How can we prevent the Earth being scorched and swallowed as the Sun grows and eventually enters its red giant phase?

Question

The Sun is slowly growing more and more luminous.

As a result of these processes, multicellular life forms may be extinct in about 800 million years, and eukaryotes in 1.3 billion years, leaving only the prokaryotes.

Earth will become uninhabitable to organisms other than thermophilic bacteria in the almost-boiling oceans and continents will be scorched down into desert wastelands. Part of the oceans will subdue to the mantle, and part will evaporate turning the atmosphere into a hellish moist-greenhouse sauna. The surviving surface water would be very near to boiling.

By 2.8 billion years from now, the surface temperature of the Earth will have reached 422 K (149 °C; 300 °F), even at the poles. At this point, any remaining life will be extinguished due to extreme conditions.

After that, Earth will be "venusified" with the destruction of its water vapor.

Then, as the Sun enters the red giant phase, it will swallow Mercury and Venus and possibly also the Earth. Even if Earth escapes being swallowed, it would be scorched and burned to a crisp beyond recognition long before that, having its surface melted into a lava ocean. It is also possibly that Earth loses the Moon if it escapes.

After the red giant phase, the Sun shrinks down and becomes a small white dwarf shedding a stellar nebula.

Now the question:

We, humans, have a lot of time (millions of years at least) to prepare for that, but nature won't wait for us.

How could we progressively move Earth to a larger orbit and prevent its natural fate until the end of the red giant phase¹ while still having trees, flowers and animals happily living² in the fields and oceans of the planet which we call home?

^{¹ Surviving the shrinking after the red giant phase would be "just" reversing the orbit enlargement. But surviving being enshrouded in the planetary nebula is topic for another question, so I will leave that out of this one.}

^{² Don't care about the Sun light being actually deep red, that isn't the problem, or at least, we have far worse concerns than that.}

Good starting points as links:

https://www.newscientist.com/article/dn14983-moving-the-earth-a-planetary-survival-guide/

http://buildengineer.com/www.paulbirch.net/MoveAPlanet.pdf

https://en.wikipedia.org/wiki/Future_of_Earth#Solar_evolution

Answer 1

Gravity Tug

Since we're talking about moving planets around as a basic axiom of the question, I'm going to assume that our energy budget is sufficiently large enough to work on that scale and not go into the nitty gritty of that side of it.

Anywho, the problem with using some sort of massive rocket motor to push the planet into a larger orbit is that that would involve lighting off a massive rocket motor in the atmosphere, probably for a fairly significant length of time. This is generally less than ideal. So let's do it to some planets or planetoids that we don't care about as much instead!

Using a lot of math(*) and some massive rocket engines, we take one or more smaller-but-still-large planetary bodies (Ceres and Mercury come to mind) and send them on carefully-calculated close passes to Earth in the same direction as its orbit. The gravitational pull of the body as it flies past will pull the Earth a bit faster in its orbit, raising its aphelion. You'll then want to arrange another close pass at aphelion to raise its perihelion as well. Do this a bunch of times and you'll eventually pull Earth into a comfortably-larger orbit.

Note that this method will play merry hell with tides, possibly alter the length of the day, and will almost certainly throw off the Moon's orbit as well. The Moon's orbit can probably be corrected with more giant rocket engines, though, and that might also help fix the length of the day as well. Good luck with the tides, though.

The math involved to do this would be absurd and the energy budget required would be literally astronomical, but we've got literally hundreds of millions of years to get ready for it. Hopefully we'll have the numbers crunched by then!

(*-Think "n-body problem over the scale of maybe decades or centuries, with the entire planet at stake if you get things wrong.")

Answer 2

THE WANDERING EARTH SOLUTION

Building on Daron the Inimitable's answer: we do what the United Earth Government in The Wandering Earth does, and use, to quote the movie's Wikipedia article, "12,000 enormous fusion-powered Earth Engines built across the Northern Hemisphere with further Torque Engines along the equator" to propel the entirety of planet Earth out of its current orbit. Fortunately, we don't have to go as far as that movie went and push the earth out of the system entirely; we just have to push it up into a safer orbit. How to get the resources for all of this is a bit of an issue, but not necessarily an unsolvable one; we could mine the resources we need to build the thrusters from another planet such as Mercury and Venus, or the asteroid belt, and use nuclear fusion to power the reactors, mining the hydrogen we need from the very sun we're trying to save ourselves from (thanks to VictorStafusa-FORABOZO for the bit about the fuel).

Answer 3

The question asks how to save Earth by moving it father from the Sun, and there are some answers how to do that.

But of course there are other ways to keep Earth cool.

Put a giant shade in the L1 Lagrange point, between Earth and the Sun. That giant shade will have to have giant engines to correct its orbit whenever it starts to drift out of the L1 point. Keeping the shade in place will take a lot of effort.

But since the planet Earth will probably have many thosuands and millions and probably billions of times the mass of the giant shade, the energy requirements to keep the shade in position will be tiny compared to the energy requirements to move the Earth farther from the Sun.

I guess that the giant shade will have a very long cable or pole which will point between the Earth and the Sun, and the gravity of the two will keep it pointed in that direction. And there were be a vast discperpendicular to the long pole or cable. The disc will have to be less wide than the diameter of the Sun and somewhat wider than the Earth's diameter in order to shade the Earth, since the Sun is much wider than the Earth but the shade will be much closer to the Earth than to the Sun. And of course the disc will probably be a lot wider than the minimum necessary diameter as a pecaution.

The disc might be made of rigid material strong enough not to crumble into a ball from its own gravity, or else it might be made of flexible material which spun around the central pole so that centrifugal force en xtends it to its full diamters.

And as a precaution there might several discs in line along the the central pole in case the disc closest to the sun fails.

The discs will probably shade theEarth by reflecting most of the sunlight and absorbing soe of it. The absorded sunlight can be used to generate electricity which will be transmitted along the central pole to the end facing the Earth, where it will power or help to power (waste not, want not) gigantic lamps aimed at the Earth which will duplicate the Sun's former and lesser illumination level, so that Earth will be lite and heated about as much as it is now, instead of as much as it would be if it was struck by the full sunlight of the future Sun.

And presumably several duplicates of the Sun shade assembly will be maintained where one can be quickly moved to the L1 point to replace the one in position if it fails.

And if the future people want to save Earth from being destroyed during the red giant phase of the Sun's evolution, eventually they will have to start moving Earth's orbit farther and farther from the Sun.

But use of a Sun shade at first may delay the time when they have to start moving the Earth by tens of millions, hundreds millions, or billions of years, and thus give them more time to accumulate resources and energy and technological knowledge for the vaster project.

And there may be other possiiblities.

Answer 4

Big Engine

Futurama had the right idea with "point your exhaust vents upwards". The only way to get the Earth into a wider orbit is to throw something in the opposite direction very hard.

The problem is Earth is Big. You need to throw whatever it is very hard or throw something very big. And you need to fight the Earth's gravity every inch of the way.

Even ignoring gravity from the Earth, I suspect that we can use secondary school physics equations to prove say an 100 km layer of gasoline over the Earth's surface is not enough to push us out of the Red Giant. I'll do the calculation later.

So you need some SciFi MagiTech to make this work.

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THE CALCULATION

The potential energy of a planet of mass $m$ a distance $R$ from a sun mass $M$ is given by the formula

$$U = - G\frac{Mm}{R}$$

Wait, the potential is negative? How can we have a negative amount of energy? Yeah Physicists do it like that just to confuse you. They write $U$ for "potential energy" too. What's up with that? It doesn't even rhyme.

It's not a problem here, since we only care about difference in potential energy between two orbits. In this case let's double the orbit distance. That gets us a bit beyond Mars. The change in potential energy is just half of the above.

$$\Delta = - G\frac{mM}{2R} \ \text{Joules}$$

where $G \simeq 6 \times 10^{-11}$ is Isaac's universal gravitational number.

Now plug in the numbers:

$$m \simeq 6 \times 10^{24}$$

$$M \simeq 2 \times 10^{30}$$

$$R \simeq 1.5 \times 10^{11}$$

and we get

$$\Delta = - G\frac{mM}{2R} = (6 \times 10^{-11})\frac{6 \times 10^{24} \times 2 \times 10^{30}}{2 \times 1.5 \times 10^{11}}$$ $$ = \frac{6 \times 6 \times 2 }{2 \times 1.5 } 10^{ 24+30 -11 -11} = \frac{72}{3 } 10^{ 32} = 24 \times 10^{ 32} $$

Joules.

IS THAT A LOT, DARON?

It is an awful lot. For example one kilo of gasoline generates about $3\times 10^7$ Joules. Meaning you would need about $10^{32-7} = 10^{25}$ kilos of petrol to double the Earth's orbit.

DARON HOW HEAVY DID YOU SAY THE EARTH WAS AGAIN?

The Earth weighs only $m \simeq 6 \times 10^{24}$ kilos! So it is worse than predicted. Not only will a 100km layer of petrol not be enough -- even a second Planet Earth made entirely of petrol is not enough.

The upshot is we need something much better than gasoline. Brinstar's "12,000 enormous fusion-powered Earth Engines" sounds closer to the mark. (The calculation is left as an exercise to the interested reader.) But you need still something to fuel all those reactors!

Edit: The calculation has been done by the interested reader gs in a comment. Using Hydrogen-Helium fusion at perfect efficiency we need only a measly $10^{20}$ kilos of fuel. That much water takes up $10^{17}$ cubic metres. The fusion fuel is about a tenth the density of water so we need $10^{18}$ cubic metres of it. Lets see how deep that much fuel would bury the planet's surface.

The volume for a thin (relative to the Earth's radius) shell of fuel is $4 \pi R^2 \epsilon$ for $\epsilon$ the shell depth. The Earth radius is $R \simeq 6 \times 10^6$ metres and so the shell has volume $4 \pi \times 36 \times 10^{12} \simeq 450 \times 10^{12} \simeq 4 \times 10^{14}$ metres. That's about 2.5 kilometres of fusion fuel needed to push the Earth into Mars' orbit.

Answer 5

SOLAR SAIL HELD IN PLACE BY GRAVITY

Since the inhabitants of the planet will know millions of years in advance that this will happen, they can settle for moving the Earth outwards very slowly. This will be needed long before the Sun reaches its red giant phase; just a billion years from new the Sun will be too hot for life on Earth to be sustained in the current orbit.

To move the sun, place a giant solar sail in the L2 Lagrange point (where the James Webb telescope is today), or rather, a bit inside it.The sail, which wil have to be of a diameter similar to Earth's or larger, will be pushed away from the Earth by the sunlight while simultaneously pulling at the Earth by their mutual gravitational attraction. The trick will be to balance the push and pull so that the sail and the Earth will be pushed out as a unit.

The acceleration of this unit will be too small to affect anything on the Earth. To move the planet to, say, the orbit of Jupiter in 10 million years will require moving it out just ca. 60 km a year or 165 m a day.