## Engines big enough....

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Ixtellor
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### Engines big enough....

to move the Earth out of its orbit. Is it possible to build such a thing using today's technology.

How much thrust would it require to move the earth/alter its orbit and based on todays tech is it even possible?

Like an engine that burns a million barrels of oil/second or some kind of nuclear explosion funneled through a sufficiently sturdy enough 'barrel'. If you could make a barrel 1 kilometer in thickness out of titanium... would it be able to contain rapid fire nuclear detonations?
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ijuin
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### Re: Engines big enough....

First of all, we need to define what constitutes moving the Earth out of its orbit. Do we need enough delta-v to completely escape from the Sun, or will altering the planet's orbit enough to add a day or two to the length of the year be sufficient?

Second, I can tell you right off that chemical-powered rockets are not going to work even if you had an infinite quantity of fuel, since the exhaust speed of any known chemical reaction is still less than Earth's own escape velocity--the exhaust would simply fall back to Earth. Even a solid-core nuclear-thermal rocket like NERVA doesn't quite reach escape velocity with its exhaust. You are going to need either a nuclear pulse engine (like Orion or Daedelaus), a fusion plasma rocket, an antimatter rocket, or just a solar sail big enough to tow the whole Earth.

Third, in order to avoid blowing away or irradiating the Earth's atmosphere, the nozzles of your engines are going to have to extend up into the thermosphere (hundreds of kilometers up). Watch out for low-orbiting space junk!

You might also want to refer to this week's "What If" entry, "Stop Jupiter", which deals with the idea of altering Jupiter's orbit.

https://what-if.xkcd.com/146/

cphite
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### Re: Engines big enough....

I've long advocated that the solution to global warming is to simply move the Earth further away from the sun. Problem solved.

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### Re: Engines big enough....

Artificial light has existed for a long time and has a plenty fast velocity to escape Earth.

You'll just have to be very patient.
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mfb
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### Re: Engines big enough....

Every rocket leaving Earth orbit alters the orbit of Earth. That's about 50 probes so far, with a typical mass of a few tons each. Let's say they all all had a mass of 5 tons (optimistic) and all left with 5 km/s in the same direction (very unrealistic). That gives a total speed change of Earth of 0.2 femtometers per second, or 1 meter in 150 million years. It can change the length of the year by about 0.2 picoseconds. We need an additional leap second in 5 trillion years. Or not, if Earth falls into the sun in 0.005 trillion years.

100 GW of directed artificial light emission over 50 years would give 88 femtometers per second. Much better. And there is a great additional effect: artificial light is mainly used during the evening, "behind" the Earth in its orbit, so it actually is somewhat directed. It increases the orbital energy of Earth, making the years a bit longer. We need the leap second in 20 billion years. Or much less, if we keep using artificial light.

If you want larger changes, build something like the StarTram to launch millions of tons into space.
If you want changes that are actually notable, make fly-bys with massive asteroids. But make sure to not hit Earth. While that would change the orbit as well, it might have unwanted side-effects.

drachefly
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### Re: Engines big enough....

If you have a space elevator (which we don't) then you can use chemical rockets attached to it at a high enough point that the exhaust can escape. That said, the mass of the earth is rather large compared to the reaction mass, so it's pretty insignificant.

Or you can use rockets attached to asteroids and guide them into the Earth. Then you get a lot of additional mass involved for free.

The problems with this latter scheme should be apparent.

Wolfkeeper
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### Re: Engines big enough....

It's theoretically possible to move the Earth, but it would be dangerous and take a long time.

Basically the trick is to use small bodies to deflect bigger bodies using multiple passes around other even bigger bodies and lots of fiddling around to keep everything on course.

In effect, you can use Jupiter as leverage to move the Earth.

The dangerous bit is that there would be many, many passes of small moons or asteroids past the Earth, and if the trajectory is off then ... they crash into the Earth. You don't actually want them to hit, you just use their gravity to move the Earth's orbit.

It would probably take centuries to do any big orbital changes.

lorb
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### Re: Engines big enough....

With current technology your best bet is very likely to just shoot stuff into space. Look into HARP and SHARP
They mostly lack funding, but the concept and the science is pretty well established and if you don't really care for the stuff you are shooting (contrary to the original goal of satellite launching where you want the projectile to stay intact) very much doable with existing technology. With this it becomes more of a question of how motivated you are (ie how much money/ressources you can pump into this) and how far/fast you want to move earth.
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wumpus
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### Re: Engines big enough....

Wolfkeeper wrote:It's theoretically possible to move the Earth, but it would be dangerous and take a long time.

Basically the trick is to use small bodies to deflect bigger bodies using multiple passes around other even bigger bodies and lots of fiddling around to keep everything on course.

In effect, you can use Jupiter as leverage to move the Earth.

The dangerous bit is that there would be many, many passes of small moons or asteroids past the Earth, and if the trajectory is off then ... they crash into the Earth. You don't actually want them to hit, you just use their gravity to move the Earth's orbit.

It would probably take centuries to do any big orbital changes.

This would be my preferred method for moving the Earth, probably with something roughly Moon-sized. The Earth is just too big to make most of our rocketry tech possible (any exhaust gasses need to be moving faster than escape velocity). You then force this "cue ball moon" into multiple "sling shot" maneuvers (in presumably something like an Aldrin Mars cycler so you don't have to keep working to get back to a slingshot possibility), then you let the "cue ball moon" slowly do its work on Earth.

Centuries is an understatement for this type of thing, but considering that Earth has been patiently orbiting the Sun at roughly this distance for 4 billion years (give or take a bit when the Moon formed), it will seem like no time at all.

- really, look at existing HARP/SHARP issues and then realize that the whole thing will require hitting escape velocity while going straight up. How much payload *isn't* going to be vaporized by hitting the atmosphere at such a speed. And even using nukes, you are still likely to have gas velocity issues slowing you down.

lorb
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### Re: Engines big enough....

wumpus wrote:- really, look at existing HARP/SHARP issues and then realize that the whole thing will require hitting escape velocity while going straight up. How much payload *isn't* going to be vaporized by hitting the atmosphere at such a speed. And even using nukes, you are still likely to have gas velocity issues slowing you down.

yes, there are issues, and yes the efficiency is terrible but compared to everything else I have seen mentioned it is the most doable at current technology levels. If you channel just 1% of the current energy output of humanity into it, and use a distributed system with space-guns all over earth, every nation doing it's part, you will have a noticeable change of earth's orbit within the next decades. It's really a lot lot lot easier than pushing the moon around in a controlled way. Or in other words: using some kind of space gun is not just theoretically possible, it is practically possible too.

edit: also you don't have to necessarily go straight up. startram which has been mentioned before in this thread is also a kind of space gun and does not shoot straight up.
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Wolfkeeper
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### Re: Engines big enough....

No, it's extremely un-doable. The gun is economically impossible to build at a scale big enough to move the Earth.

Although you'd think that moving moons isn't possible either, we actually already know how to move small asteroids using gravity tractors, and if you think about it, that means that by coordinating asteroids we can move bigger objects using those asteroids, and then using big asteroids we can move moons, and using moons we can move planets. Going up that scale you're employing exponentially more energy each time- in a literal, non figurative sense.

That's how Freeman Dyson proposed to do this in his book; Perturbing the Universe.

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### Re: Engines big enough....

lorb wrote:edit: also you don't have to necessarily go straight up. startram which has been mentioned before in this thread is also a kind of space gun and does not shoot straight up.

You don't have to shoot straight up. It might even work better, but understand that a significant chunk of the energy will only effect the Earth's rotation and not the orbit. I understand why space guns rarely shoot straight up. This one has different goals.

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### Re: Engines big enough....

Wolfkeeper wrote:No, it's extremely un-doable. The gun is economically impossible to build at a scale big enough to move the Earth.

One gun for sure. But it is extremely doable to build many guns that fire a payload of say 100kg into space. The reasons SHARP/HARP didn't get more funding and weren't continued are that: 1) can't do humans, they don't survive the acceleration. 2) very hard to do big payloads. 3) stuff vaporizes and it's very hard to get satellites to space without damaging them this way.
All of those are non-issues when your goal is to move earth. Issue 1 doesn't matter for obvious reasons. Issue 2 doesn't matter because there is no difference between building 1000 small guns that shoot 100kg to space or a big gun that shoots 100 tonnes. and you can shoot as many times as you want, you can do the change slowly over time. and 3 is also obvious.
You just need to build a huge amount of small space guns, that shoot small amounts of matter into space, in a controlled, regular and coordinated fashion, and as time passes earths orbit will gradually change. Sure the logistics and costs are bad, but not prohibitive if humanity really wants this done in the foreseeable future with current technology.
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Wolfkeeper
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### Re: Engines big enough....

I don't care how many guns you make, how big, or how small you make them, the economics isn't going to work out.

Energy costs money, where would you get the money from to make the energy to move the Earth?

The whole point of using asteroids is that you can effectively exchange energy/momentum between the Earth and Jupiter, so you're leveraging a massive, massive source.

Big guns don't do that, you'd be stuck with something like nuclear power, but nuclear power has loads of problems and is pretty expensive.

lorb
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### Re: Engines big enough....

Already said it really depends on how hard humanity wants to do this. The energy is already there. Just use 1% of all the energy humanity is generating, pump it into a lot of space guns for a few decades. Done.
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Wolfkeeper
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### Re: Engines big enough....

OK, you're going to force me to Math you.

Mass of Earth = ~6e24.

Orbital speed = ~30km/s

Orbital energy = 0.5*m*v^2 = 0.5 * 6e24 * 30000^2 = 2.7e33 J

Let's say we want to add 1m/s to the orbital speed. How much energy would the Earth then have?

Orbital energy2 = 0.5 *m*v2^2 = 0.5 * 6e24 * 30001^2 = 2.7002e33 J

Difference = 180e27 J

Total energy consumption of mankind per year =~ 5.67e27 J

(https://en.wikipedia.org/wiki/World_energy_consumption)

Divide one by the other, and it would take 31 years if mankind used ALL the energy it makes each year, to increase the orbital speed of the Earth by a single metre per second.

So your estimate is out by 4 orders of magnitude; and that would only increase the orbital speed by about 0.01%.

And I think you'll find they're using that energy for other things: requisition request denied.

lorb
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### Re: Engines big enough....

You are right. I underestimated human energy output a lot.
I retract my claim that it is the most doable and best or fastest way.
It is possible (at current technological level) but looking at either the cost or the time it takes to achieve a meaningful result there are better ways.
Still not convinced moving asteroids/moon(s) around would be a lot cheaper and faster.
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Wolfkeeper
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### Re: Engines big enough....

The reason it can work is because the solar system is a chaotic system, so with great care, and not quickly or easily either, it's possible to move stuff around; by moving the small stuff you can move the bigger stuff, and by moving the bigger stuff you can move the even bigger stuff. It would still take centuries though, or even longer.

gmalivuk
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### Re: Engines big enough....

Wolfkeeper wrote:OK, you're going to force me to Math you.

Mass of Earth = ~6e24.

Orbital speed = ~30km/s

Orbital energy = 0.5*m*v^2 = 0.5 * 6e24 * 30000^2 = 2.7e33 J

Let's say we want to add 1m/s to the orbital speed. How much energy would the Earth then have?

Orbital energy2 = 0.5 *m*v2^2 = 0.5 * 6e24 * 30001^2 = 2.7002e33 J

Difference = 180e27 J
That's how much energy would need to be input from a source at rest relative to the Sun. But from the reference frame of something moving at 30km/s already, you're just going from 0m/s to 1m/s, which is a difference of 3e24J for something as massive as Earth.

---

Really, though, if you're going to treat Earth like a rocket then you should be using the rocket equation instead, and concerning yourself with how momentum gets transferred. And even with the most mass-efficient solution possible (a photon drive), you'd use up 2e16kg (in this case by converting it entirely into energy) for 1m/s delta-v.

A more "modest" solution where launched projectiles have a velocity at infinity of 10km/s would require launching 0.01% of the planet's mass into space, or 6e20kg. And if we're launching it from the ground, we also need to factor in the velocity it'll lose due to Earth continuing to pull on it, so launch velocity will need to be sqrt(11.2^2+10^2)=15km/s (assuming no air resistance). The energy for the whole program will be 0.5*6e20*15000^2 = 6.75e28 J.

(The most energy efficient technique seems to be when the velocity at infinity is equal to escape velocity from the surface, but that only brings us down to 6.72e28J.)
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Wolfkeeper
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### Re: Engines big enough....

gmalivuk wrote:That's how much energy would need to be input from a source at rest relative to the Sun. But from the reference frame of something moving at 30km/s already, you're just going from 0m/s to 1m/s, which is a difference of 3e24J for something as massive as Earth.

Oh. Doh! Yes of course, that's Oberth effect, but there's the Earth escape velocity difficulty that you deal with below that prevents it from going that low...
Really, though, if you're going to treat Earth like a rocket then you should be using the rocket equation instead, and concerning yourself with how momentum gets transferred.

Not exactly.
And even with the most mass-efficient solution possible (a photon drive), you'd use up 2e16kg (in this case by converting it entirely into energy) for 1m/s delta-v.

Actually photon rockets are extraordinarily inefficient; they are about the worse possible form of rocket it is possible to come up with. To be fair, we need to stick to energy for comparisons. I make that 2e16 kg = 1e33 J, so less reaction mass, but wayyyyyy more energy.
A more "modest" solution where launched projectiles have a velocity at infinity of 10km/s would require launching 0.01% of the planet's mass into space, or 6e20kg. And if we're launching it from the ground, we also need to factor in the velocity it'll lose due to Earth continuing to pull on it, so launch velocity will need to be sqrt(11.2^2+10^2)=15km/s (assuming no air resistance). The energy for the whole program will be 0.5*6e20*15000^2 = 6.75e28 J.

(The most energy efficient technique seems to be when the velocity at infinity is equal to escape velocity from the surface, but that only brings us down to 6.72e28J.)

Sounds about right. So only ~10 years of all mankinds primary energy then! What a relief!

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### Re: Engines big enough....

Wolfkeeper wrote:
Really, though, if you're going to treat Earth like a rocket then you should be using the rocket equation instead, and concerning yourself with how momentum gets transferred.

Not exactly.
Not exactly what?

And even with the most mass-efficient solution possible (a photon drive), you'd use up 2e16kg (in this case by converting it entirely into energy) for 1m/s delta-v.

Actually photon rockets are extraordinarily inefficient; they are about the worse possible form of rocket it is possible to come up with. I make that 2e16 kg = 1e33 J.
There's a reason I said "mass-efficient". As in, photon rockets give you the lowest mass ratio (since they have the highest "exhaust" velocity).

In a conventional rocket, the lowest energy requirement is when exhaust velocity is 62.75% of the required delta-v, but that requires a lot more propellant mass than a higher exhaust velocity rocket does.

In a rocket with non-negligible escape velocity of its own, things are rather more complicated. (The forum doesn't like the link, but go to Wolfram Alpha with "Solve[D[(exp(x/v)-1)(v^2+y^2),v]==0]", and note that x is delta-v and y is escape velocity.)
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Wolfkeeper
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### Re: Engines big enough....

gmalivuk wrote:
Wolfkeeper wrote:
Really, though, if you're going to treat Earth like a rocket then you should be using the rocket equation instead, and concerning yourself with how momentum gets transferred.

Not exactly.
Not exactly what?

And even with the most mass-efficient solution possible (a photon drive), you'd use up 2e16kg (in this case by converting it entirely into energy) for 1m/s delta-v.

Actually photon rockets are extraordinarily inefficient; they are about the worse possible form of rocket it is possible to come up with. I make that 2e16 kg = 1e33 J.
There's a reason I said "mass-efficient". As in, photon rockets give you the lowest mass ratio (since they have the highest "exhaust" velocity).

Again, we're talking about energy, and you're cheating, since you're assuming 100% mass-energy conversion. If you can pull that off, then we don't need to worry about energy at all!
In a conventional rocket, the lowest energy requirement is when exhaust velocity is 62.75% of the required delta-v, but that requires a lot more propellant mass than a higher exhaust velocity rocket does.

No, with standard rockets the highest efficiency obtainable tends to 100% of the rocket engine efficiency. The ~63% result assumes that the exhaust velocity is fixed, which we have no reason to do in this situation. It mainly depends on how much propellant you're willing to use, the more you use, and the slower it leaves the Earth, the less energy it takes.

In a rocket with non-negligible escape velocity of its own, things are rather more complicated. (The forum doesn't like the link, but go to Wolfram Alpha with "Solve[D[(exp(x/v)-1)(v^2+y^2),v]==0]", and note that x is delta-v and y is escape velocity.)

In the case of the Earth, the escape velocity is extremely problematic; the first 11.2km/s of the exhaust velocity is wasted; Earth's gravity pulls on the exhaust and slows it down, and only the speed above escape velocity makes any difference.

It would be much, much more effective to mount the engines on the moon, due to its lower escape velocity.

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### Re: Engines big enough....

Wolfkeeper wrote:
gmalivuk wrote:
Wolfkeeper wrote:
Really, though, if you're going to treat Earth like a rocket then you should be using the rocket equation instead, and concerning yourself with how momentum gets transferred.

Not exactly.
Not exactly what?

And even with the most mass-efficient solution possible (a photon drive), you'd use up 2e16kg (in this case by converting it entirely into energy) for 1m/s delta-v.

Actually photon rockets are extraordinarily inefficient; they are about the worse possible form of rocket it is possible to come up with. I make that 2e16 kg = 1e33 J.
There's a reason I said "mass-efficient". As in, photon rockets give you the lowest mass ratio (since they have the highest "exhaust" velocity).

Again, we're talking about energy, and you're cheating, since you're assuming 100% mass-energy conversion. If you can pull that off, then we don't need to worry about energy at all!
You were talking about (only) energy, but the rest of the thread has been more generally about what it would take to move Earth's orbit, and even if the energy is there, the needed mass becomes a serious issue for something as large as Earth.

In a conventional rocket, the lowest energy requirement is when exhaust velocity is 62.75% of the required delta-v, but that requires a lot more propellant mass than a higher exhaust velocity rocket does.

No, with standard rockets the highest efficiency obtainable tends to 100% of the rocket engine efficiency. The ~63% result assumes that the exhaust velocity is fixed, which we have no reason to do in this situation. It mainly depends on how much propellant you're willing to use, the more you use, and the slower it leaves the Earth, the less energy it takes.
Sure, which is why I'm saying we can't keep treating propellant mass like it's nothing.

In a rocket with non-negligible escape velocity of its own, things are rather more complicated. (The forum doesn't like the link, but go to Wolfram Alpha with "Solve[D[(exp(x/v)-1)(v^2+y^2),v]==0]", and note that x is delta-v and y is escape velocity.)

In the case of the Earth, the escape velocity is extremely problematic; the first 11.2km/s of the exhaust velocity is wasted; Earth's gravity pulls on the exhaust and slows it down, and only the speed above escape velocity makes any difference.
I know the effect escape velocity has, and why. I did after all include it inmy calculations multiple times.

My point was not that it's more practically difficult (which we've all known since the first replies to the thread), but that the math also becomes a lot more complicated.
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Wolfkeeper
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### Re: Engines big enough....

The Earth seems to have no particular lack of reaction mass. You can sinter together rock of all sorts and lob it into the sky; but 100% definitely, mankind has practical limits on the amount of energy it can access.

That's why f'ing big guns do not seem to be a viable plan for moving the Earth.

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### Re: Engines big enough....

Wolfkeeper wrote:The Earth seems to have no particular lack of reaction mass. You can sinter together rock of all sorts and lob it into the sky; but 100% definitely, mankind has practical limits on the amount of energy it can access.

That's why f'ing big guns do not seem to be a viable plan for moving the Earth.
Except, we can acquire energy indefinitely (on human scales) from the Sun, if slowly. We do not have unlimited reaction mass. The figure I mentioned earlier (of 0.01% of the planet's mass) amounts to 2e17m^3 of crust, and there are 1.5e14m^2 of land on Earth.

Using 1% of our current energy budget for the next thousand years seems a *lot* more practical that lobbing the top 1.3km of the continents into space.

Edit: For a rocket operating at 100% propulsive efficiency (relative to some frame), the mass ratio is the same as the velocity ratio (relative to that frame). As you pick frames closer and closer to the initial rest frame of the rocket, then, the mass ratio gets closer and closer to infinity.

If you fix the starting mass (by saying it's the entire Earth for example), then the enrgy requirement drops all the way toward zero. In the end you just lob a rock *forward*, and call that rock the payload with everything else being the "propellant".
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Wolfkeeper
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### Re: Engines big enough....

gmalivuk wrote:Using 1% of our current energy budget for the next thousand years seems a *lot* more practical

What we have at your end is a complete lack of understanding of the scale of the problem.

The calculation we just did showed that 100% of the human primary energy use for 10 years would increase the speed of the Earth by 1m/s.

1% for a thousand years is the same energy you would need to move the Earth by 1m/s.

To do anything very useful you would need thousands of metres per second of delta-v.

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### Re: Engines big enough....

Wolfkeeper wrote:
gmalivuk wrote:Using 1% of our current energy budget for the next thousand years seems a *lot* more practical

What we have at your end is a complete lack of understanding of the scale of the problem.

The calculation we just did showed that 100% of the human primary energy use for 10 years would increase the speed of the Earth by 1m/s.

1% for a thousand years is the same energy you would need to move the Earth by 1m/s.

To do anything very useful you would need thousands of metres per second of delta-v.
I'm not misunderstanding the scale of the problem, I'm pointing out that even for changing Earth's velocity by a paltry 1m/s, you'd need to strip the top 1300 meters off all the continents (or 400m of rock everywhere, or fully drain half the oceans).

However much energy you'd need, we can lengthen the timescale of the project and be fine (up to 9ish orders of magnitude, at least). But we can't magically generate additional continents to launch, on any timescale. If anyone lacks understanding of the scale of the problem, it's you, because you keep just focusing on energy and treating reaction mass as effectively unlimited.
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Wolfkeeper
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### Re: Engines big enough....

gmalivuk wrote:
Wolfkeeper wrote:
gmalivuk wrote:Using 1% of our current energy budget for the next thousand years seems a *lot* more practical

What we have at your end is a complete lack of understanding of the scale of the problem.

The calculation we just did showed that 100% of the human primary energy use for 10 years would increase the speed of the Earth by 1m/s.

1% for a thousand years is the same energy you would need to move the Earth by 1m/s.

To do anything very useful you would need thousands of metres per second of delta-v.
I'm not misunderstanding the scale of the problem, I'm pointing out that even for changing Earth's velocity by a paltry 1m/s, you'd need to strip the top 1300 meters off all the continents (or 400m of rock everywhere, or fully drain half the oceans).

However much energy you'd need, we can lengthen the timescale of the project and be fine (up to 9ish orders of magnitude, at least). But we can't magically generate additional continents to launch, on any timescale. If anyone lacks understanding of the scale of the problem, it's you, because you keep just focusing on energy and treating reaction mass as effectively unlimited.

That is how the rocket equation works though; reaction mass is essentially infinite, you just end up with a smaller planet in the relevant orbit.

But of course, yes you'd be stripping the outside off the Earth, revealing a red-hot molten core that would boil all the oceans.

The whole thing is just absolutely, stupidly, completely ridiculous; which is my fundamental point; it's not even plausible on energetic grounds, never mind any of the other practicalities.

Meanwhile, you probably could move the Earth by choreographing the asteroids and using them as gravity tractors via repeated slingshots, over a few hundred years or so.

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### Re: Engines big enough....

gmalivuk wrote:But we can't magically generate additional continents to launch, on any timescale.
Earth does that naturally over geological timescales.

A space elevator provides a way to launch stuff to space without wasting energy if you find some clever way to store and redistribute energy over the trip well beyond geostationary orbit. Well, over time you slow down the rotation of Earth - currently at an energy of 2*1029 J.

Greenhouse gases can change the radiation balance a bit. You have to change the difference in emitted radiation between evening and morning for an effect.