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I have one idea for a planet which may turn out to be a massively reengineered and terraformed Mars, which would have double its mass to be more Earth-like, an installed magnetosphere, etc. It would have a massive endoskeleton which expands the surface (with trenches everywhere of course) to an Earth-like volume as well. (Let's assume the megascale engineers delivered the extra mass from the asteroid belt.)

How would Mars' orbit be affected by such a project? I like to imagine the engineers and settlers of this reformed Mars would've also been able to synchronize the planet's days with that of Earth's, but would Mars fall closer towards the sun from the extra mass?

Also, would it even be necessary to engineer Mars on such a massive scale? Practically aside, is it important for Mars to have the same mass and size of Earth for long-term human colonization, or could humans live pretty much the same long-term (without health problems or morphological changes after hundreds of generations)?

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  • $\begingroup$ A planet's orbit does not really depend on its mass. It does a little bit, but the amount is so small it's negligible. $\endgroup$ – Phiteros Oct 22 '17 at 20:37
  • $\begingroup$ Depends 100% on how that mass is delivered. If you can deliver the mass you can also deliver the kinetic energy to change orbit. $\endgroup$ – Peter - Unban Robert Harvey Oct 22 '17 at 21:47
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The effect on orbit would be negligible. The relevant factors for the orbit are the combined mass of the Sun and Mars and the ratio of the masses of the Sun and Mars. These would change slightly so the orbital period would be decreased, the year would be shorter, but stars are so much more massive than planets that the effect is negligible and not really worth calculating. Especially since the process required to add the extra mass would have had a bigger, and not predictable by us, effect on the orbit. Basically it depends on how the engineers dropped all that extra mass to Mars.

It probably is not necessary to make Mars same mass and size. Both are linked to two relevant factors, surface gravity and ability to retain atmosphere, but it is better to focus on the directly important things and ignore the details. At least for fiction.

Increasing surface gravity and giving Mars an atmosphere would be separate questions (that probably have been asked?) IMHO.

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  • $\begingroup$ Great answer! Thanks. Let's say there was a steady stream of mass from the asteroids being delivered to Mars over a long period of time, so Mars is very gradually gaining in mass. $\endgroup$ – Jason Perry Oct 22 '17 at 22:38
  • $\begingroup$ Mars needs to maintain its orbit so the stream of asteroids will have to be balanced to avoid acceleration or deceleration $\endgroup$ – Slarty Oct 22 '17 at 23:05

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