Are there cases or conditions in which an object orbiting another one (e.g. a satellite and its planet) does not get tidally locked to it?

For example, in this question, it is mentioned that a large moon orbiting a planet could prevent it getting tidally locked both to its star and its moon? If there are situations where a planet getting tidally locked to its star can be prevented, can this happen in planets with multiple moons?

  • $\begingroup$ It's probably not to difficult to set up a three body system in which the satellite would inevitably escape from the planet and go into orbit around the sun before it could get tidally locked. $\endgroup$
    – James K
    Jul 23 at 16:33
  • $\begingroup$ Hyperion has entered the chat. $\endgroup$
    – notovny
    Jul 25 at 12:30
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    $\begingroup$ How is this question different from the one referenced? $\endgroup$
    – eshaya
    Jul 25 at 16:16

3 Answers 3


In short, the answer is: no.

On some occasions, the solar torque will overpower the lunar torque, and will synchronise the planet with the star. Be mindful though that, for the star to win, it is NOT sufficient for the peak value of the solar torque to supersede the peak value of the lunar torque. Owing to the frequency-dependence of the planet's $k_2/Q$, the situation is more complex, see this discussion.

On other occasions, even a very massive moon may fail to synchronise the planet's rotation because the moon may leave the reduced Hill sphere (when synchronisation is carried out my a tidally receding moon) or fall through the Roche limit (when the synchronisation is performed by a tidally descending moon).

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    $\begingroup$ Nice answer. It also may bear mentioning that tiny moons at distant orbits have enormous times to lock to small planets at the edge of a stellar system. Also, Pluto will never lock to our Sun since it is locked to Charon. $\endgroup$
    – Connor Garcia
    Jul 24 at 0:31
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    $\begingroup$ Are we considering Mercury to be tidally locked? It's locked in a 2:3 resonance but it isn't "always one face to the sun" which is what we usually mean by tidal lock. It can't get out of that resonance because while the 1:1 lock would be a lower energy state, it would require adding energy to get out of the current resonance. I don't know if there's any planet/moon systems that have a similar resonance relationship but it's at least theoretically possible. $\endgroup$ Jul 24 at 20:24
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    $\begingroup$ @DarthPseudonym The term "tidal locking" is oftentimes used as a synonym to tidal synchronism ( = 1:1 spin-orbit resonance). In reality, the meaning of tidal locking is broader. It includes other spin-orbit resonances (3:2, in the case of Mercury). $\endgroup$ Jul 24 at 23:00
  • $\begingroup$ @DarthPseudonym The fact that the energy of rotation in a 3:2 spin-orbit resonance is higher than that in a 1:1 resonance does not warrant that the rotator must eventually leave the higher resonance and despin towards synchronism. The capture of a rotator in one or another spin state is defined by the frequency-dependence of the tidal torque. The torque due to the permanent triaxiality of the body also participates in the process -- but this is a higher level of complexity. For the theory of tidal capture see this work. $\endgroup$ Jul 24 at 23:35
  • $\begingroup$ @DarthPseudonym You have pointed very rightly that it would require adding energy to get out of a higher spin state. In higher spin states, the tidal bulge is running over the circumference of the body, making tidal heating much more intense than under synchronism. For very close planets, this may result in thermal runaways. This study demonstrates that in these situations overheating leads to a change in rheology, which in its turn enables the body to slip out of the higher spin state, and to evolve towards synchronism. $\endgroup$ Jul 24 at 23:41

Yes, at least for a 2-body system, all gravitationally interacting bodies will eventually stop rotating, given enough time. This applies even to perfect spheres. This is because the finite speed of gravity (which is equal to the speed of light c in the theory of Relativity) leads to a retarded interaction, which makes the receding part of the rotating body appear to have a higher density, and the approaching part of the body a lower density as illustrated in the below diagram

retarded density of rotating object

This figure shows the density of an initially (when at rest) uniform distribution when it rotates counter-clockwise with a speed of $0.5 c$ as it appears for an observer on the positive x-axis at x=3. A mass at this point would therefore experience a stronger gravitational attraction to the upper half than the lower half, therefore slowing the rotation down.

The same effect due to the presence of other satellites will be a) much smaller due to their much smaller mass (and thus gravitional force) compared to that of the planet, and b) it will be periodical. So they will only lead to a small wobble around the mean locked position. After all, the Earth's Moon is not perfectly locked either but shows a small periodical wobble due to disturbances by other bodies.

For more details regarding the retardation effect see my paper "Computational Visualization of Retardation Effects on Observed Particle Distributions" published in European Journal of Physics (you can download a full text version here ).

  • $\begingroup$ but as the other answer and its comments show, there are systems where tidally locking can be avoided @Thomas $\endgroup$
    – vengaq
    Jul 24 at 23:34
  • $\begingroup$ @vengaq The other answers did not take the retardation efffect into account I mentioned. This will eventually stop any rotation. and thus lock the satellite and eventually also the planet. Due to their much smaller mass compared to the planet, other satellites could only cause a small periodical wobble around the locked position (similar to the libration of the Moon). $\endgroup$
    – Thomas
    Jul 25 at 21:44

A moon in an elliptical orbit may come into a tidal resonance with its planet instead of a full tidal lock. Or, a moon may not lock to its planet if it is in resonance with another moon. In addition tidal influences with the spin of the planet or with other moons in the system can cause a moon to be pushed out of the system, tidal acceleration, or pulled into the planet,tidal deceleration, before it ever has a chance to fully lock.

An alternative to tidal locking is orbital resonance, which occurs when celestial bodies exert periodic gravitational influence on each other, with their orbital periods related by a ratio of small integers. This phenomenon can be considered a form of tidal locking. A classic example of orbital resonance is seen in Jupiter's Galilean moons, where the orbital periods of Ganymede, Europa, and Io are in a stable 1:2:4 resonance configuration, effectively countering the planet-moon tidal effects. Callisto will someday also come into resonance.

Another type of resonance is spin-orbit resonance, where the spin periods of moons are multiples of orbit period multiples.

Over extremely long timescales, on the order of hundreds of billions of years, the Sun's gravitational influence on Jupiter (raising a tidal bulge locked onto the Sun) will inevitably lead to energy dissipation through friction, gradually slowing down Jupiter's rotation until it becomes nearly locked to the Sun, completing one rotation in its solar year.

If a planet's spin were already tidally locked to the orbital period of a large moon, the interaction of the Sun's tidal forces and interacting of tidal bulges with winds in the atmosphere over an immensely long timescale would lead to the gradual removal of orbital energy from the moon's orbit. As a consequence, the moon's orbit would slowly decay (while spinning up the planet and maintaining the tidal lock), and it would eventually fall into the planet. It would then be free to start locking to its host star.

As for the moons of Jupiter, due to its rapid rotation, the gravitational drag from the bulges raised on Jupiter will cause the moons to slowly spiral outward, gradually increasing their orbital periods, likely maintaining their orbital period ratios as the periods lengthen. Eventually, they may escape the system altogether. If the moons have not been lost by the time Jupiter becomes nearly locked to the Sun, they will be pulled inward until they are either torn apart or burn up in Jupiter's atmosphere.

This generalizes to any planet with moons orbiting a star.

However, after ONLY 30 billion years or so, the entire solar system will probably have disintegrated from the disturbances of passing stars. See, “The Great Inequality and the Dynamical Disintegration of the Outer Solar System,” Jon K. Zink et al 2020 AJ 160 232.

On the longest time frame, gravity waves emitted by orbiting objects will remove their energy causing them to spiral in; moons into planets, planets into stars, stars into black holes at the center of galaxies, clusters and groups into their own centers.


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