Recently I watched a documentary about Io, a moon around Jupiter. Io has volcanic activity even though it is small and can't retain heat well because gravity from Jupiter and a nearby moon cause it to sort of speed up from the opposing gravitational pulls, causing heat, when they are all lined up.

My question is why this same principle doesn't cause satellites orbiting Earth to heat up when they line up with the Earth and the Moon; or why the Moon doesn't heat up when it lines up with the Earth and the Sun; or why the Earth doesn't heat up when it lines up with the Sun and other planets, etc.

Is it because the scale factor of the distances and sizes are different? If so what scale factors would be needed to produce this affect?

Example(Obviously not to scale):


1 Answer 1


The simple answer: Satellites do feel this force, but obviously don't get ripped apart. The tidal forces are simply too small (for the satellites' materials) to actually rip them apart.

The Why: Tidal forces happen because one side of an object feels such a larger huge difference in force than the other side. The magnitude of the force not only has to deal with the size of things pulling on each other, but the distance. Even massive things (like the sun or jupiter) have relatively little pull when very far away. If you get close, you feel the effects of them much more strongly and the effects increase more quickly!

Due to the fact that Io is a moon very close to other large bodies, this makes the difference of the force of gravity on one side is VERY different than that on the other. (Try plugging in correct values for Io, Jupiter, and the distance between them; then try calculating the force of jupiter on Io as seen from one side of Io to the other.)

Most man-made satellites around earth are much, much smaller than moons and they are surrounded by very distant or very small things. This makes the difference between the force of gravity on one side of the satellite is almost the same as the force of gravity on the other.

This being said, if a man-made satellite were to be in the same situation as Io, either being very large or being very close to other big things, it could get ripped to shreds.

In short: It's all about what forces are being applied. Distant objects apply small forces, close objects apply bigger forces. Tidal forces happen when gravity changes wildly between your head and your feet.

  • $\begingroup$ Your explanation seems to imply the other moon's gravitational pull has little effect on Io and the heat is caused more by it's proximity to Jupiter. The documentary said that the speeding up(causing the heat) only happened when Jupiter, Io, and another certain moon lined up. Also I think the documentary implied that the heat wasn't caused so much as by the opposing forces ripping Io different ways but by the gravitational pulls giving Io a speed boost, which caused Io to heat up from the friction from within itself(so in a static example it would act differently.) $\endgroup$
    – Sam Sabin
    Apr 15, 2014 at 22:55
  • $\begingroup$ Sorry, I think my title was a bit misleading. $\endgroup$
    – Sam Sabin
    Apr 15, 2014 at 22:58
  • $\begingroup$ Ah, I didn't mean to imply that heating was from Jupiter alone, nor do I mean to imply that Jupiter does not exert tidal forces. The other moons exert forces on Io, too. It may be that these effects are maximized during different times, because of the positions of all the moons. $\endgroup$
    – PipperChip
    Apr 15, 2014 at 23:24
  • $\begingroup$ But I think the heat has something to do with speeding up and causing friction, not just from being pulled different ways. So if Jupiter was lined up with Io and another moon but all the globes were stuck in one place and not moving I think the heat on Io would be significantly less. $\endgroup$
    – Sam Sabin
    Apr 16, 2014 at 21:43
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    $\begingroup$ It's a little complicated. The heating of Io is caused by the fact that Io's orbit is not perfectly circular. As the distance to Jupiter varies, the gravitational force between the side away from Jupiter and the side facing Jupiter changes. This causes the shape of Io to change, and the flexing causes heat. In the long run, this ought to cause the orbit to become perfectly circular, so no more flexing. But Europa and Ganymede are in a particular set of orbits that keep distorting Io's orbit. And "the globes stuck in one place" can't happen as long as the orbits have different diameters. $\endgroup$ Apr 17, 2014 at 3:03

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