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Inspired from the game "Planetary Annihilation" a Halley Delta V Engine is a engine built in the ground that is big enough to move the planet when used. This can be used for two things in my mind. War, and a savior from global warming (as it can be used to move us further from the sun like scientist are trying to use meteors for). It can be used for war by smashing planets into each other. But if used to save us from global warming, what could be the backlashes of such an engine on the environment, and would such an engine even be possible?

Edit: So basically I'm seeing this engine would not be safe for use in a planet we plan on keeping alive. In that I conclude its best use is for warfare in the destruction of planets and possibly damaging planets nearby as chunks of the two planets collide. And for those complaining about not having enough info about the engine, it would most likely be powered by some sort of electrical motion such as repulsion rather than something such as fossil fuels considered that might not be available on all planets.

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    $\begingroup$ If you had the technology to build an engine that could move a planet you would have already solved the problem of global warming and would no longer need it. However, the reason I voted to close this question as too broad is that you've provided no information about the engine. Without that information, it is impossible to address the issue of its impact on the environment (however, simply moving the planet would have an enormous impact on the environment.) Finally, as an old friend of mine once said, "given enough time and money, anything is possible." $\endgroup$ – JBH Dec 8 '17 at 14:28
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    $\begingroup$ @JBH Just so you know, you don't need any information about the engine. I'm going to demonstrate that the waste heat from moving the planet will boil off the oceans. So this question is still answerable using Thermodynamics. $\endgroup$ – kingledion Dec 8 '17 at 14:43
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    $\begingroup$ Any engine is powerful enough to move the planet. Even a horse is powerful enough! Or even one hamster! It will move the planet very slowly, but move it it will. $\endgroup$ – AlexP Dec 8 '17 at 15:40
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    $\begingroup$ The only reason why it might be feasible in game, is that there is no life left in the universe. The war is waged between unfeeling robots to whom time and the environment are meaningless. Who knows how many thousands of years that 1 hour match really took? Additionally, their tech level is such that everything is measured in mass and energy. The game has Death Stars... Very fun to play, but not the best model to base your ideas on. $\endgroup$ – user2259716 Dec 8 '17 at 15:55
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    $\begingroup$ @JBH Did you read my answer? Thermodynamics doesn't care about the specifics of your engine. If you generate any waste heat, you will fry the planet due to the magnitude of the energy involved. And if you aren't using magic, you will generate waste heat. QED. Question answered :) $\endgroup$ – kingledion Dec 8 '17 at 16:51
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It takes a lot of energy to move the Earth

In this question, I calculate the energy needed to move the Earth. In order to move the Earth by 1m in orbit, you will need to expend about $2\times10^{22}$ Joules. This is about five orders of magnitude greater than the largest atomic bomb, and only one order of magnitude less than the Chicxulub asteroid that killed the dinosaurs.

If you want to move the Earth 1% farther away from the sun, the required energy expenditure is more like $1\times10^{28}$ Joules. That is roughly equivalent to the orbital energy of the Moon, the total output of the Sun for 26 seconds, or 1800 years of sunlight on Earth.

There will be waste heat.

The main problem with your device, is that it will generate waste heat. That is simply thermodynamics, no process is 100% efficient. Even if we assume a 99% efficiency, which is probably closer to magic than science, there will still be $1\times10^{26}$ Joules of waste heat released. And since your engine is mounted into the Earth, the waste heat will be dissipated into the Earth's ground and atmosphere.

Needless to say, that amount of waste heat is unbelievable. The oceans are about $1\times10^{18}\text{ m}^3$. The volumetric heat of seawater is about $4 \text{ MJ}/\text{m}^3\text{C}$. That means, that even a 99% efficient planet-mover moving the planet only 1% farther from the sun would increase the ocean's temperature by about 20 C. If you drop the efficiency to a less magical 50%, the oceans would be boiled away and our atmosphere stripped into space.

Even if you only moved the planet a little distance to combat global warming, you'd be doing much more harm than good.

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    $\begingroup$ You have the solution right in your answer. The needed energy is equivalent to "1800 years of sunlight on Earth." So if you use a sunshade to block 100% of incoming sunlight, you can move earth 1% further out in 1800 years with a 50% efficient engine or in 18 years with a 99% efficient engine. Of course you could also just use a much smaller permanent sunshade to block 2% of the suns light, and accomplish the same thing. $\endgroup$ – Lex Dec 8 '17 at 17:34
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    $\begingroup$ I've never been able to think of a way to easily estimate the severity, but I've always thought something like that would cause massive earthquakes. The vibrations of a machine pumping out energies of that magnitude would not just gently vibrate. $\endgroup$ – MozerShmozer Dec 8 '17 at 18:00
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    $\begingroup$ @Lex If you can block the energy from the Sun with a sunshade why we do need to take away from it in the first place? =) $\endgroup$ – jean Dec 8 '17 at 18:20
  • $\begingroup$ @jean - because a sunshade will need maintenance (orbital adjustments, etc), while moving the planet is a permanent solution. $\endgroup$ – Jules Dec 8 '17 at 19:45
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    $\begingroup$ @Aliden, I'm not suggesting a problem with thermodynamics at all. In an electrically controlled environment, heat is a consequence of resistance, usually identified as material sheet rho. However, in theory, superconducting transistors would have nearly no sheet rho. No resistance, no heat. Thermodyanmics is preserved. While I like Kingledion's answer, I'm having trouble choking down today's limitations for a far-future solution. $\endgroup$ – JBH Dec 8 '17 at 22:28
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You actually can do it without completely destroying all life. The trick is to put the engines on a moon, and hover the moon near Earth in a position where the moon would fall to Earth if you turned the engines off for a few days. The gravity of the moon will slowly pull Earth towards it.

If you mount three Halley engines on the moon at right angles to each other, it's a colossal waste of energy but you'll be able to hover near Earth without directing your exhaust directly at Earth.

I suggest using Earth's moon, because it's already in the neighbourhood, plus its orbit could be messed up by this plan anyway and at least then you control where it goes and can put it back in a good orbit when you're done.

Obviously this plan will change the height and timing of ocean tides while you're carrying it out.

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    $\begingroup$ The idea is good, especially since it also avoids firing the engine through the atmosphere somehow. But you don't need to actually hover the moon, shifting its orbit so that it's eccentric to the earth's gravity center would suffice. Much less engine power required, in exchange for some more precise control of how the engine is fired, and a longer operation time. $\endgroup$ – cmaster Dec 8 '17 at 21:45
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    $\begingroup$ Plus one for heading off the follow up question of how do we bring the moon with us, because life on this planet requires it to normalize Earth's tilt. $\endgroup$ – Mazura Dec 8 '17 at 22:37
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Such an engine is not safe

The amount of thrust needed to move a planet would likely cause a heat blast to ripple through the atmosphere (like a super volcano only bigger). This would likely kill off most life as well as temporarily increase global temperature. That being said if you find a way to isolate the ejection stream from the atmosphere, like a 'magical force field', then it would be less catastrophic.

The risk in changing Earth's orbit is the likely possibility of changing it too much. Too far and you risk getting too cold. And/or entering Mars's path, setting you up for an inevitable collision.

Another interesting consequence is the tectonic strain of using the engine. Exerting enough force to move a planet is akin to the force of 2 planet size objects colliding. Because of Earth's rotation, you can't do gradual accelerations. You would have to do bursts. That would cause massive earthquakes all over the world. The effect on the tectonic plate it is sitting on would be interesting. In one respect it could push the plate lower into the mantle which would result in flooding on the surface as well as eruptions on the borders. At the same time, on the opposite side of the world, you would also see similar volcanic activity.

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    $\begingroup$ Changing it too much? Are we to believe that people capable of making this engine haven't mastered simple orbital mechanics? $\endgroup$ – timuzhti Dec 8 '17 at 22:20
  • $\begingroup$ @Alpha3031 or a simple feedback loop. Four words: Just stop in time. $\endgroup$ – John Dvorak Dec 9 '17 at 0:47
  • $\begingroup$ "Exerting enough force to move a planet is akin to the force of 2 planet size objects colliding." That's like saying that exerting enough force to move a car is akin to the force of two car size objects colliding, which is completely wrong. Any net external force will move any object it's applied to. If you want to accelerate it hard, you need a big force. If you want to accelerate the planet as hard as a planetary impact would, you need a force as big as a planetary impact. But who said anything about accelerating that hard? $\endgroup$ – David Richerby Dec 9 '17 at 9:53
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Here is a nice article about that. Go to chapter "push from the sun". It is using a solar sail. The sail reflects the light and provides enough thrust to catch-up with sun's increasing luminosity. The sail is not in regular orbit, but the sail's thrust balances with Earth's gravity. It is gravity that transfers the thrust to the earth. One point to consider: do not set the sail so it reflects the sunlight at Earth (It's a huge sail!). That will aggravate global warming and will also disrupt the day-and-night cycle

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I think there are several answers as to why using classical physics to solve this problem is futile.

Fortunately, we have subspace field theory. In the 23rd century, Chief O'Brien is able to relocate Deep Space 9 to be closer to the Bajoran wormhole using just a handful of thrusters. This procedure accomplished in mere minutes what otherwise would have taken several months.

So, all we have to do is produce a low-level subspace field around the Earth to reduce it's inertial mass, and voila, we can just set up a few thrusters to solve the problem. It may still take several months, but at least we can use 23rd century technology!

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Rather than move the earth farther from the sun, which is probably the hardest way to effect global cooling, why not put a giant shade at the La Grange point between earth and the sun to keep the earth cooler?

Not the least reason would be you could easily undo the effect if your scientists miscalculated the move: Much easier to move the shades than move the earth. Even without miscalculations, undo will be likely necessary when the next ice age hits. A good solution would be instead of using shades use a giant lens to diverge the sun's rays when cooling is wanted, or converge the sun's rays when heating is wanted.

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Trying to push a planet will require MASSIVE amounts of energy. Let's assume here that you are able to produce that much.

Now you are trying to push a hot liquid ball with an extremely thin and fragile layer of semi solid crust. Such a thruster may even sink to the core of the planet, taking with it a massive chunk of the crust, thereby drowning the planet in a layer of magma.

So you can only move a planet with gravity. To speed up the planet in the orbit, you can have a massive rocket about the mass of the moon orbiting the sun just ahead of the Earth in a slightly bigger orbit. The earth would then leech of the energy of this planet.

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