How to keep the earth in the habitable zone

[mrspeedybob]

This article speaks of the Earth leaving our solar systems habitable zone in a billion years or so. I would like to propose a serious discussion of what we, as a species, can do about it.

A billion years is a large amount of time, but the scope of the problem is also very large.

One thought I had was to build large arrays of mirrors which would reflect sunlight back into space. The short term effect would be that less solar radiation is absorbed, thus cooling the planet. The long term effect would be that if we reflected the light in a rearward direction along our orbit, the force of the radiation pressure would gradually accelerate the planet into a higher orbit. I realize it would be very gradual, but we do have a billion years.
There would be certain interesting problems to overcome as we approach the orbit of mars.

Any thoughts on this idea, or any other ideas which might be more do-able?

 

[stargazer3]

I think this method isn't very effective. The Earth already receives the light from the Sun, putting some mirrors on the surface shouldn't help much — the radiation pressure will be doubled at best, assuming that all the sunlight is currently absorbed by our planet.

 

[bahamagreen]

I ran across something a long time ago that suggested the Earth can be moved; and can also be moved out of and back into orbit if necessary (to avoid an impact).

The bad news was that to move the Earth we would use brute force, and it is brutal; a large equatorial area would be burned - started quickly and stopped quickly because the force would be normal to the surface and vary direction as the Earth rotates... so it might need to be repeated at the same time of day for some days in a row to get a net relocation.
To do the move out and back in (collision avoidance) would take advantage of the Earth's rotation and a 24 hour burn so that the Earth made a little epicycle in its orbit around the hazard.
It was recommended that to burn the better part of a continent would need thermonuclear assets to get the timing right - a lot of them... the risk would be high and the results of success quite horrible under even the best planned and calculated effort.

On a happier note, I'm thinking that if we had plenty of time or forewarning, a better plan for tomorrow might be to use the same approach, but do it on the Moon rather than the Earth... the idea being to have the calculations and assets in place so that a shift in the Moon's orbit - and thereby a shift in the barycenter about which both the Moon and Earth orbit - could be much more gracefully executed to either temporarily move the Earth to dodge an impact, or move the Moon in place to take the impact itself, or move the Moon to slingshot the Earth into a new orbit... and probably losing the Moon as a satellite.

I'm sure the possible solutions will get easier and less drastic with future technology... a billion years is a long time to figure it out.

 

[Mordred]

Use a similar proposed method to move large asteroids.

have a large craft (large enough to create a gravitational influence ) have it orbit the planet in the direction you wish to move the planet. This would probably take centuries to complete dependant on the amount of influence the craft can have upon the Earth. The good news is that we would have plenty of time for it. By then we should be able to build extremely large spacecraft assuming we are still around.

 

[stargazer3]

Use a similar proposed method to move large asteroids.

have a large craft (large enough to create a gravitational influence ) have it orbit the planet in the direction you wish to move the planet. This would probably take centuries to complete dependant on the amount of influence the craft can have upon the Earth. The good news is that we would have plenty of time for it. By then we should be able to build extremely large spacecraft assuming we are still around.

Sorry, can you explain it a little bit? How does it change the orbit? The centre of mass of the planet-craft system would be "wobbling" slightly, what of it?

 

[mrspeedybob]

I ran across something a long time ago that suggested the Earth can be moved; and can also be moved out of and back into orbit if necessary (to avoid an impact).

I'd love to read the original paper if you know where it can be found.

I'm sure we could do something similar, but less destructive, with the amount of time we have. Perhaps build a mass driver that would launch an iron projectile into space at reletivistic speeds once a day for millions of years.

After some calculations on my original reflector idea, it seems as though the effect would be several orders of magnitude too small, even over a billion year time frame.

 

[Mordred]

Considering how far into the future this discussion is descibed in its quite reasonable to assume we would be able to build a large enough craft in space to create a gravity well. You can then get that craft. To use its gravity well for the Earth to follow. Who knows by then our technology may be sufficient to create a sufficient gravity well via other means. This would have a reduced impact upon Earth Enviroment.

 

[glappkaeft]

Perhaps build a mass driver that would launch an iron projectile into space at reletivistic speeds once a day for millions of years.

To a first approximation this is pretty much equivalent to bombarding the Earth with relativistic iron projectiles, i.e. a very bad day for anyone on the same continent as the launcher.

 

[mrspeedybob]

To a first approximation this is pretty much equivalent to bombarding the Earth with relativistic iron projectiles, i.e. a very bad day for anyone on the same continent as the launcher.

I believe it is very different for the same reason that it is very different to fire a gun then to be shot. Δp is the same for both the shooter and the target but most of the energy is in the KE of the projectile, therefore the target absorbes more energy then the shooter.

That is fortunate for those living close to the mass driver but also means this would turn the Earth about the least energy efficient rocket immaginable. I'll do some math later on today to see just how bad it would be, unless someone else beats me to it.

 

[glappkaeft]

I believe it is very different for the same reason that it is very different to fire a gun then to be shot. Δp is the same for both the shooter and the target but most of the energy is in the KE of the projectile, therefore the target absorbes more energy then the shooter.

The problem is that anytime a significant mass traveling at relativistic velocity hits something as dense as 1 standard atmosphere (and in this case this happends at the end of the muzzle) it's going to shed an insane amount of energy, vaporizing the projectile, increasing the crossection of what is now a relativistic plasma cone which will increase the amount of energy that is shed. Every square meter of the Earth's surface has about 10 tons of atmospheric mass above it that the projectile has to interact with.

The problem is very similar to the first XKCD "What If" comic and similar scenarios ("The Killing Star, "The Relativistic Ravioli projectile" etc.).