# Do we know enough about orbital mechanics and gravity to safely move a planet? **URL:** \ **Category:** Factual Questions\ **Created:** [March 28, 2019, 8:34pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681 "2019-03-28T20:34:32Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [March 28, 2019, 8:34pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/1 "2019-03-28T20:34:32Z")
Leave aside how we’d move the planet. What I want to know is, if we could move, say, the Earth to a different orbit or if we wanted to chuck it right out of the solar system (or whatever), do we know enough about orbital mechanics and gravity to do so without sending the Earth into another planet or into anything else (unless that was the plan)? Could we move the Earth to a further orbit, or maybe swap it with Mars without crashing it into something or destabilizing the whole planetary system? Again, not asking how we would move it, asking if we could move it, could we do so as we intended, or do we not know enough to, even if we could move it, to do so as we intend? I get that a lot of this, even what I’m asking, is probably speculative and might not be best suited for GQ, but I’d really like a GQ type answer if possible. I’ve played with the universe sandbox game, so I know in the game I can move things around, change the mass, and that by doing so it can change orbits and destabilize the system, but that’s just a game. For reference or maybe context this came up in a discussion I was having with some friends about having to move the Earth in a billion or so years gradually outward as the sun expands and the habitable zone move outward. My thought was…well, even if we COULD move the Earth, could we do so without smashing it into another planet or other large object defeating the purpose, and could we do it without destabilizing the system? I had no idea, so thought I’d ask here. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [March 28, 2019, 9:07pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/2 "2019-03-28T21:07:16Z")
Fundamentally, yes. This isn’t materially different than plotting the trajectories of spacecraft to some particular orbit or intercept. There are some nuances in dealing with a mass significant enough to affect other major bodies, and in assuring the long term stability of a new arrangement in accounting for orbital resonances and perturbations, but then, if you have the ability to move an intact planet into an entirely new orbit then you can surely correct for the occasional irregularities. Earth at the orbit of Mars should be fine; but in between, say, Jupiter and Saturn would probably not be stable in the long term and would require periodic adjustment. The practicalities of doing so are another matter. Although it was specified in the o.p. that it is assumed that there is black box technomagic to move planets around, one thing that needs to be appreciated is that at the scale of a planet a body can no longer assumed to be even approximately solid, particularly one with an active core; it is more of a viscous fluid held in shape by gravity with a tenuous crust around it. There is no way to directly push or pull on it with physical contact, and even using another mass to tractor it is going to create enormous tidal stresses at any significant rate of acceleration. So, you have to have some way of applying an acceleration to the entire mass or somehow negating inertia. The only “practical” way of doing this would be by directly manipulating spacetime to form a geodetic path that causes the Earth to falls away from its orbit into a new trajectory; if you can do that kind of business, you probably don’t have a great need to be futzing around with moving planets into habitable orbits. Also, don’t forget the Moon. It isn’t particularly necessary, but it adds aesthetic appeal and we’d miss it if we left it behind. Stranger ---
**Author:** ![XT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/xt/32/456_2.png) [@XT](https://boards.straightdope.com/u/XT)\ **Post date:** [March 28, 2019, 9:32pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/3 "2019-03-28T21:32:25Z")
Thanks for the answer. Appreciate it. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [March 28, 2019, 10:09pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/4 "2019-03-28T22:09:53Z")
By the way, probably the best text to reference for planetary astrodynamics is Murray’s [_Solar System Dynamics_](https://www.amazon.com/Solar-System-Dynamics-Carl-Murray/dp/0521575974/). I don’t have it but it has been recommended to me and reading through the table of contents it looks to cover all fundamentals of planetary astrodynamics including spin-orbit resonance, the disturbing function, and the various perturbations that can affect system stability. [Here is a presentation on the disturbing function that references the book.](https://www.ast.cam.ac.uk/~wyatt/lecture2_planetarysystemdynamics.pdf) My go-to reference on astrodynamics is Vallado’s [_Fundamentals of Astrodynamics and Applications_](https://www.amazon.com/Fundamentals-Astrodynamics-Applications-Technology-Library/dp/1881883183/) but this is a thick and very dense book that immediately jumps into equations and solution algorithms that took me several months to process and understand, and is really geared toward planetary and interplanetary trajectory design rather than celestial mechanics in general, but I found it far more useful than BMW and some of the other basic astrodynamics texts that are typically used as class texts. Stranger ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [March 28, 2019, 10:26pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/5 "2019-03-28T22:26:59Z")
If you are trying to mitigate the effects of the increase in brightness of the Sun, you would need to move Earth’s orbit closer to Mars. Eventually the Earth would start to interact with Mars, and would either displace that planet into an eccentric orbit, or eventually collide with it. So you’d want to move Mars as well. Trouble is, moving Mars into a wider orbit would take it into the Asteroid Belt, scattering asteroids all over the Solar System. Sometimes it is possible for two planets to swap orbits- such a thing may have happened four billion years ago, if the Nice model is correct. > **[Nice model](https://en.wikipedia.org/wiki/Nice_model)** > > The Nice (/ˈniːs/) model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Côte d'Azur Observatory—where it was initially developed in 2005—in Nice, France. It proposes the migration of the giant planets from an initial compact configuration into their present positions, long after the dissipation of the initial protoplanetary disk. In this way, it differs from earlier models of the Solar System's formation. This planetary migration is use The orig... In this scenario, Uranus and Neptune swapped places, an event which would have caused havoc with all the minor objects in the Solar System. Moving planets is not something that can be easily done without chaotic consequences. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [March 28, 2019, 10:33pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/6 "2019-03-28T22:33:56Z")
Incidentally, here’s a proposal by the late Paul Birch that could allow the adjustment of a planet’s orbit over time; it uses a continuous stream of small objects, rather than a few large ones, so it largely free of the tidal effects noted by **Stranger**. > **[MoveAPlanet.pdf](https://orionsarm.com/fm_store/MoveAPlanet.pdf)** > > 475.95 KB ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 28, 2019, 10:38pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/7 "2019-03-28T22:38:33Z")
Any process that can move planets would necessarily involve manipulation of energies on a literally astronomical scale. Whatever that process is, I’d be worried about how safe it is. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [March 28, 2019, 10:53pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/8 "2019-03-28T22:53:26Z")
Birch’s proposal uses solar power; of course, on a planetary scale, solar power is vast and abundant resource, and could be very dangerous if misused. ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [March 28, 2019, 11:33pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/9 "2019-03-28T23:33:14Z")
> [@Chronos](#): > > Any process that can move planets would necessarily involve manipulation of energies on a literally astronomical scale. Whatever that process is, I’d be worried about how safe it is. Probably would require goggles and gloves for sure. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [March 28, 2019, 11:33pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/10 "2019-03-28T23:33:29Z")
> [@Chronos](#): > > Any process that can move planets would necessarily involve manipulation of energies on a literally astronomical scale. Whatever that process is, I’d be worried about how safe it is. That’s what the [Omega-13 device](https://m.youtube.com/watch?v=cMho5_Fl_rs) is for. Stranger ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 28, 2019, 11:38pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/11 "2019-03-28T23:38:37Z")
Always wear your PPE (Planetary Protective Equipment)! ---
**Author:** ![msmith537](https://avatars.discourse-cdn.com/v4/letter/m/d9b06d/32.png) [@msmith537](https://boards.straightdope.com/u/msmith537)\ **Post date:** [March 28, 2019, 11:47pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/12 "2019-03-28T23:47:49Z")
> [@Stranger\_On\_A\_Train](#): > > The practicalities of doing so are another matter. Although it was specified in the o.p. that it is assumed that there is black box technomagic to move planets around, one thing that needs to be appreciated is that at the scale of a planet a body can no longer assumed to be even approximately solid, particularly one with an active core; it is more of a viscous fluid held in shape by gravity with a tenuous crust around it. There is no way to directly push or pull on it with physical contact, and even using another mass to tractor it is going to create enormous tidal stresses at any significant rate of acceleration. So, you have to have some way of applying an acceleration to the entire mass or somehow negating inertia. The only “practical” way of doing this would be by directly manipulating spacetime to form a geodetic path that causes the Earth to falls away from its orbit into a new trajectory; if you can do that kind of business, you probably don’t have a great need to be futzing around with moving planets into habitable orbits. I think that kind of handwaves away a fundamental problem with the question, don’t you? It seems to me that the biggest component of risk in this exercise is imparting enough force on the Earth (or whatever planet) to move it without destroying the planet or killing everything on it. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [March 28, 2019, 11:56pm UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/13 "2019-03-28T23:56:48Z")
You could do a [long slow trip](https://arxiv.org/abs/astro-ph/0102126), say to allow for the Sun’s gradual heating up. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [March 29, 2019, 12:24am UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/14 "2019-03-29T00:24:39Z")
> [@dtilque](#): > > You could do a [long slow trip](https://arxiv.org/abs/astro-ph/0102126), say to allow for the Sun’s gradual heating up. This method is very similar to Birch’s method, except that the asteroids are used in their natural state rather than ground up into smaller pellets. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [March 29, 2019, 2:06am UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/15 "2019-03-29T02:06:48Z")
> [@eburacum45](#): > > Incidentally, here’s a proposal by the late Paul Birch that could allow the adjustment of a planet’s orbit over time; it uses a continuous stream of small objects, rather than a few large ones, so it largely free of the tidal effects noted by **Stranger**. > [https://orionsarm.com/fm\_store/MoveAPlanet.pdf](https://orionsarm.com/fm_store/MoveAPlanet.pdf) And the Birch method is basically the same thing as the aforementioned Nice method (and much safer than the Nasty method.) ---
**Author:** ![Mijin](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mijin/32/9369_2.png) [@Mijin](https://boards.straightdope.com/u/Mijin)\ **Post date:** [March 29, 2019, 3:24am UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/16 "2019-03-29T03:24:16Z")
There was a recent movie where the whole earth is moved to another star system: [The Wandering Earth](https://en.wikipedia.org/wiki/The_Wandering_Earth). Probably not that much help with the OP though…in terms of science it makes \*Armageddon \*seem like a Stanford Astrophysics lecture. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [March 29, 2019, 4:21am UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/17 "2019-03-29T04:21:23Z")
> [@Stranger\_On\_A\_Train](#): > > There is no way to directly push or pull on it with physical contact, and even using another mass to tractor it is going to create enormous tidal stresses at any significant rate of acceleration. Eh, it’s not as bad as all that. Suppose our engineers decide to use the Moon as the tractor mass. We live with lunar tides now and they aren’t so bad. The Moon exerts about 2e20 N on the Earth: 6.7e-11 m^3/(kg-s^2) \* 6e24 kg \* 7.3e22 kg / (3.8e8 m)^2 = 2e20 N Earth is thus accelerated at: 2e20 N / 6e24 kg = 3.3e-5 m/s^2: Over a day, that’s 2.9 m/s. Over a year, that’s 1052 m/s. Earth’s orbital velocity is 30 km/s. Mars’ is 24 km/s. So, a bit under 6 years to move Earth into Mars orbit. Another 24 years to reach escape velocity. Of course, being able to wrangle a lunar mass is non-trivial, but nevertheless that seems a more tractable challenge than direct manipulation of spacetime. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [March 29, 2019, 4:50am UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/18 "2019-03-29T04:50:30Z")
> [@eburacum45](#): > > If you are trying to mitigate the effects of the increase in brightness of the Sun, you would need to move Earth’s orbit closer to Mars. Eventually the Earth would start to interact with Mars, and would either displace that planet into an eccentric orbit, or eventually collide with it. So you’d want to move Mars as well. Trouble is, moving Mars into a wider orbit would take it into the Asteroid Belt, scattering asteroids all over the Solar System. > > Sometimes it is possible for two planets to swap orbits- such a thing may have happened four billion years ago, if the Nice model is correct. > [Nice model - Wikipedia](https://en.wikipedia.org/wiki/Nice_model) > In this scenario, Uranus and Neptune swapped places, an event which would have caused havoc with all the minor objects in the Solar System. Moving planets is not something that can be easily done without chaotic consequences. Consequences, schmonsequences, as long as I’m rich. ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [March 29, 2019, 6:16am UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/19 "2019-03-29T06:16:12Z")
> [@Chronos](#): > > Any process that can move planets would necessarily involve manipulation of energies on a literally astronomical scale. Whatever that process is, I’d be worried about how safe it is. I have had the same thought as the OP. Won’t it be easier to simply add/remove material to the sun to maintain it at somewhere like present levels? ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [March 29, 2019, 6:23am UTC](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681/20 "2019-03-29T06:23:40Z")
Either approach (adjusting the orbit of the Earth or changing the composition of the Sun) would require essentially magical technology and a command of energy that vastly exceeds what could be produced by any conventional means including nuclear fusion. It may be reasonable assumed that we will have some future understanding of physics that may allow for either but any projection of how we could do this based upon current understanding of physics is like Victorian ‘natural philosophers’ envisioning manipulation of the luminiferous aether using cavorite. Stranger [Next page](https://boards.straightdope.com/t/do-we-know-enough-about-orbital-mechanics-and-gravity-to-safely-move-a-planet/831681.md?page=2)