Not “when the sun becomes a red giant”, because red giants are variable on a much too short time scale, but, as others mentioned, we can probably keep the earth in a habitable zone for another 5 billion years or so. We have more than enough hydrogen on earth to provide the necessary potential energy increase with fusion-based propulsion, though building something like a 100 petaWatt engine is problematic at this point, (for comparison, it is a significant fraction of the total solar radiation hitting the earth).
EDIT: I suspect that terraforming Mars (and/or cooling down the Earth more efficiently when the Sun gets brighter) would require less energy than moving the Earth to the Mars orbit. My calculations could be off, though, hopefully someone can do them independently.
Only major problem I know of with terraforming Mars is how to give it a magnetic field. We’d have to somehow re-melt the interior of the planet. Otherwise, we could just put up with constant intense solar radiation, and atmosphere off-gassing into space. Maybe if we built a big fusion reactor in the middle of the planet...?
I recall estimating the power required to run an equatorial superconducting ring a few meters thick 1 km or so under the Mars surface with enough current to simulate Earth-like magnetic field. If I recall correctly, it would require about the current level of power generation on Earth to ramp it up over a century or so to the desired level. Then whatever is required to maintain it (mostly cooling the ring), which is very little. Of course, an accident interrupting the current flow would be an epic disaster.
Let’s do a quick estimate. Destroying a Mars-like planet requires expending the equivalent of its gravitational self-energy, ~GM^2/R, which is about 10^32J (which we could easily obtain from a comet 10 kn in radius… consisting of antimatter!) For comparison, the Earth’s magnetic field has about 10^26J of energy, a million times less. I leave it to you to draw the conclusions.
Not “when the sun becomes a red giant”, because red giants are variable on a much too short time scale, but, as others mentioned, we can probably keep the earth in a habitable zone for another 5 billion years or so. We have more than enough hydrogen on earth to provide the necessary potential energy increase with fusion-based propulsion, though building something like a 100 petaWatt engine is problematic at this point, (for comparison, it is a significant fraction of the total solar radiation hitting the earth).
EDIT: I suspect that terraforming Mars (and/or cooling down the Earth more efficiently when the Sun gets brighter) would require less energy than moving the Earth to the Mars orbit. My calculations could be off, though, hopefully someone can do them independently.
Only major problem I know of with terraforming Mars is how to give it a magnetic field. We’d have to somehow re-melt the interior of the planet. Otherwise, we could just put up with constant intense solar radiation, and atmosphere off-gassing into space. Maybe if we built a big fusion reactor in the middle of the planet...?
I recall estimating the power required to run an equatorial superconducting ring a few meters thick 1 km or so under the Mars surface with enough current to simulate Earth-like magnetic field. If I recall correctly, it would require about the current level of power generation on Earth to ramp it up over a century or so to the desired level. Then whatever is required to maintain it (mostly cooling the ring), which is very little. Of course, an accident interrupting the current flow would be an epic disaster.
Wouldn’t it be more efficient to use that energy to destroy Mars and build start building a Dyson swarm from the debris?
Let’s do a quick estimate. Destroying a Mars-like planet requires expending the equivalent of its gravitational self-energy, ~GM^2/R, which is about 10^32J (which we could easily obtain from a comet 10 kn in radius… consisting of antimatter!) For comparison, the Earth’s magnetic field has about 10^26J of energy, a million times less. I leave it to you to draw the conclusions.