 1.5 times its orbital, this would take a change of 1/3 of its rotational momentum.

//This could be accomplished by nuclear bombardment, which might have the added advantage of pre-fracking the areas to be mined and do a fair amount of smelting in the process.// Assuming the current nuclear arsenals of humankind, no, it couldn’t.

Mercury has a mass of about 3.3e23 kg, a radius of about 2440000 m, and a rotational speed of about 1.24e-6 rad/s. Approximating via the assumption it’s a uniform density solid sphere, so has a moment of inertia of 2/5 m r^2, its rotational momentum is about 9.745e29 kg m^2/s

Assuming an near optimally efficient rocket with exhaust velocity equal to Mercury’s escape velocity of 4250 m/s, you’d need to eject about 3.1e19 kg of reaction mass. Assuming 100% efficiency, this would require about 2.8e29 J. The largest nuclear bomb ever built had about 2.1e17 J. So, assuming you could make a 100% efficient nuclear bomb rocket, you’d need aver 1,400,000,000,000 of them. To date, we humans have managed to build about 100,000 nuclear bombs of various sizes, of which about 17,000 are still usable. So we’re on the order of a few 1,000,000 times short of having enough to put Mercury in Solar tidal grav-lock.

I hate to see a good planet-scale engineering idea go down in flames, though, so suggest forgetting about nuclear explosives, and thinking in terms of orbiting bodies’ natural tendency to tidally lock with their primaries. This is caused by their lumpiness (irregularity). So, to get Mercury grav-locked you could make it very, very lumpy, via stuff like big satellites anchored with super-strong cables. You’d have to be smart about it, though, as Mercury’s unusually eccentric orbit might make a static such arrangement just follow its current 3:2 resonance.

Of course, there’s always the old super-space-engineering standby of crashing big bodies into planets, which has the mining bonus of ejecting much of them into big debris clouds that last 100s of years or more, but that’s too far off the original idea of relatively gentle rotation changing..
-- CraigD, Mar 29 2013


The original idea is pretty far off to begin with. You may be so far off that you're coming back around and approaching 'near' from the other end.
-- Alterother, Mar 29 2013


//good planet-scale engineering idea go down in flames,

That would seem to be Mercury's eventual fate anyway as it's next to the big, hot melty thing.

Update - stardate Saturday the errrm..seems we just have to be patient and Mercury will come to us, eventually. See Mercury meteorite (maybe) link.
-- not_morrison_rm, Mar 30 2013


Very cool link.

[CraigD] Thank you so much for your analysis. I can refute not one stitch of your math. I don't get it, you may as well be speaking a foreign language and I'll have to take your word for it.
Your numbers may be correct but the premise apon which those numbers are based is flawed.

//It doesn’t take less energy to change the axis of rotating body by about 90 degrees than to stop its rotation.//

Are you sure about that? I know this to be false. A spinning body can be made to change the orientation of its axis using much less energy than it takes to stop its spin entirely... just ask any cat.

//To cause Mercury to keep the same face toward the Sun, you wouldn’t want to do anything but change its rotation to match its orbit. Since its rotational velocity is almost exactly 1.5 times its orbital, this would take a change of 1/3 of its rotational momentum.//

I don't want to stop its spin, I want to use its spin to tip it... which will take much less than 1/3 of its rotational momentum to accomplish.

// Mercury has a mass of about 3.3e23 kg, a radius of about 2440000 m, and a rotational speed of about 1.24e-6 rad/s. Approximating via the assumption it’s a uniform density solid sphere, so has a moment of inertia of 2/5 m r^2, its rotational momentum is about 9.745e29 kg m^2/s//

Again the numbers are meaningless for me but the assumption that Mercurys' core is solid renders them invalid from the get-go doesn't it?

See, if the core is liquid, then fluid dynamics come into play, wave amplification plays a greater role and I see none of these numbers contained in your calculations...
not that I would understand them if you had included them of course... just that you haven't, right?

I don't want to bully the planet into tipping. I want to nudge it... the way a pre-schooler would nudge a swing into motion from a standstill because their feet weren't long enough to reach the ground.

See?
-- 2 fries shy of a happy meal, Apr 01 2013


// Mercury has a mass of about 3.3e23 kg, a radius of about 2440000 m, and a rotational speed of about 1.24e-6 rad/s. Approximating via the assumption it’s a uniform density solid sphere, so has a moment of inertia of 2/5 m r^2, its rotational momentum is about 9.745e29 kg m^2/s// //Again the numbers are meaningless for me but the assumption that Mercurys' core is solid renders them invalid from the get-go doesn't it?// Because Mercury has a magnetic field (about 1% as strong as Earth’s, but a big discovery when it was first measured by Mariner 10 in the 1970s), we’re pretty certain it either has an all-liquid iron core, or, a solid inner core surrounded by a liquid outer one like Earth. However, for momentum calculations, being something other than a rigid body means that more energy is needed to change it, because some of that energy will be converted to heat as the outer part of the body accelerates the inner part via fluid drag. That Mercury’s core is denser than its crust does reduce the amount of energy needed (because angular momentum is mass times radius), but not greatly, as Mercury has fairly large core relative to its total size (it’s hypothesized it lost much of its outer parts in a giant impact, but, unlike Earth, didn’t keep any detectable part of it as a moon or ring) and less density differences than a larger, compositionally similar body like Earth.

So assuming a uniform density rigid body should be within a factor of 2 of accurate, good enough for these sorts of “on the order of” energy requirement estimates. We might need a 100% efficient rocket system powered by 3,000,000,000,000 of the biggest nukes ever made, or 700,000,000,000 – it’s still many times more than we have.

//I don't want to stop its spin, I want to use its spin to tip it... which will take much less than 1/3 of its rotational momentum to accomplish.// You can’t tip something that’s not experiencing force opposing gravitational (or equivalent, such as in a constantly accelerating vehicle) force (that is, experiences a single “up/down” direction) and has gravitational potential energy (that is, isn’t already tipped) Things sitting upright on the surfaces of planets can be tipped. Planets can’t. They have no single up/down direction.

A key to visualizing the momentum of rotating bodies is to understand that they can be broken down into smaller bodies, all accelerating at different constant rates toward points on a common axis of rotation line. It takes no less energy to change the axis of rotation of a rotating body than it takes to change the direction of each of the smaller bodies it can be broken down into. So changing the axis of a rotating body by 90° has the same energy requirement as changing the direction of each moving part of it by 90°, exactly 2 times that of stopping it/them.

//I don't want to bully the planet into tipping. I want to nudge it... the way a pre-schooler would nudge a swing into motion from a standstill because their feet weren't long enough to reach the ground.// The swing is a good example to consider. It _seems_ like a short-legged kid moving a swing by leaning and tucking their legs is using less energy to accomplish the same final swinging state as a long-legged one with the same mass m who does it with one mighty push against the ground, but (ignoring funky frictional and biomechanical stuff), they’re each using the same amount, equal to m*v^2, where v is their maximum swinging speed. Nudging and bullying spend the energy more and less gradually, but spend the same amount of energy. And you’ve got to have an up/down for swings to work, so their mechanics don’t model the rotation of planets.
-- CraigD, Apr 09 2013


hmmmm,
// Things sitting upright on the surfaces of planets can be tipped. Planets can’t. They have no single up/down direction.//
//Nudging and bullying spend the energy more and less gradually, but spend the same amount of energy. And you’ve got to have an up/down for swings to work, so their mechanics don’t model the rotation of planets.//
//So changing the axis of a rotating body by 90° has the same energy requirement as changing the direction of each moving part of it by 90°, exactly 2 times that of stopping it/them.//

Well, there 'is' a distinct up/down direction for orbital bodies though.
Down corresponds with the position of the body around which it orbits minus the lag-time of the speed of gravity.
What I am saying though is that, even without a gravitational source to act against, a spinning body can be made to tip with much less force than it takes to stop it from spinning. I only know this from having been strapped into a human gyroscope and figuring out how to control its rotational rate and orientation from inside of the gyroscope itself.

I managed to atain four or five revolutions per second and, (when it began to shake loose from its mounts and looked about to go trucking through the fairgrounds), it took almost the same amount of energy to bring it back to a standstill as it took to get it up to speed, but I could change it's orientation while it was spinning to whatever I wished, also I could, and did, tilt its axis 180 degrees within a single revolution with hardly any effort at all from the inside.

I am saying that the same physics would have worked even in microgravity where there appears to be no up or down.
and I can prove it... just not with math.
-- 2 fries shy of a happy meal, Apr 09 2013


To change the axis of rotation of a body, you need to change the angular momentum. The law of conservation of angular momentum means that you must impart an equal-and-opposite angular momentum to the rest of the universe. You were able to do that with the human gyroscope by exchanging angular momentum with the earth, through gravity and structural contact. In that instance, you can indeed change your axis of rotation with very little loss of energy.

But unless you can similarly couple Mercury with a massive body, such as the sun, you will need to expend vast amounts of energy. Worse, with a pole pointing towards the sun, you will need to be constantly exchanging angular momentum. There's a reason why bodies are never found orbiting in that configuration.
-- spidermother, Apr 10 2013


Ah, I see.
Uranus points one pole at the sun but then half of its year later the other pole faces the sun.
Even if a planet had no spin and was far more dense at one pole than the other, the oscillations caused from travelling around the sun itself would eventually induce spin. You are right, it would take constant adjustment.

Crap!

Y'know, one of these days that ol' drawing board's gonna tell me it doesn't want me back anymore...
-- 2 fries shy of a happy meal, Apr 10 2013


//There's a reason why bodies are never found orbiting in that configuration.//

There's an episode from a forensic science show in there somewhere.
-- pertinax, Apr 11 2013


Once you've tunneled down a thousand feet or so beneath the planet's surface, it wouldn't matter which side faced the Sun.

<edit>

Apparently Mercury has a liquid iron core. That's going to complicate matters.
-- whlanteigne, Apr 11 2013


I think you'd find a terrestrial planet devoid of internal thermal fluctuation to be a pretty cold and barren place...something like Mars, perhaps. According to Morgan Freeman (presenting on behalf of anyone willing to appear on his show), the insidy-parts of gas giants are in pretty dramatic flux at all times.

My knowledge of geology/seismology/planetary engineering fairly is limited, so I won't deliver a lecture on the mechanics of the Earth's mantle, but I'd lay down good money that somebody else will.
-- Alterother, Apr 11 2013


It would probably be just as easy to change the orbit of Mercury and capture it as a second moon of the Earth by directing asteroids to impact Mercury in such a way as to increase its orbital speed, then redirecting the altered orbit to be captured by the Earth-Moon system.
-- whlanteigne, Apr 11 2013



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