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The scenario

Many science fiction novels are concerned with aliens visiting Earth, but what if aliens have no interest in Earth at all and yet they arrive at the Solar system for the sole purpose of refueling by harvesting as much hydrocarbons as possible from Saturn's moon Titan (e.g. methane, ethane, etc. stored in polar lakes and other hydrocarbons in the dust on the surface or perhaps even underground).

The question

What would be the impact of such harvest on Titan itself, the Saturn system and the entire Solar system?

Is the moon's orbit going to be changed significantly enough due to its mass reduction so that it collides with other moons or perhaps it even falls down to the planet?

What is a reasonable timeline for the impact to become apparent?

Additional info

I'm developing an idea for a science fiction novel which is meant to show that just as we humans destroy entire ecosystems with complete disregard for other lifeforms due to mining operations, an alien civilization could do the same to us by simply using the Solar system as a gas station on the way.

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Is the moon's orbit going to be changed significantly enough due to its mass reduction

Mass reduction will have zero effect on the moon's (Titan's) orbit. Mass of the moon is not part of the orbital calculations (mass of the planet is). It is possible that the method of removing mass might shift the orbit, but since you don't give the method, we can't really describe that.

It will affect other orbits, but mostly by affecting them less. Currently Titan distorts other orbits with its mass. Removing mass will decrease this effect, but it's not that big an effect in the first place. This will not cause observable changes in the time scale of a typical story. It might cause noticeable changes if your story is lasting millions or billions of years.

Changes to Titan's mass would have minimal effects on the rest of the solar system.

If your goal is to exhibit changes, consider having the aliens harvesting something from Titan's core rather than its atmosphere and seas. Chunks of Titan could become asteroids and comets, flying all over the solar system and bumping into things. Or have them pull Titan out of its orbit while they extract what they need, letting it go when done. Either could give you a Deep Impact or Armageddon style story (asteroid impact with Earth).


Newton's law of universal gravitation says that the force with which two objects are pulled together is proportional to the product of their masses. However, that force is not part of orbital mechanics. What matters in terms of orbits is the acceleration.

Essentially what happens is the mass of the satellite is divided out of the force equation to give just the acceleration. It's combining two equations:

$$F=G\frac{m_1m_2}{r^2}$$ $$F=ma$$

to get

$$g=a=G\frac{m_p}{r^2}$$

Where

  • $g$ or $a$ is the acceleration due to gravity.
  • $G$ is the gravitational constant.
  • $m$ and $m_1$ both represent the mass of the satellite/moon.
  • $m_2$ and $m_p$ represent the mass of the planet.
  • $r$ is the orbital radius, the distance between the centers of mass of the two objects.

An orbit exists when the acceleration due to gravity balances the inertia of the tangential velocity in such a way as to maintain the tangential velocity's magnitude even while changing its direction. Neither the acceleration nor the velocity is dependent on the mass of the satellite.

The mass of the satellite does affect how much the planet revolves around the satellite. But in a typical planet/satellite relationship, the masses are so different that we ignore the effect of the planet revolving around the satellite. Reducing the mass of the satellite essentially makes the planet wobble a little less.

There is a slight effect on the satellite's orbit from its mass. When the satellite moves the planet, it changes the orbital radius. That in turn changes the acceleration equation. But the change will generally cancel out over the course of an orbit.

If Titan were larger, increasing the mass might pull Saturn out of its orbit around the Sun. But decreasing the already small mass is unlikely to have a large effect on Saturn. Even at the worst possible time, it's unlikely to be a big deal. I'd actually be more worried that the alien spacecraft, which is apparently huge (large enough to carry off a substantial portion of Titan's mass), would disrupt Saturn's orbit.

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  • $\begingroup$ Thanks for the rapid answer. I'm unfamiliar with orbital mechanics. I thought the mass reduction would have an effect due to Newton's law of universal gravitation. Guess I'm wrong though. $\endgroup$ – toncha Oct 23 '18 at 4:09
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    $\begingroup$ Nitpick: The orbit expands slightly. It's not the mass of the planet that determines the orbit, but the mass of the system--planet + moon. Since the moon is a tiny fraction of the planet's mass we normally ignore it, but it is part of the equation. $\endgroup$ – Loren Pechtel Oct 23 '18 at 7:30
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    $\begingroup$ Thank you very much @Brythan for the clarification. This additional information makes it totally clear to me now. I really appreciate the equations. And I find it very helpful that you also provide little hints (e.g. harvest method and location, spaceship mass) as to how the story might develop given that the scenario described by me wont work. $\endgroup$ – toncha Oct 24 '18 at 13:59
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Short answer is not much.

For a start, a planet (or moon)'s atmosphere is only a small percentage of its mass. Titan's Mass is substantial, around 1.4x 1023 Kg. I'm not sure how massive the atmosphere of Titan is, but it's a lot less than that.

On top of that, most of that atmosphere is Nitrogen. Only around 1.5% of the atmosphere is hydrocarbons or methane, meaning that you don't even get a large difference in the atmosphere if you remove it, let alone in the mass of the moon itself.

In point of fact, they may be there right now doing it (hence the low hydrocarbon count) and we'd be very unlikely to detect them unless they have a great radio station broadcasting for their workers.

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    $\begingroup$ That final sentence... Why am i picturing the opening scene of Independence Day, with REM's end of the world as we know it playing on the radio and waking up the techie to listen into transmissions coming from afar... $\endgroup$ – Blade Wraith Oct 23 '18 at 7:00

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