In an old science fiction story. Two sides are involved in a forever type war. Technology had advanced to the point that they couldn't actually hurt each other so one side had an idea.

Build giant rocket motors to slow the rotation of the planet, the idea being to stop the planet with their enemies on the dark side.

Fantastic plan but the question is, given enough power what would be the effect of slowly reducing a planets rotation to zero?

Probably not a very bright idea as, if this fictional planet is any thing like our home world Earth. Slowing to a stop would not be easy to do. To calculate the energy input required for the mass involved.. All do able. BUT! . If it were this Earth that we stopped the effects would be dire....The dark side would freeze while the solar facing side would cook. The atmosphere could boil off. The oceans would freeze/boil. If we lived through that we would most certainly die of radiation burns as the van-alien belts would be gone. No rotation, no magnetic field to protect us. This is a very bad idea. Please cancel planes for altering orbital velocity. Or rotation speed.The Eco system that has allowed life to flourish is a delicate balance of many criteria. Changing any could result in irreversible consequences. What would be the effect,? Our death.

Of course this means that the planet has one side always facing the star, and the other side always facing the depths of space. That is not "stopped" rotation; the planet is rotating, it just has an axial rotation period that is equal to its annual orbital period. In general, one would expect the effect to be that the night side folks get really cold and the dayside folks get really hot. This is because all of the radiant heat falls always on the one side of the planet, and it does not get a chance to rotate into the dark and cool off, so it gets baked. Likewise, the other side does not get a chance to rotate into the day and warm up.

But that is not always necessarily the case. A planet with a dense, heavy atmosphere, can move heat from the day side to the night side by superrotation of the atmosphere. This is what happens on Venus, which rotates very slowly with respect to the sun, but its day side & night side temperatures are the same. And if the planet is far enough from its star, it may not get enough heat to bake the dayside anyway.

That kind of rotation can only be maintained by tidal locking. If the planet is not at the proper distance from its star, that will be an unstable situation. The planet will begin to "rotate" again, due to tidal forces. So constant force will have to be applied to keep it locked.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell

If you want to stop the planet do it very gently. Else the kinetic energy might be converted into heat, and you´d end up with an inferno of molten rock all over.

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What brings us together is stronger than what pulls us apart

If it's done by a machine at the surface, you'd only be directly pushing on the crust... probably just one plate. The momentum of the mantle, core, and other plates would cause some nifty geological "disruptions". Of course, for the sake of argument, we could postulate that they have the technology to stop the whole thing from rotating together instead of pushing on one part and counting on that to push/pull the rest...

Temperatures on the light side would end up too high. And at the horizon line, the only place where the average temperature might remain what it was before, the conflict between the temperatures on the light and dark sides would result in rather chaotic wind and weather.

A non-rotating core would cease to create a magnetic field.

This is an excellent example of the power of hatred motivating people to go to extraordinary lengths to destroy their opponent. I vote we send a peace delegation to that planet and reconcile the two parties before the whole race is exterminated through hate.

I would suggest not sending a delegation, and allowing them to destroy themselves. They may decide to put aside their own differences and focus on us!

If it were this Earth that we stopped the effects would be dire....The dark side would freeze while the solar facing side would cook. The atmosphere could boil off. The oceans would freeze/boil. If we lived through that we would most certainly die of radiation burns as the van-alien belts would be gone. No rotation, no magnetic field to protect us. This is a very bad idea. Please cancel planes for altering orbital velocity. Or rotation speed.The Eco system that has allowed life to flourish is a delicate balance of many criteria. Changing any could result in irreversible consequences.

That's a rather dire prediction, and I'd suggest flawed as well.

The dark side would freeze while the solar facing side would cook. The atmosphere could boil off. The oceans would freeze/boil.

Not necessarily. As the atmosphere and oceans cooled and heated it would cause high pressure and low pressure zones resulting in hot air moving into the dark side and cold air travelling to the hot. While the darker side would be colder, this exchange of air would prevent the extreme changes in temperature. We would have extreme weather however. Note that Venus has regular winds of over 300km/h.

If we lived through that we would most certainly die of radiation burns as the van-alien belts would be gone. No rotation, no magnetic field to protect us. This is a very bad idea.

Again, unlikely. It might not be a wonderful idea to stand outside when there was a solar flare that sent a CME in our direction, but as a rule there is little solar radiation in space. The upper Van Allen Belt is created because the passing solar wind gets caught in it and then builds up to a large amount while the lower one is actually made up of ioniozed hydrogen from our own atmosphere, created by the solar wind and then trapped in the magnetic fields. Without a magnetic field we'd lose the belt, but life wouldn't end. Remember that our magnetic field dies in strength, stops, and then switches over every hundred-thousand odd years and life didn't get wiped out by that.

The Eco system that has allowed life to flourish is a delicate balance of many criteria. Changing any could result in irreversible consequences.

Well while it certainly would be interesting, I think life would carry on regardless, creatures and plants would adapt.

On a side note, stopping the planet actually would cause about 3 months of total darkness, 6 months of twilight and 3 months of sun, similar to what occurs in our polar regions now, rather than one side being in permanent darkness. To do that you would need to slow the rotation rate to equal the orbit about the sun. This would mean a slow rotation.

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Howling from the Shadows

It must be fun to lead a life completely unburdened by reality. --- JayUtah

You can't reason an irrational person out of an irrational belief. --- Noclevername

Apollo: The History and the Hoax
Enter the World of Athran

I bet the liquid core would carry on rotating, so you'd still have a magnetic field. I don't know how long it would take to stop due to friction.

(Note many planets and our sun have different rotation angular velocity at different latitudes, and that situation has persisted (presumably) for billions of years.)

I bet the liquid core would carry on rotating, so you'd still have a magnetic field. I don't know how long it would take to stop due to friction.

The rotating core doesn't cause the magnetic field. Remember there is a website (now two) attached to this forum.

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Howling from the Shadows

It must be fun to lead a life completely unburdened by reality. --- JayUtah

You can't reason an irrational person out of an irrational belief. --- Noclevername

Apollo: The History and the Hoax
Enter the World of Athran

I'm not sure that would work; something to do with conservation of angular momentum

The earth's rotation is slowing, but that is because it is coupled to something external (the moon). I guess it would depend on where/how the rocket motors are located. But if you imagine some rocket motors on the equator, fixed to the earth and pointed towards the west, firing constantly...they would not slow the earth's rotation

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PW -- Plant Whisperer

But if you imagine some rocket motors on the equator, fixed to the earth and pointed towards the west, firing constantly...they would not slow the earth's rotation

Conservation of angular momentum isn't relevent when you are apply a torque to an object, the rocket motors would be doing just that, burning up their fuel to provide the energy required to apply the torque.

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Howling from the Shadows

It must be fun to lead a life completely unburdened by reality. --- JayUtah

You can't reason an irrational person out of an irrational belief. --- Noclevername

Apollo: The History and the Hoax
Enter the World of Athran

The rocket motor applies a torque to the earth, transferring angular momenturm to the earth, through the crust. At the same time, the rocket is accelerating exhaust gases to high velocity, imparting same quantity of momenturm to gases in atmosphere, but in opposite direction. However, the earth and its atmosphere are a closed-system, i.e. atmosphere transfers momentum back to earth.

Result? No net change: the total angular momentum of earth/rocket/atmosphere does not change, ergo rotation rate does not change.

To slow earth's spin, you must couple the braking force to something external to the earth.

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PW -- Plant Whisperer

A rocket doesn't push again the air though. While the gases it is expell do enter that atmosphere, not all of that momentum is returned to the earth. Some will be lost with gases escaping the atmosphere as it heats up, other parts will remain in the atmosphere itself as part of the weather system.

__________________
Howling from the Shadows

It must be fun to lead a life completely unburdened by reality. --- JayUtah

You can't reason an irrational person out of an irrational belief. --- Noclevername

Apollo: The History and the Hoax
Enter the World of Athran

I think that this argument is basically correct - if you assume that the rocket is on the surface and exhaust gases are retained in the atmosphere (essentially a closed system) - the rotation change to the earth surface would be limited to amount of angular momentum that could be "transfered" to the atmosphere. For this impossible scenario to work, the rocket "exhaust" gases would need to escape the earth's atmosphere.
Compared to obtaining the energy and mass that would be required to modify earth's rotation, engineering a system to deliver the exhaust to space would be trivial!

Ha! Another 1940(?) Science fiction short story debunked!

Thanks for the interesting replies.