Is there any plausible way the earth could be moved away from the sun and into an orbit which would keep the earth habitable when the sun becomes a red giant?
This would correspond to a change in specific orbital energy of 132712440018/(2(1 AU)) to 132712440018 / (2(7 AU)). (where the 12-digit constant is the standard gravitational parameter of the sun. This is like 5.6 10^10 in Joules / Kilogram, or about 3.4 10^34 Joules when we restore the reduced mass of the earth/sun (which I’m approximating as just the mass of the earth).
Wolframalpha helpfully supplies that this is 28 times the total energy released by the sun in 1 year.
Or, if you like, it’s equivalent to the total mass energy of ~3.7 * 10^18 Kilograms of matter (about 1.5% the mass of the asteroid Vespa).
So until we’re able to harness and control energy on the order of magnitude of the total energetic output of the sun for multiple years, we won’t be able to do this any time soon.
There might be an exceedingly clever way to do this by playing with orbits of nearby asteroids to perturb the orbit of the earth over long timescales, but the change in energy we’re talking about here is pretty huge.
I think you have something there. You could design a complex, but at least metastable orbit for an asteroid sized object that, in each period, would fly by both Earth and, say, Jupiter. Because it is metastable, only very small course corrections would be necessary to keep it going, and it could be arranged such that at every pass Earth gets pushed out just a little bit, and Jupiter pulled in. With the right sized asteroid, it seems feasible that this process could yield the desired results after billions of years.
Hah, thanks for pointing this out. I must have read or heard of this before and then forgotten about it, except in my subconscious. Looks like they have done the math, too, and it figures. Cool!
Ignoring the concept of “can we apply that much delta-V to a planet?”, I’d be interested to know whether it’s believed that there exists a “Goldilocks zone” suitable for life at all stages of a star’s life. Intuitively it seems like there should be, I’m not sure.
Of course, it should be pointed out that the common understanding of “when the sun becomes a red giant” may be a bit flawed; the sun will cool and expand, then collapse. On a human time scale, it will spend a lot of that time as a red giant, but if you simply took the Earth when its orbit started to be crowded by the inner edge of the Goldilocks zone and put it in a new orbit, that new orbit wouldn’t be anywhere close to an eternally safe one. Indeed, I suspect that the outermost of the orbits required for the giant-stage sun would be too far from the sun at the time we’d first need to move the Earth.
The sun’s luminosity will rise by around 300X as it turns into a giant. If we wish to keep the same energy flux onto the earth at that point, we must increase the earth’s orbit a factor of sqrt(300) = 17X. The total energy of the earth’s current orbit is 2.65E33 J. We must reduce this to 1⁄17 of its current value. or reduce it by (16/17)*2.65E33 J = 2.5E33 J. The current total annual energy production in the world is about 5E17 J. The sun will be a red giant in about 7.6E9 years. So we would need about a million times current global energy production running full time into rocket motors to push the earth out to a safe orbit by the time the sun has expanded.
But it is worse than that. The Sun actually expands over a scant 5 million years near the end of that 7.6E9 years. So to avoid freezing for billions of years because we have started moving away from the sun too soon, we essentially will need a billion times current energy production running into rocket engines for those 5 million years of solar expansion. But the good news is we have 7.6E9 billion years to figure out how to do that.
If we use plasma rockets which push reaction mass out at 1% the speed of light, then we will need a total of about 6E16 kg reaction mass, or about 0.000001% of the earth’s total mass. The total mass of water on the earth is about 1E21 kg so we could do all of this using water as reaction mass and still have 99.99% of the water left when we are done.
I wonder what the exhaust plume of an engine like that would look like, and how far away from it you’d have to be standing to still be capable of looking at anything after a second or two.
I’m curious about the thought process that led to this being asked in the “stupid questions” thread rather than the “very advanced theoretical speculation of future technology” thread. =P
As a more serious answer: Anything that would effectively give us a means to alter mass and/or the effects of gravity in some way (if there turns out to be a difference) would help a lot.
I wasn’t sure there was a way to do it within current physics.
Now we get to the hard question: supposing we (broadly interpreted, it will probably be a successor species) want to move the earth outwards using those little gravitational nudges, how do we get civilizations with a sufficiently long attention span?
[...] how do we get civilizations with a sufficiently long attention span?
I heard Ritalin has a solution. Couldn’t pay attention long enough to verify. ba-dum tish
On a serious note, isn’t the whole killing-the-Earth-for-our-children thing a rather interesting scenario? I’ve never seen it mentioned in my game theory-related reading, and I find that to be somewhat sad. I’m pretty sure a proper modeling of the game scenario would cover both climate change and eaten-by-red-giant.
I don’t see the connection to killing the earth for our children. Moving the earth outwards is an effort to save the earth for our far future selves and our children.
I think “for our children” means “as far as our children are concerned” and failing to move the earth’s orbit so it doesn’t get eaten by the sun (despite being able to do it) would qualify as “killing the earth for our children”. (The more usual referents being things like resource depletion and pollution with potentially disastrous long-term effects.)
If we haven’t gotten one by then, we’re doomed. Or at least, we don’t get a very good planet. We could still have space-stations or live on planets where we have to bring our own atmosphere.
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.
Yes, I saw an article a few years ago a back of the envelope estimate that suggested this would be doable if one could turn mass on the moon more or less directly to energy and use the moon as a gravitational tug to slowly move Earth out of the way. You can change mass almost directly into energy by feeding the mass into a few smallish blackholes.
Black holes feel gravity just like any other massive body. And they can be electrically charged. So you can move them around with strong enough gravitational and/or electric fields.
It can, as long as you don’t mind that you won’t get it back when you’re done. You have to constantly fuel the black hole anyway. Just throw the fuel in from the opposite direction that you want the black hole to go.
Throwing mass into a black hole is harder than it sounds. Conveniently sized black holes that you actually would have a chance at moving around are extremely small, much smaller than atoms, I believe. I think they would just sit there without eating much, despite strenous efforts at feeding them. The cross-section is way too small.
To make matters worse, such holes would emit a lot of Hawking radiation, which would a) interfere with trying to feed them, and b) quickly evaporate them ending in an intense flash of gamma rays.
The problem is throwing mass into other mass hard enough to make a black hole in the first place.
Hawking radiation isn’t a big deal. In fact, the problem is making a black hole small enough to get a significant amount of it. An atom-sized black hole has around a tenth of a watt of Hawking radiation. I think it might be possible to get extra energy from it. From what I understand, Hawking radiation is just what doesn’t fall back in. If you enclose the black hole, you might be able to absorb some of this energy.
Yes, making them would be incredibly hard, and because of their relatively short lifetimes, it would be extremely surprising to find any lying around somewhere. Atom sized black holes would be very heavy and not produce much Hawking readiation, as you say. Smaller ones would produce more Hawking radiation, be even harder to feed, and evaporate much faster.
I don’t really know if it’s plausible, but Larry Niven’s far-future fiction A World Out of Time (the novel, not the original short story of the same name) deals with exactly this problem.
His solution is a “fusion candle”: build a huge double-ended fusion tube, put it in the atmosphere of a gas giant, and light it up. The thrust downwards keeps the tube floating in the atmosphere. The thrust upwards provides an engine to push the gas giant around. In the book, they pushed Uranus to Earth, and then moved it outwards again, gravitationally pulling the Earth along.
This is a fascinating question. Very speculatively, I could imagine somehow using energy gained by pushing other objects closer to the Sun, to move the Earth away from the Sun. Like some sort of immense elastic band stretching between Mars and Earth, pulling Earth “up” and Mars “down”.
Is there any plausible way the earth could be moved away from the sun and into an orbit which would keep the earth habitable when the sun becomes a red giant?
According to http://arxiv.org/abs/astro-ph/0503520 we would need to be able to boost our current orbital radius to about 7 AU.
This would correspond to a change in specific orbital energy of 132712440018/(2(1 AU)) to 132712440018 / (2(7 AU)). (where the 12-digit constant is the standard gravitational parameter of the sun. This is like 5.6 10^10 in Joules / Kilogram, or about 3.4 10^34 Joules when we restore the reduced mass of the earth/sun (which I’m approximating as just the mass of the earth).
Wolframalpha helpfully supplies that this is 28 times the total energy released by the sun in 1 year.
Or, if you like, it’s equivalent to the total mass energy of ~3.7 * 10^18 Kilograms of matter (about 1.5% the mass of the asteroid Vespa).
So until we’re able to harness and control energy on the order of magnitude of the total energetic output of the sun for multiple years, we won’t be able to do this any time soon.
There might be an exceedingly clever way to do this by playing with orbits of nearby asteroids to perturb the orbit of the earth over long timescales, but the change in energy we’re talking about here is pretty huge.
I think you have something there. You could design a complex, but at least metastable orbit for an asteroid sized object that, in each period, would fly by both Earth and, say, Jupiter. Because it is metastable, only very small course corrections would be necessary to keep it going, and it could be arranged such that at every pass Earth gets pushed out just a little bit, and Jupiter pulled in. With the right sized asteroid, it seems feasible that this process could yield the desired results after billions of years.
I thought this sounded familiar
Hah, thanks for pointing this out. I must have read or heard of this before and then forgotten about it, except in my subconscious. Looks like they have done the math, too, and it figures. Cool!
Ignoring the concept of “can we apply that much delta-V to a planet?”, I’d be interested to know whether it’s believed that there exists a “Goldilocks zone” suitable for life at all stages of a star’s life. Intuitively it seems like there should be, I’m not sure.
Of course, it should be pointed out that the common understanding of “when the sun becomes a red giant” may be a bit flawed; the sun will cool and expand, then collapse. On a human time scale, it will spend a lot of that time as a red giant, but if you simply took the Earth when its orbit started to be crowded by the inner edge of the Goldilocks zone and put it in a new orbit, that new orbit wouldn’t be anywhere close to an eternally safe one. Indeed, I suspect that the outermost of the orbits required for the giant-stage sun would be too far from the sun at the time we’d first need to move the Earth.
The sun’s luminosity will rise by around 300X as it turns into a giant. If we wish to keep the same energy flux onto the earth at that point, we must increase the earth’s orbit a factor of sqrt(300) = 17X. The total energy of the earth’s current orbit is 2.65E33 J. We must reduce this to 1⁄17 of its current value. or reduce it by (16/17)*2.65E33 J = 2.5E33 J. The current total annual energy production in the world is about 5E17 J. The sun will be a red giant in about 7.6E9 years. So we would need about a million times current global energy production running full time into rocket motors to push the earth out to a safe orbit by the time the sun has expanded.
But it is worse than that. The Sun actually expands over a scant 5 million years near the end of that 7.6E9 years. So to avoid freezing for billions of years because we have started moving away from the sun too soon, we essentially will need a billion times current energy production running into rocket engines for those 5 million years of solar expansion. But the good news is we have 7.6E9 billion years to figure out how to do that.
If we use plasma rockets which push reaction mass out at 1% the speed of light, then we will need a total of about 6E16 kg reaction mass, or about 0.000001% of the earth’s total mass. The total mass of water on the earth is about 1E21 kg so we could do all of this using water as reaction mass and still have 99.99% of the water left when we are done.
I wonder what the exhaust plume of an engine like that would look like, and how far away from it you’d have to be standing to still be capable of looking at anything after a second or two.
I’m curious about the thought process that led to this being asked in the “stupid questions” thread rather than the “very advanced theoretical speculation of future technology” thread. =P
As a more serious answer: Anything that would effectively give us a means to alter mass and/or the effects of gravity in some way (if there turns out to be a difference) would help a lot.
I wasn’t sure there was a way to do it within current physics.
Now we get to the hard question: supposing we (broadly interpreted, it will probably be a successor species) want to move the earth outwards using those little gravitational nudges, how do we get civilizations with a sufficiently long attention span?
I heard Ritalin has a solution. Couldn’t pay attention long enough to verify. ba-dum tish
On a serious note, isn’t the whole killing-the-Earth-for-our-children thing a rather interesting scenario? I’ve never seen it mentioned in my game theory-related reading, and I find that to be somewhat sad. I’m pretty sure a proper modeling of the game scenario would cover both climate change and eaten-by-red-giant.
I don’t see the connection to killing the earth for our children. Moving the earth outwards is an effort to save the earth for our far future selves and our children.
I think “for our children” means “as far as our children are concerned” and failing to move the earth’s orbit so it doesn’t get eaten by the sun (despite being able to do it) would qualify as “killing the earth for our children”. (The more usual referents being things like resource depletion and pollution with potentially disastrous long-term effects.)
Thanks. That makes sense.
If we haven’t gotten one by then, we’re doomed. Or at least, we don’t get a very good planet. We could still have space-stations or live on planets where we have to bring our own atmosphere.
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.
Yes, I saw an article a few years ago a back of the envelope estimate that suggested this would be doable if one could turn mass on the moon more or less directly to energy and use the moon as a gravitational tug to slowly move Earth out of the way. You can change mass almost directly into energy by feeding the mass into a few smallish blackholes.
How do they propose to move the blackholes? Nothing can touch a blackhole, right?
Black holes feel gravity just like any other massive body. And they can be electrically charged. So you can move them around with strong enough gravitational and/or electric fields.
It can, as long as you don’t mind that you won’t get it back when you’re done. You have to constantly fuel the black hole anyway. Just throw the fuel in from the opposite direction that you want the black hole to go.
Throwing mass into a black hole is harder than it sounds. Conveniently sized black holes that you actually would have a chance at moving around are extremely small, much smaller than atoms, I believe. I think they would just sit there without eating much, despite strenous efforts at feeding them. The cross-section is way too small.
To make matters worse, such holes would emit a lot of Hawking radiation, which would a) interfere with trying to feed them, and b) quickly evaporate them ending in an intense flash of gamma rays.
The problem is throwing mass into other mass hard enough to make a black hole in the first place.
Hawking radiation isn’t a big deal. In fact, the problem is making a black hole small enough to get a significant amount of it. An atom-sized black hole has around a tenth of a watt of Hawking radiation. I think it might be possible to get extra energy from it. From what I understand, Hawking radiation is just what doesn’t fall back in. If you enclose the black hole, you might be able to absorb some of this energy.
Yes, making them would be incredibly hard, and because of their relatively short lifetimes, it would be extremely surprising to find any lying around somewhere. Atom sized black holes would be very heavy and not produce much Hawking readiation, as you say. Smaller ones would produce more Hawking radiation, be even harder to feed, and evaporate much faster.
I don’t really know if it’s plausible, but Larry Niven’s far-future fiction A World Out of Time (the novel, not the original short story of the same name) deals with exactly this problem.
His solution is a “fusion candle”: build a huge double-ended fusion tube, put it in the atmosphere of a gas giant, and light it up. The thrust downwards keeps the tube floating in the atmosphere. The thrust upwards provides an engine to push the gas giant around. In the book, they pushed Uranus to Earth, and then moved it outwards again, gravitationally pulling the Earth along.
This is a fascinating question. Very speculatively, I could imagine somehow using energy gained by pushing other objects closer to the Sun, to move the Earth away from the Sun. Like some sort of immense elastic band stretching between Mars and Earth, pulling Earth “up” and Mars “down”.
That is essentially what would happen if you used gravitational assistance and orbited asteroids between Mars and Earth.