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Our Solar system poses a significant problem for colonisation insofar that the rocky planets are predominantly on the... small scale; at least as far as mass is concerned (Earth is the exception rather than the rule). However, there are many rocky planets, dwarf planets, moons, and asteroids scattered throughout the system. Excluding the severe logistical difficulties in generating such giant impact events in the first place, are there theoretical means by which the masses of different bodies can be combined, rather than their collisions simply shearing vast amounts of debris off the larger body?

Back in the early stages of the Solar system when the planets where hot and less solid, such combinations seem to have been a real possibility. Is it still hypothetically possible in a mature system?

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  • $\begingroup$ "Settling" is a better word than "colonization". The latter is politically and historically loaded and deters most non-space enthusiasts who get very different associations than we space nuts do when we hear the term. We don't expect to find any poor hottentots out there to oppress, so we should not talk as if we did. $\endgroup$ – LocalFluff Aug 5 '15 at 10:04
  • $\begingroup$ You'd have to achieve very gentle touchdown. For large primary bodies, you'd be thrusting against their gravity, expending a continentload of propellant. Not really practical. $\endgroup$ – Deer Hunter Aug 5 '15 at 10:24
  • $\begingroup$ @DeerHunter If the asteroid to be relatively soft landed, is icy, which I think is what is important to terraforming, and if the propulsion system placed on it uses that local water ice as fuel, wouldn't much of the exhaust also land on the target planet in the end, since it is directed towards its gravity well? Given the fuel factory and engine, which might lift off in the last minute to be reusable, there's maybe not much to lose but time? $\endgroup$ – LocalFluff Aug 5 '15 at 10:45
  • $\begingroup$ " Excluding the severe logistical difficulties in generating such giant impact events in the first place" Wouldn't there be a significant overlap in solutions? If you can generate the energy needed to move entire planets orbits, and apply it in a controlled way, what difficulty is a few debris? $\endgroup$ – NPSF3000 Aug 5 '15 at 10:56
  • $\begingroup$ It's worth noting that at impact velocities measured in km/s solid/liquid ceases to matter. It's the principle of "water is has hard as concrete when falling at terminal velocity" - which isn't quite true, but the point is that liquids can't move out of the way fast enough to matter at those velocities so liquids and solids end up acting more or less identically. $\endgroup$ – Blake Walsh Aug 5 '15 at 11:29
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Absolutely, in fact, many of these objects have been discovered. This is what is known as a "Contact Binary". It happens when two objects collide very slowly. In fact, the comet that Rosetta is orbiting, 67P, is in fact believed to be such an object. It is expected that 10-15% of all larger (200m+) asteroids will be contact binaries. Wikipedia has a list of all known contact binaries.

enter image description here

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  • $\begingroup$ I cannot believe how difficult it was to google an image of a Nimbus. Don't people know about this African God-thing of wealth and luck and whatever else is good? $\endgroup$ – LocalFluff Aug 8 '15 at 14:16
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If I'm reading your question right, this is a crazy idea and I love crazy ideas.

Our Solar system poses a significant problem for colonisation insofar that the rocky planets are predominantly on the... small scale; at least as far as mass is concerned (Earth is the exception rather than the rule).

Size is important, cause you want a magnetic field, requires a solid/liquid inner core (less likely if the planet is too small), you want the planet to retain it's atmosphere (again, size helps - gravity being stronger), and you probably want plate tectonics (heat in the core is important there too) and you need the right temperature, but presumably if we can move enough mass to build a 2nd earth, we can move that mass as well.

However, there are many rocky planets, dwarf planets, moons, and asteroids scattered throughout the system. Excluding the severe logistical difficulties in generating such giant impact events in the first place, are there theoretical means by which the masses of different bodies can be combined, rather than their collisions simply shearing vast amounts of debris off the larger body?

When you're talking about moving millions, billions and even trillions of tons, on objects that orbit the sun at many thousands of miles per hour, that's no easy trick, add to that, most of these objects are very far apart, so, as you say, logistical difficulties do exist.

By creating an atmosphere around the object, that will buffer to some extent any debris fly off. Glancing blows vs direct hits can help too and lower velocity impact, such as starting from mostly stable Trojan points might help.

Back in the early stages of the Solar system when the planets where hot and less solid, such combinations seem to have been a real possibility. Is it still hypothetically possible in a mature system?

Another problem that perhaps you haven't though of is the mass of all the stuff out there might not be as much as you think.

The total mass of the entire asteroid belt, including dwarf planet Ceres is about 4% of the mass of our moon. (Source). The mass of Jupiter's Trojans might be around that much as well (I read that somewhere).

The mass of all Kuiper belt objects might be significantly larger than that, but those objects are more icy, less rocky, not as good for building a rocky planet and enormously spread out.

Even the mass of all moons wouldn't be enough. Ganymede is larger than Mercury, but less than half it's mass, all the moons together would give you a bit more than the mass of Mars, and many of the larger moons are far out in the solar system and have more water, less dense elements. Pull too many moons together and you'll have a planet covered with oceans, which might not be all bad, but it's probobly not what you're going for.

enter image description here

Source: https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size

So, if you really want to build another Earth like planet with objects in our solar system, you'd probobly have to combine Venus, Mars and Mercury (and maybe Europa for water). Those 3 planets combined would have a mass almost equal to the Earth. There's probobly enough Iron in there to create the desired Iron core, once the planet after the heat of smashing together cooled enough to have a solid metal center and a liquid outer metal core. Mars has a lower Iron ratio but Mercury has a lot of Iron.

Once they smash into each other, the newly formed planet would need a long time to cool, likely tens of millions of years or so. But you might be able to build an earth that way. I don't believe you can build an earth out of dwarf planets, moons and asteroids though. There's simply not enough material.

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  • $\begingroup$ Every planet is more massive than the sum of masses of all smaller planets. I doubt a "world builder" would have the imagination to create something like that. But here we are. $\endgroup$ – LocalFluff Aug 8 '15 at 13:51
  • $\begingroup$ That's interesting. I'd never thought of it like that before but that's true. It would also take an insane amount of energy to move Venus out into Mars' orbit, but if you had 10 or 20 million years to do it, it would have a small chance of working and creating a 2nd earth. $\endgroup$ – userLTK Aug 8 '15 at 15:40

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