
Ask HN: What would it take to move the Earth out of its orbit? - YeGoblynQueenne
According to wikipedia, the Earth orbits the sun at an average distance of 149.60 million km.<p>Say we wanted to move the Earth slightly outside its current orbit so that its average distance from the sun was a full 150 million km.<p>What would that take?<p>In particular: would a collision with something really big, like the moon, or a dwarf planet, do it? How big would &quot;big&quot; have to be (say, in Earth masses)? Would the moon do it?<p>How about if a huge mass magically materialised on the surface of the Earth? Would that somehow knock the Earth out of orbit? If so, how big would that mass have to be?<p>I&#x27;m sorry but I can&#x27;t reveal the real motivation behind my question. I am definitely not planning to build a doomsday device so that I can rule the world.<p>(Seriously, I&#x27;m just wondering and I&#x27;m not a physics geek so I don&#x27;t know how to start thinking about this).
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FiatLuxDave
Here is a relevant paper which may answer some of your questions:
[https://arxiv.org/abs/astro-ph/0102126](https://arxiv.org/abs/astro-
ph/0102126)

I've been slowly writing a paper about how to move planets by using repeated
gravitational assists from masses moving in orbits which repeatedly cycle
between inner and outer planets, conveying momentum from one planet to
another. Unfortunately, I just don't have time to finish it right now. Feel
free to email me if you want to discuss this stuff in more depth.

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YeGoblynQueenne
Very intersting, thanks. This could make a great sci-fi story, btw.

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athriren
There is a Chinese science fiction movie that is about this (although instead
of slightly shifting the earth’s orbit, they make earth a spaceship itself)
called The Wandering Earth. I believe it is on Netflix. The subtitles for the
theatrical release were not the best I’ve ever seen but it was a fun movie.

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ColinWright
You'd need an injection of delta-vee to move to an elliptical orbit where
perihelion is the current distance, and aphelion is the desired distance. So
you need to accelerate the Earth by some amount. When it gets to aphelion it's
not moving fast enough to stay there, so you need another injection of delta-
vee to increase the speed to maintain the new orbit. Note that the new speed
in orbit is less than the old speed, despite adding speed twice, because the
Earth will lost speed as it climbs up the gravity well.

Then you remember that Kepler's law is R^3 is proportional to P^2, where R is
the distance and P is the period. So you can compute the new P based on the
old P, and start doing sums from there.

Could probably be done on the back of an envelope with a little more work, but
I'm not in a position to do so now. If you're serious, get no better answer,
and can wait a day or two, email me.

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YeGoblynQueenne
"If I'm serious"? You mean, serious about planetary anhiliation, right? :)

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ColinWright
No, by "if you're serious" I mean about having an answer. Sometimes people ask
questions on a whim and, after an hour or two, don't really care. But
sometimes people really want an answer, so they are serious about finding out.

Equally: "... would a collision with something really big, like the moon, or a
dwarf planet, do it?"

As always, it depends on the speed and direction, as much as the mass. But if
it happened, I suspect we'd have bigger problems to worry about than what out
new orbit would be.

Or no problems at all.

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YeGoblynQueenne
Thanks for taking the time to answer. I think my curiosity has been satisfied.
I did a bit of reading after posting the question here and it seems to me that
the simplest way to move the planet out of its orbit would be to pull it by
having something really large pass by it, rather than hit it directly with
something very large. If I understand correctly, that seems to be the way that
scientists think that rogue planets leave their solar systems and wander
around. Collisions with large bodies seem to either result in moons being
formed or planets breaking up completely?, so it seems that a collision is not
a "safe" method to move the planet.

I'm not sure about mantaining the new orbit, rather than falling back towards
the sun, as per your comment, but I'm guessing that if the Earth was pulled
away from the sun by something as large as another star then it would just
start falling towards that other star.

Anyway to be honest I didn't need an exact calculation, more a general
intuition. But thanks for the offer :)

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ColinWright
> _" I'm not sure about maintaining the new orbit ..."_

OK, so you give it a boost. Assuming you've boosting along the orbital path,
it's now travelling exactly in the same direction as it was, but faster. Its
new path is an ellipse that rises to a higher peak (aphelion), slowing as it
goes, but when it gets there it has lost a lot of speed.

And in fact, it now doesn't have enough speed for that new, higher orbit (even
though the speed for that orbit is less than the original), and because it's
an ellipse that passes through that original position, it will not stay at
this higher altitude, but will trace the second half of the ellipse back to
the original position.

The magic term is "Hohmann Transfer":

[https://en.wikipedia.org/wiki/Hohmann_transfer_orbit](https://en.wikipedia.org/wiki/Hohmann_transfer_orbit)

Again, let me know if you want more, but I see others have made similar offers
so I won't be offended if you don't ask.

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antstrangler
You have to add velocity to the earth along it's orbit to raise it.

Would the moon do it: Size does not matter (despite what my ex may tell you).
You can accelerate a bowling ball by throwing a tennis ball at it hard enough.
You can do the same with the earth and moon if you throw it hard enough.

Lets give it a whack to increase perihelios by 0.89M km so that we average
150M km and a more circular orbit. We'll need a velocity increase of about
0.33 km/s. If we assume that it's an inelastic collision we'd need to throw
the moon at the earth at about 27km/s.

It's popular to measure energy in comparison to nuclear weapons. That is about
10E14 times the energy of the most powerful nuclear bomb ever designed.

You should dobule check my calculations before sending out RFQ for your
doomsday device. If you just send this out to a contractor you will run the
risk of looking silly to people who know what they're doing.

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YeGoblynQueenne
Isn't there a danger that the Earth will just break up instead of accelerating
it if you hit it with a very large mass moving at a very great speed?

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antstrangler
Yes. The good news is that most of the mass will probably not reach escape
velocity and eventually gravity will do the job of pulling it together.

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wreet
As someone who is building a doomsday device I am curious about this too.

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Piskvorrr
I'm pretty sure there's an XKCD what-if on this. Or ought to be.

