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It's said the earth is approaching the sun. Can we push it away little by little?

Although we can emit rocket towards the sun, but it's too expensive to make a rocket:

step1

So I think maybe we can recycle the rocket, at first, the rocket is send out with a very fast speed, so give back a big force to the earth. And then, we let the rocket slowly comes back to the earth, and re-send it again:

step2

Well, this solution sounds a little inefficient. Still I don't know if it works. Better ideas?

EDIT

Umm.. It seems the earth is leaving the sun. That's ok. Then, the question is how can we push the earth towards the sun, i.e., correct the orbit by human force? Even if the sun turns into a red giant someday, we may finally find a way to put the earth at a safe position far enough from heat.

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Besides the inherent problems in differences in mass (it's worse than a human trying to move a skyscraper on their own power), your idea has a major problem - if you launch the rocket, it's only going to be moving the earth for a short period of time - you'd be 'far better off' to leave the rocket on earth, and just point it towards the sky. You should be able to get a much longer sustained burn that way. Of course, any rocket powerful enough to move the earth would likely crack the crust, so... –  Clockwork-Muse Jun 8 '12 at 17:15

3 Answers 3

up vote 8 down vote accepted

The Earth is pretty heavy. No rocket that we can currently make is going to have much effect on it's motion.

The only remotely feasible suggestion I've seen is to divert asteroids from the asteroid belt so they pass close to the Earth in a slingshot orbit. The idea is to accelerate the asteroid and decelerate the Earth, which causes the Earth to move into a larger orbit. Any one asteroid isn't going to have a big effect, but there are a lot of asteroids out there so you would do it repeatedly.

There are potential problems of course. You still need a fair bit of energy to divert the asteroid, though you could do it with a solar sail if you had enough time. Also you'd need to be pretty sure you could divert the asteroid accurately as an error in the asteroids trajectory could prove embarrassing.

Anyhow, who told you the Earth is approaching the Sun? As far as I know it isn't, though because the Earth's orbit is an ellipse the Earth-Sun distance does decrease for for six months then increase again for the next six months.

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"an error in the asteroids trajectory could prove embarrassing" Good one. –  user973810 Jun 8 '12 at 14:05
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Define 'larger' orbit. Correct me if I'm wrong, but wouldn't decelerating the Earth drop it in closer to the sun? –  Clockwork-Muse Jun 8 '12 at 17:27
    
Planets in larger orbits move more slowly, so after you've moved the earth outwards it would be moving more slowly that it was. However you'd actually do it by accelerating the Earth to put it in an elliptical orbit with the major axis larger than the current orbit. Then when the earth is at perihelion you accelerate it again to make the orbit circular. So you actually accelerate it twice, but it ends up moving more slowly. That's because after the first acceleration the Earth slows as it approaches perihelion and the second acceleration doesn't make up all the speed lost. –  John Rennie Jun 8 '12 at 17:38
    
However you're correct that if you decelerate the earth you'd put it in an elliptical orbit with the semi-major axis equal to the current orbital distance and the semi-minor axis less than the current orbit. Just to confuse matters the Earth would on average be moving faster even though you decelerated it! –  John Rennie Jun 8 '12 at 17:41
    
Actually, there is nothing confusing about it. At the point in the orbit where you apply the acceleration the Earth will be moving faster. On the other side of the orbit the Earth will be moving slower, though. Of course, the distance from the Sun at that point will be larger, and that is where you see the extra energy. You can then perform a second manoeuvre at that slower, further point to push the opposite (closer, faster point) out further from the Sun. –  dotancohen Jun 9 '12 at 8:44

There exist sun tides as well as moon tides. The moon is receding from the earth due to the tides , because of energy and angular momentum balances.

The same is happening very slowly to the distance of the orbit of the earth from the sun.

No, we are not approaching the sun, we are slowly distancing ourselves from it, as the moon is distancing from the earth.

There is an effect which is making us move very slowly away from the Sun. That is the tidal interaction between the Sun and the Earth. This slows down the rotation of the Sun, and pushes the Earth farther away from the Sun.

But how big of an effect is this? It turns out that the yearly increase in the distance between the Earth and the Sun from this effect is only about one micrometer (a millionth of a meter, or a ten thousandth of a centimeter). So this is a very tiny effect.

and it continues:

There is another effect which is also small, but somewhat bigger than the tidal effect. The Sun is powered by nuclear fusion, which means the Sun is continuously transforming a small part of its mass into energy. As the mass of the Sun goes down, our orbit gets proportionally bigger. However, over the entire main sequence lifetime of the Sun (about 10 billion years), the Sun will only lose about 0.1% of its mass, which means that the Earth should move out by just ~150,000 km (small compared to the total Earth-Sun distance of ~150,000,000 km). If we assume that the Sun's rate of nuclear fusion today is the same as the average rate over those 10 billion years (a bold assumption, but it should give us a rough idea of the answer), then we're moving away from the Sun at the rate of ~1.5 cm (less than an inch) a year

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Thank you for adding that, Anna. I have heard both arguments: that the Earth is approaching and that the Earth is receding from the Sun. –  dotancohen Jun 9 '12 at 8:45

Why do you think that we need to push the Earth farther from the Sun? The rate at which the Earth is approaching the Sun in so minimal as to be inconsequential. The Sun will red giant before the Earth gets close enough to matter, and at that stage the increased distance will not help us.

In any case, to get farther from the Sun we would not push the Earth out from the direction of the Sun. Rather, at aphelion we would push the Earth in the direction of the Earth's travel around the Sun. If you are interested, you might want to read a bit about orbital mechanics.

Note also Anna's answer that the Earth may actually be receding from the Sun.

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Do you mean apply the force at the direction of the tangent vector, is more efficient then at the perpendicular vector? –  Xiè Jìléi Jun 8 '12 at 12:35
    
Our sun will never nova... –  Biff MaGriff Jun 8 '12 at 15:19
    
Thanks, Bill, you are right. The Sun will turn into a red giant, not nova. I'm all embarrassed now! –  dotancohen Jun 9 '12 at 7:51
    
@Xiè: It is not a matter of efficiency but rather changing the shape (eccentricity) of the orbit and therefore the distance from the Sun (two manoeuvres). You should read the linked article on orbital mechanics. –  dotancohen Jun 9 '12 at 7:54

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