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It seems to me, that if we want to make Mars as Earthlike as possible, we should increase its mass. The long term affects of lower gravity environments on humans is not fully known, but we have enough information to assume we would have lower bone density on Mars and smaller, weaker muscles (including our hearts). In order to create a place where Earth life and Martian life could be exchanged back and forth, again, it seems that we need to increase the mass of Mars.

If we redirected enough asteroids to collide with Mars and increase its mass, could that affect the orbit and rotation of the planet itself? Could it endanger the orbits of the rest of the solar system?

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    $\begingroup$ Are you asking whether this is possible, or whether it's an inevitable result of the given course of action (which ridiculously difficult, by the way)? $\endgroup$
    – Nathan Tuggy
    Jun 30, 2016 at 1:15
  • $\begingroup$ Maybe not mass itself, but many of the asteroids would contain a lot of water and other gases. A few scattered over the surface might go a long ways to returning water to the surface of Mars, plus some green-house gases. Longer-term the question is how to retain some kind of atmosphere. $\endgroup$
    – Paul
    Oct 20, 2017 at 18:55

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Planets are much, much bigger than you think.

The total mass of all the bodies in the main asteroid belt is about half of 1% of Mars's present mass. There's no realistic way to round up enough loose matter to appreciably increase the mass of a planet.

38% of Earth's gravity is pretty substantial. Humans have managed to survive a year in microgravity without severe long term ill effects. It would not surprise me to learn that 38% g is physiologically more similar to 100% than to 0%. Mars-born hearts won't develop the same strength as Earth-born, but they may not need to.

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  • $\begingroup$ I was thinking about this also, you wouldn't need just rock you'd also need water so what about the Kuiper Belt beyond Pluto it is said to have hundreds of thousands of icy bodies larger than 100 km across and an estimated trillion or more comets (the comets would contain loads of water) some scientists believe it could contain up to 30 Earth masses. So it seems to me their would be plenty of mass & water out their to turn mars into a 2nd earth but it would also need a much larger moon to protect in from jupiters gravitational forces like the earths moon does for the earth. $\endgroup$
    – Harrison evans
    Nov 30, 2016 at 21:43
  • $\begingroup$ According to Wikipedia, the total mass of the current Kuiper belt is 1/25 to 1/10 of the mass of Earth; models of the solar system's formation suggest there was more mass at one time. The collection and redirection of an Earth mass worth of KBOs would be a significant engineering challenge. $\endgroup$
    – Russell Borogove
    Dec 1, 2016 at 0:13
  • $\begingroup$ Just plant a massive fusion drive at the core of Ganymede, hollow out a shaft and use the internal mass as propellant, creating a nuclear rocket. Execute a soft orbital capture at Mars, where, given their like masses, the two bodies become a binary orbiting the common COG. Then just use a mass driver to deplete Ganymede down to the surface of Mars over a generation or two. You'd end up with a body about .8 the mass of Earth. Easy peasy :) $\endgroup$
    – Chris B. Behrens
    Feb 11, 2019 at 20:22
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As Russell Borogove pointed out, that plan is quite impossible because you need to get that mass from somewhere, and all the other mass is already concentrated on even larger planets. And even when we would have enough asteroids, the plan would still not be even remotely feasible due to the huge energy requirements it would take to change the orbits of such huge amounts of mass.

But let's assume that this plan would be possible and examine it theoretically.

When there is a plastic collision (two bodies crash into each other and turn into one), not only their masses combine, but also their momentum. Depending on how they hit, this will result in a rotation and velocity change. So yes, every asteroid impact changes the planet's speed (and thus its orbit) and its rotation.

However, when you plan the asteroid strikes properly, you can plan them in a way that they cancel each other out and the resulting body has the same orbit and rotation as the original one.

Also, another question remains: Is the resulting body still Mars? To get a surface gravity of Earth, you would have to increase the mass by factor ten. So just 10% of the mass would be original Mars material, the rest from the asteroids you scooped up. So the composition of the new planet's soil and atmosphere would depend far more on your choice of asteroids than on the original properties of Mars. All the asteroid impacts would also generate a huge amount of heat, so you would end up with a hot, geologically unstable mess of a proto-planet, which will take millions of years to cool down.

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The mass of Mars is 6.419 * 10E23 kg, the mass of the largest object in the asteroid belt, the dwarf planet Ceres is 9.393 * 10E20 kg. Adding Ceres to Mars would result in 6.4284 * 10E23 kg, a very small change, only from 100 to 100.146%. But moving Ceres to Mars using our present propulsion methods would require a lot more mass of propellants than the mass of Ceres itself. The idea of increasing the mass of Mars is not realizable using our present level of science and technology.

The change of gravity would be measurable using very sensitive gravimeters, but not noticeable by astronauts.

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