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Was reading Which planet or moon would be most feasible to terraform? and specially https://space.stackexchange.com/a/206/12379 . I am not really good at science or maths so please excuse below and look at the figures and ideas shared as a simple person's analogy to what he thinks.

While I don't know if we have technology to add mass to a planet. Let's say we decide to go for it so to have somewhat like Earth's gravity (from 1/5th say it becomes 1/3rd or better due to doubling size of Mars as well as having more real-estate for Mars colonists) while also presuming or assuming that such technology is not expensive to sustain (which probably is/would be wrong) wouldn't it change the rotational planet of Mars itself vis-a-vis in relation to other planets due to its attraction and repulsion by it, thereby also changing rotations of other planets, which will have unknown ripple effects on the solar system as a whole (at least the nine planets that are there ?), maybe also changing/risking Earth's atmosphere in the process for the worse?

This is a what-if fantasy idea probably might have tried in some sci-fi B-Movie.

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    $\begingroup$ There is no technology existing or even reliably hypothesized to be able to add mass other than literally taking that mass and moving it over to Mars. $\endgroup$ – Nathan Tuggy Nov 16 '15 at 20:43
  • $\begingroup$ Terraforming Mars does not necessitate adding mass! Also, are you presuming you have a magical technology that actually creates mass out of nothing -- violating conservation of mass? Or is your technology converting energy to mass? What I am getting at is: where does the mass come from? $\endgroup$ – Brian Lynch Nov 16 '15 at 20:45
  • $\begingroup$ You are getting ahead of yourself - it is really hard to ask speculative questions like this without knowing more about the background science. Planets affect each other's orbits very little because they are so far apart, and there is no repulsion. If we could move around that incredibly huge amount of mass, we'd probably build huge space colonies anyhow. Maybe look at O'Niell Cylinders. $\endgroup$ – kim holder Nov 16 '15 at 23:58
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There are issues with trying to added mass to Mars, or any other celestial object.

Sourcing the material to add to Mars is the main problem. Getting the material to Mars will be the next problem and financing the operation will be another major problem.

Ignoring those problems, material would have to be added uniformly to the surface. A high concentration of very dense material in one location may introduce a very slight wobble in the spin of Mars, not its orbit.

Considering the time frames you are assuming, any material added to it the would most likely resemble unconsolidated sand or rubble on the surface of Mars and would not necessarily increase the density of the planet due to the density of the material used and the void spaces between the particles of sand/rubble.

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