Assume 2 planetoids (one of which could be Earth), similar to Earth (says similar radius, and a gravity force ranging from half to twice the gravity of Earth). Is it plausible they are tidally locked near enough so that we could "climb" (using aircrafts) from one to another (say 20 km of distance, which is reasonable I think; much lower gravity would allow also for higher buildings ^^ ).

As far as I know there are two main constraints:

  1. Roche Limit (any planetoid closer than that limit would measure an "upward" gravity force at some surface point).

  2. Centripetal force (any planetoid near enough would rotate around the other at so great a speed that there are points on its surface where you could measure an "upward" gravity force).

In both cases the upward gravity would make the planet disintegrate (because rocks would just float into the space).

Given the above constraints, is such a scenario plausible? The Roche limit approximate formula seems simple enough to show that is not possible (at least for planets similiar to Earth). The other "duplicated question" also take as accepted answer "no" wich is wrong because there exists binary stars systems tidally locked wich start close enough to each other to have reciprocal atmosphere exchange.

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    $\begingroup$ I've made a few edits to your question to improve clarity. If you disagree, feel free to roll them back to the original. $\endgroup$
    – Frostfyre
    Commented Jun 11, 2015 at 13:00
  • $\begingroup$ I totally agree with your changes, you seemed to read into my mind ;) thanks! $\endgroup$ Commented Jun 11, 2015 at 13:03
  • $\begingroup$ 20 km is within the earths athmosphere. If one of the planetoids where earth this would make this clearly impossible, since the other planet would orbit within the athmosphere and would experience drag. $\endgroup$
    – Taemyr
    Commented Jun 11, 2015 at 13:30
  • $\begingroup$ Since tidally locked means "showing same face" there would be no air drag (there will be the same drag on both planets they would have if they were 20 milion kilometers apart). Anyway air drag is not in the given constraints, the purpose of the question is to determine if such gravitational/cetrifugal equilibrium can exists, dropping all other factors such as real possibility to such condition happens, air/tidal drag, magnetic fields etc.). When we see that is possible we could examine other factors in another question $\endgroup$ Commented Jun 11, 2015 at 13:36
  • $\begingroup$ The question is not duplicate, it has "hard-science" tag wich requires to show why this is not possible (so taking 2 pair of planets, one pair has 2G and 0.5G and the other one has 1G and 1G gravities, I need an approximation of Roche Limit distance from each planet considering the other in the pair) $\endgroup$ Commented Jun 12, 2015 at 12:37

2 Answers 2


The problem would be that they would be well inside the Roche limit. And would need to be well inside the Roche limit for a very long time for the tidal locking to occur.

Tidal locking actually takes a very very long time to happen. The process it is thought to happen through is explained quite well in this wikipedia entry.

That's with out taking into account the atmospheric drag of the second planet being inside the atmosphere of the first causing it to slow its orbit and then smash together reasonably quickly.

To pull this off you would need some form of magic involved.

As another small side, though the moon's orbital period and rotation are tidally locked we don't always see exactly the same chunk of it. It moves slightly and we get to see about 9% 1 more surface area than we would if it was completely locked to us. Unless the two planets were exactly locked together you would not be able to build a building across the gap as it would crack if there was even the smallest movement.

  • $\begingroup$ Thanks' that interesting, I removed "buildings" (I think it is possible to make some rope elastic bulding that won't break) and left aircrafts $\endgroup$ Commented Jun 12, 2015 at 12:46
  • $\begingroup$ @DarioOO - please read the duplicate question and answers. The satellite will be a band of rubble, $\endgroup$ Commented Jun 12, 2015 at 12:51
  • $\begingroup$ Already readed those answers ^^ It gives no numeric limit, it just says that limit exists without saying how far it will be in example if it is 10 kilometers then 20 kilometers would be enough to "climb on other planet".. of course I would use the simplified formula for rigid bodies wich is as much as twice more permissive than real formula.. Currently for the Earth-Moon pair the limit is 10.000 kilometers and knowing earth is 6000 kilometers in radius we get only 4000 free kilometers, so it is likely with some particular density we get very near (4000 km are not very much in astronomy).. $\endgroup$ Commented Jun 12, 2015 at 13:08
  • $\begingroup$ The Roche Limit depends on the mass, size and density of the two objects. See: en.wikipedia.org/wiki/Roche_limit#Determining_the_Roche_limit for some maths of how to work it out for two bodies, or look at the table near the end for stuff in the solar system. $\endgroup$ Commented Jun 12, 2015 at 14:09
  • $\begingroup$ Another problem would be the surface of the planet would have to be below orbital velocity, whilst the satellite has to be (by definition) above orbital velocity. This would mean the planet would be nearly spinning itself apart. $\endgroup$
    – Aron
    Commented May 29, 2016 at 15:06

I think there are Many problems when imagining this construction.

I would look into how this works for:

  • Pluto and Charon
  • Twin neutron stars
  • Jupiter and its effect on its moon Io

In short, I think tidal locking requires either a huge distance, or an enormous speed (with all the consequences that come with it).

Perhaps when you don't imagine rocky planets but rather gas planets (large & light = surfaces are close together), this might work. And without a surface the people on it would live in floating cities.


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