# How to move a planet? **URL:** \ **Category:** Factual Questions\ **Created:** [July 10, 2007, 7:07pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244 "2007-07-10T19:07:14Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Captain\_Carrot](https://avatars.discourse-cdn.com/v4/letter/c/f0a364/32.png) [@Captain\_Carrot](https://boards.straightdope.com/u/Captain_Carrot)\ **Post date:** [July 11, 2007, 5:14pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/21 "2007-07-11T17:14:54Z")
> [@Stranger on a Train](#): > > Just don’t go spinning a giant habitable ring around a central star, there being a well-known stability problem with that particular solution. Really? I didn’t know that. What is 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:** [July 11, 2007, 5:22pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/22 "2007-07-11T17:22:53Z")
[QUOTE=Napier] As Stranger suggests, you could use an external massive object to pull a planet. However, a slingshot effect would be brief and would require an object to be pretty close - therefore there’d be tidal hell to contend with, a little rough on the trophy. [/QUOTE] It depends on how close you get and how massive it is. We can pretty much assume that introducing _any_ new force, even one from a massive, distant body with small gradients is going to cause some seismic disruption. Bodies the size of planets (or even small moons) just can’t be considered rigid objects. It’s like trying to catch a raw egg without breaking the shell. If you want to make Venus habitable (at least from the perspective of limiting Sunlight impinging upon it) you’d be better off building an orbital shield placed at the L1 libration point (with a keel pointed back at Venus to keep it properly oriented) to absorb the excess solar energy. (You’ll still have to do something with the energy–perhaps side-mounted radiators that exhaust in shadow tangent to the orbit, or converting the bulk of it into a maser that beams energy down to the surface for storage and use.) Now, all you have to cope with is the crushing atmospheric pressure and highly reactive reducing atmosphere. Good luck with that. Stranger ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [July 11, 2007, 5:26pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/23 "2007-07-11T17:26:06Z")
[QUOTE=Stranger On A Train] (You’ll still have to do something with the energy–perhaps side-mounted radiators that exhaust in shadow tangent to the orbit, or converting the bulk of it into a maser that beams energy down to the surface for storage and use.) [/QUOTE] Wouldn’t a reflective surface sending most of back in the general direction of the Sun have the effect of dealing with most of the energy? ---
**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:** [July 11, 2007, 5:38pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/24 "2007-07-11T17:38:04Z")
[QUOTE=Captain Carrot] Really? I didn’t know that. What is it? [/QUOTE] The [Wikipedia article on the Ringworld](http://en.wikipedia.org/wiki/Ringworld#Instability) doesn’t really go into sufficient detail, but basically the Ringworld is not in a free orbit about its central star; it’s a rotating object under tension which is spun at 770 mi/s (to achieve ~1 Earth gravity of centrifugal acceleation, simulating the presumed gravity of the Pak homeworld) which is vastly faster than its free orbiting speed. If it were just sitting there it would have neutral stability–that is, if no force pushed it off course it would merely drift, with gravitational forces from the star within cancelling each other out–but because it is rotating so bloody fast there are dynamic components that make it significantly unstable, like a Hula Hoop. (You’ll have the same problem with a rotating Dyson shell.) The Ringworld has other problems, including that the tensile strength of the ring floor material has to be absurdly stronger than anything we can conceive of. I’ve always wondered why Niven didn’t just make out out of a single artificially strengthened macromolecule similar to GP hulls, or enclose it in a statis field, since these are two available technologies in Known Space that would allow such a structure. (Stasis fields, of course, have serious problems with Special Relatiivty, but that’s another issue entirely.) Building a bunch of tiny Ringworlds (like Culture Orbitals) makes a lot more sense. But of course, they wouldn’t be as grandiose as a single macrostructure. Stranger ---
**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:** [July 11, 2007, 5:55pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/25 "2007-07-11T17:55:47Z")
[QUOTE=Giles] Wouldn’t a reflective surface sending most of back in the general direction of the Sun have the effect of dealing with most of the energy? [/QUOTE] Sure, but then you’re going to have a net force tending to push it out of the metastable L1 point. For an object the size of a planet this force is negligible, but I’m imagining a shield made of vacuum-cemented dust or somesuch on the order of about 1m thickness (which will only require a largish asteroid), so the amount of mass per facing area may be small enough that light pressure could destabilize it, even with a large tidal keel to keep it oriented with the planet. Plus, even if you coat it with a very high albedo material, we’ll have to assume that the coating will degrade fairly rapdily from impingment of solar wind; it’ll be receiving almost 4kW/m[sup]2[/sup] at Venus’ orbit, which will rapidly degrade most materials. It’s probably easier to pump away the heat and radiate it away (or exhaust it via a thermally-energized propellent analogous to evaporation in terrestrial environments) than to try to make the shield perfectly reflective. Stranger ---
**Author:** ![JayRx1981](https://avatars.discourse-cdn.com/v4/letter/j/71e660/32.png) [@JayRx1981](https://boards.straightdope.com/u/JayRx1981)\ **Post date:** [July 11, 2007, 6:14pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/26 "2007-07-11T18:14:10Z")
Theoretically, would an Einstein-Rosen bridge be a possibility for moving a planet? Place one planet-sized end in the direct path of the moving planet’s orbit and the other positioned in the orbit you want the planet to set in? And if you get Venus into an Earth distance orbit, why couldn’t you move it out of the plane of Earth’s orbit (essentially angle the plane of the orbit at say 30 degrees relative to earth’s) to avoid the collision potential? Since we’re talking pure theory here, that is… ---
**Author:** ![mlees](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@mlees](https://boards.straightdope.com/u/mlees)\ **Post date:** [July 11, 2007, 6:14pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/27 "2007-07-11T18:14:38Z")
[QUOTE=athelas] Let’s pretend you had a planet which whose rotation is slaved to its revolution (so that the same side always faced the sun). To move the planet, I build a huge rocket engine on the sunward side which expelled its gases into space. In order to move it into a larger orbit, do I need to have constant thrust? Or can I just maintain thrust over a single revolution and then cut the engine? [/QUOTE] Mercury is “tide locked” to the sun in this fasion. This is because the rotation of the planet matches the time needed to complete one orbit around the sun. If you plant a motor powerful enough to puch Mercury away from the sun, the Mercurian Year becomes longer, and falls out of sync with the day/night rotational period, and your rocket motor ends up pointing in all kinds of skewed directions, if not corrected. (Mercury as a balloon let go in the solar system. Pffffttftftft!) To answer your last question, though, depends on how strong your motor is, and how much force the planet’s crust can withstand. I would opt for a slow but steady apprach, adjusting for errors as they are observed, rather than a big blast that would be much harder to fix when it fouls up. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [July 11, 2007, 6:27pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/28 "2007-07-11T18:27:49Z")
[QUOTE=Stranger On A Train] The [Wikipedia article on the Ringworld](http://en.wikipedia.org/wiki/Ringworld#Instability) doesn’t really go into sufficient detail, but basically the Ringworld is not in a free orbit about its central star; it’s a rotating object under tension which is spun at 770 mi/s (to achieve ~1 Earth gravity of centrifugal acceleation, simulating the presumed gravity of the Pak homeworld) which is vastly faster than its free orbiting speed. If it were just sitting there it would have neutral stability–that is, if no force pushed it off course it would merely drift, with gravitational forces from the star within cancelling each other out–but because it is rotating so bloody fast there are dynamic components that make it significantly unstable, like a Hula Hoop. (You’ll have the same problem with a rotating Dyson shell.) [/QUOTE] Actually, the rotation has nothing to do with it. A non-rotating ringworld is gravitationally unstable, just as a rotating one it. As soon as it gets a tiny bit off-center (so the sun is no longer on the axis of the ring), gravity will pull it more and more off center (until one edge hits the sun and we assume either the ring or the sun break). [Non-rigorous proof here:[Why Niven rings are unstable](http://www.alcyone.com/max/writing/essays/why-niven-rings-are-unstable.html)] For what it’s worth, a ringworld is stable in the other direction: if the sun stays exactly on the axis, but just above or below the plane of the ring, gravity will pull the ring back into place (not that this does much good when it’s unstable the other way). A non-rotating Dyson sphere, on the other hand, is gravitationally neutral. So if it gets a little off-center it just stays there, and gravity doesn’t pull it back into place or farther out. (And if you give it a tiny velocity relative to the sun, it just keeps moving). Non-rigorous proof at the above link, or get a first-year calculus student to prove it to you. A planetary orbit is sort of neutral, but with ‘brakes’ in the sense that the orbit stops moving when you stop pushing on it. ---
**Author:** ![Anne\_Neville](https://avatars.discourse-cdn.com/v4/letter/a/b9e5f3/32.png) [@Anne\_Neville](https://boards.straightdope.com/u/Anne_Neville)\ **Post date:** [July 11, 2007, 7:23pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/29 "2007-07-11T19:23:22Z")
[QUOTE=mlees] Mercury is “tide locked” to the sun in this fasion. This is because the rotation of the planet matches the time needed to complete one orbit around the sun. [/QUOTE] No, it’s not. We used to think it was, but [we learned from radar observations in 1965 that Mercury is in a 3:2 spin-orbit resonance](http://en.wikipedia.org/wiki/Mercury_%28planet%29#Orbit_and_rotation). That means three days on Mercury are the same as two years. ---
**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:** [July 11, 2007, 7:24pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/30 "2007-07-11T19:24:47Z")
Dang, **Quercus** beat me to the Ringworld stability question. At least I still get to cover Mercury’s lock. It is locked to the Sun, but resonantly, not rigidly. Its rotation period is exactly 2/3 of its year, so every time it reaches perihelion, the opposite side is facing the Sun. This is stable because Mercury has a fairly elliptical orbit, and the planet itself is somewhat prolate. Every perihelion, the long axis of the planet is pointing towards and away from the Sun. ---
**Author:** ![mlees](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@mlees](https://boards.straightdope.com/u/mlees)\ **Post date:** [July 11, 2007, 7:25pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/31 "2007-07-11T19:25:18Z")
[QUOTE=Anne Neville] No, it’s not. We used to think it was, but [we learned from radar observations in 1965 that Mercury is in a 3:2 spin-orbit resonance](http://en.wikipedia.org/wiki/Mercury_%28planet%29#Orbit_and_rotation). That means three days on Mercury are the same as two years. [/QUOTE] Doh! Thanks. I need to research more before opening my big yapper. ---
**Author:** ![groman](https://avatars.discourse-cdn.com/v4/letter/g/73ab20/32.png) [@groman](https://boards.straightdope.com/u/groman)\ **Post date:** [July 11, 2007, 9:23pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/32 "2007-07-11T21:23:29Z")
Could you magnetize Venus and use magnetism to drag the core (assuming the rest of the planet would follow)? ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [July 12, 2007, 12:48am UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/33 "2007-07-12T00:48:29Z")
[QUOTE=Napier] As Stranger suggests, you could use an external massive object to pull a planet. However, a slingshot effect would be brief and would require an object to be pretty close - therefore there’d be tidal hell to contend with, a little rough on the trophy. To move the planet without wrecking it, you’d have to use a much larger mass much further away, so the field strength doesn’t change much from point to point on the earth. The Sun, for instance, is close enough that the difference in field strength between the sides of the Earth facing toward it versus facing away from it generate significant ocean tides (I don’t mean the much bigger Moon effect, but rather the small solar effect that IIRC is the difference between spring and neap tides). [/QUOTE] As pointed out in a link in one of **Anne Neville** links, you can use an [ordinary asteroid](http://www.space.com/scienceastronomy/planetearth/earth_move_010207.html) to move a planet; you don’t need anything massive. Now, doing so in the OP’s timeframe of mere thousands of years would likely require the massive object you proposed, or many, many asteroids. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [July 12, 2007, 8:01am UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/34 "2007-07-12T08:01:05Z")
[QUOTE=Stranger On A Train] I’ve always wondered why Niven didn’t just make out out of a single artificially strengthened macromolecule similar to GP hulls, or enclose it in a statis field, since these are two available technologies in Known Space that would allow such a structure. (Stasis fields, of course, have serious problems with Special Relatiivty, but that’s another issue entirely.) Stranger [/QUOTE] Because the Pak never discovered either technology. Similarly, neither the Puppeteers nor the Outsiders nor anyone else seem to have reinvented the _cziltang brone_. Earth humans independently rediscovered stasis fields though - that’s how they let Kzanol out of the bag. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [July 12, 2007, 8:35am UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/35 "2007-07-12T08:35:19Z")
If we move Venus and terraform it to be like Earth, We may need to give it a moon for terrestrial species dependent on lunar navigation and tides. Mercury fills in that role well. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [July 12, 2007, 9:32am UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/36 "2007-07-12T09:32:04Z")
I’d rather park Mars nearby so you had a cool double-planet system, plus, as stated, somewhere to dump Venus’s excess atmosphere. Long term, I suppose they’d tidally brake each other, but we could correct that when we felt like 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:** [July 12, 2007, 3:36pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/37 "2007-07-12T15:36:52Z")
[QUOTE=Malacandra] Because the Pak never discovered either technology. Similarly, neither the Puppeteers nor the Outsiders nor anyone else seem to have reinvented the _cziltang brone_. Earth humans independently rediscovered stasis fields though - that’s how they let Kzanol out of the bag. [/QUOTE] Yeah, but _scrith_ was a conceit that was never adequately described (unless he did so somewhere in the post-_Ringworld Engineers_ oeurve). In general, the Ringworld is an impossibility for mostly conventional technology of the Pak, from both a materials standpoint, and from the energy and effort required to construct it. How did Pak engineers–lacking FTL capability and other “indistinguishable from magic” technology–manage to build something on the scale of the Ringworld, then transport their broods there? And why would they build something so hideously indefensible? Never mind giant magnetically pumped solar lasers; all you’d need is to blow a few million tons of radioactives onto the surface, or a rich cloud of negative ions to foul their Bussard attitude jets, or even a relatively small amount of antimatter to damage the Ring structure; it must be under enormous tension and even a small weakness would cause it to shred apart. And the Pak should have known that the ginormous size of the Ringworld would almost certainly result in balkinization and (without regular culling) mutation and diversification of Pak breeders into bastard species. The Ringworld honestly doesn’t make a lot of sense as a Pak artifact. Stranger ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [July 12, 2007, 3:48pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/38 "2007-07-12T15:48:59Z")
Perhaps they stole it then. 🙂 We’ve only Louis Wu’s guess for it that the Pak built the Ringworld - we know the place was lousy with Pak once upon a time, but not that the protectors built it; even the evidence of the Repair Centre can’t be taken as gospel, 'cos there’s no reason why it couldn’t be a Pak add-on to an existing structure. And the Pak, on the evidence, would have been confident in their ability to improvise some means of defending the Ringworld once they’d half-inched it, and addressing the other concerns. ---
**Author:** ![Anaamika](https://avatars.discourse-cdn.com/v4/letter/a/5f9b8f/32.png) [@Anaamika](https://boards.straightdope.com/u/Anaamika)\ **Post date:** [July 12, 2007, 4:29pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/39 "2007-07-12T16:29:31Z")
I’m glad to see we’re working on this. We’ll need it for when the Vogons decide to build their bypass right through our orbit. ---
**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:** [July 12, 2007, 4:32pm UTC](https://boards.straightdope.com/t/how-to-move-a-planet/411244/40 "2007-07-12T16:32:07Z")
[QUOTE=Malacandra] Perhaps they stole it then. 🙂 [/QUOTE] Ohhh…that’s a novel idea. I hadn’t even considered that. Of course, then you’d have to explain how it fit the needs of the Pak just so (right distance from a G2 star, correct “gravity”, et cetera), and why the worlds of Known Space, which is a couple hundred light-years away IIRC, are mapped on the Great Ocean, but that proposal does have merit. Stranger [Previous page](https://boards.straightdope.com/t/how-to-move-a-planet/411244.md?page=1) [Next page](https://boards.straightdope.com/t/how-to-move-a-planet/411244.md?page=3)