# Make a cracked planet whole again

If a flourishing planet was cracked into several parts by some external force like a weapon of mass destruction, would it be possible to put it back together again? What possible ways can be used to make the planet whole again?

So to add, the parts of the planet didn't get far away from each other but did separate from each other. There's no more core and the center is only empty space. The parts didn't collide with each other and are floating away very slowly.

• How did the planet crack? Do the cracks go right through to the core? If so are the separate pieces moving with any outward velocity? Commented Oct 14, 2017 at 8:41
• Gravity will pull together planet chunks quite sharply, so either "mass destruction" imparted enough energy to have pieces (at least) orbit or they will quite rapidly bump against each other and coalesce into very few masses. Please state starting conditions. Commented Oct 14, 2017 at 8:42
• Lots of duck tape? Commented Oct 14, 2017 at 10:41
• In your planetary slicing and dicing thought experiments don’t think of a planet as a giant cake that can be sliced up. Think of it more like a very big floating ball of water (not a perfect analogy but it’s far better than the cake example). You can imagine how difficult it would be to cut up, because a knife passing through it would not easily divide it into two and if you did manage to split it, surface tension would squash the two hemispheres into two smaller spheres. (surface tension playing the role of gravity) Commented Oct 14, 2017 at 19:24
• "There's no more core" sounds like handwavium to me. You've just left any reality behind. Commented Oct 14, 2017 at 23:52

gravity will do the trick of putting the pieces together. It is believed something similar already happened when a body of about the size of Mars hit the proto-Earth, generating the Moon.

Or, even further back, that's how planets got formed.

• But will gravity still exist there? Commented Oct 14, 2017 at 8:48
• Yes gravity will still exist. All of the pieces will exert a gravitational attraction on each other. Commented Oct 14, 2017 at 10:39
• @SovereignSun not only will gravity exist there, but assuming no bits of the planet reached escape velocity it’s inevitable that they’ll reform. Might be pretty cataclysmic though, but I figure your planet cracker covered that already. Commented Oct 14, 2017 at 12:20
• Whatever cracked the planet will probably leave it hot. Or pieces will definitely get hot when they collide with each other again. The heating aspect produced by both the cracking and the recolliding will leave this planet a very different place. Your cabana might not be as nice as it was. Commented Oct 14, 2017 at 13:07
• Also look at the Caloris basin on Mercury. Major impact, you can still see the damage to the crust but no cracks leading deep into the world. Commented Oct 15, 2017 at 6:09

Keeping the planet together and fussing it together are two different tricks.

One of the moons of Mars, I believe, is just a collection of rocks held together by gravity.

If just surface chunks are hewed off, like they were chipped, they would just form another tectonic plate, I can imagine.

However, if the planet actually split, you have the problem of what to do with the molten core (if it had one - some planets are solid). Unconstrained by the crust, things would be a mess. If this molten core stayed around, it could re-solidify on the crust and potentially fuse the parts back together. If, as you stated, it dispersed and the core became hollow, there would be a lot less gravity, and certainly the center of gravity would change with the changing profile. If it re-solidified in clumps the geometry of the planet is certainly altered, and so would its spin, and potentially the length of days and years would change.

But there would be tremendous instability in the ground until this happened, earthquakes, volcanoes, shifting parts until they found equilibrium, not to mention extreme climate instability and maybe loss of atmosphere, a completely altered magnetic field, changes in the ionosphere, and potential loss of oceans, if your planet had any of these. I would not expect prospects of survival would be high except for cockroaches. Cockroaches can survive anything..

• Other than naturally would it be possible to do it using technology and science? I mean quicker. Commented Oct 14, 2017 at 17:13
• With handwaving anything is possible. If the technology were very, very advanced, maybe. I suppose an advanced civilization could enclose it in a sphere and use nanobots or similar. There are not many ways to artificially cool down a planet in a short period of time. Commented Oct 14, 2017 at 17:23
• @SovereignSun sure, put rockets on each piece and push them towards each other. Commented Oct 14, 2017 at 21:03

For convenience this answer will assume an Earth mass planet. If it has been cracked into several parts by a weapon of mass destruction and those parts moving slowly away from each other, then this suggests something like the following has happened.

Whatever the weapon was it has carved the planet into sections and moved those sections apart out to a distance roughly that of the planet's former diameter. Normally that those sections would be expected to fall back together under their mutual gravitation. The fact that they have not suggests the sections of the planet are moving at orbital velocity around the centre of mass of the planet. Even if the planet is in pieces its centre of mass and, naturally, its gravity will remain where it was before the sectioning of the planet. The fragments of the planets are moving at an orbital velocity of about 8 km/s. However, with respect to each other they might only have very small velocity differences. Hence, their slowly floating apart.

Digression: the OP suggested the planet's core might have been removed. If this is the case, then what remains of the planet will be less massive than its original condition gravitywise. This also means the orbital velocity of the planet's sections will be correspondingly than 8 km/s. This is something that may need to be factored in later.

This indicates the situation of a planet that has been taken apart, those sections of the planet have been accelerated to orbital velocity and they are all moving around what was the planet's former barycentre (its centre of mass & gravity).

If the technology exists that take a planet apart, then possibly a different application of the technology might put the planet back together again. This might be safest way of doing it.

Otherwise it will be necessary to slow down the pieces of the planet to allow gravity to bring them together. Basically this will involve banging together extremely large lumps of planetary matter.

The deceleration of the orbital velocity could involve using solar or magnetic sails, but this will probably take an extremely long time to do so. Many millions of years, at least. Installing rockets in the sections could be used for their deceleration. Preferably, nuclear rockets of some kind, for example, thermonuclear propulsion. Adding gigantic pusher plates to the most stable parts of the sections and detonating nuclear explosive devices would provide nuclear pulse propulsion on the grandest scale possible.

Whatever propulsion system was used because of the colossal masses involved it will take an extremely long time to achieve anything. The more the powerful the propulsion system used the more probable is the destruction and disruption of the sections of the planet.

The simplest method of bringing together the mass of the planet would be to do destructively. Blasting off large amounts of matter from the sections to collide with the other sections and thereby reduce their overall orbital velocity. If clouds of dust and fragments were orbiting in the opposite direction to the flock of planetary fragments this will gradually decelerate the lot of them. Probably reducing all of the pieces to dust, then gravity would coalesce the dust into a planetary mass again. The original planet will, of course, be totally smashed, but the planet will have reformed.

As was said previously, if the super-scientific technology exists to pull a planet apart, then there must exist the super-scientific technology to reassemble it again. Everything else maybe more technically feasible, but it will take too long and be too messy.

• That's interesting. I'm looking for some interesting ideas that may be useful. Commented Oct 15, 2017 at 4:05
• @SovereignSun You're welcome. Use them as starting point and see where that takes you. Commented Oct 15, 2017 at 4:07
• I doubt there's enough viable fissile material in the Solar System to do significantly planetary scale engineering with atomic Orion rockets. Fusion torches or go home. Or just wait for nature to take its course. Commented Oct 15, 2017 at 15:55