Agreed. In all likelihood, the seven parts would collide a bit, heat up a lot, and generally lump back together into a new moon. On that scale, the moon is really soft. So much so that each of the seven individual lumps would probably flow into a roughly spherical shape before colliding with anything.
Right. What most people don't realize is that at planetary scale (or even at Moon scale) things are fluid. There are no solids at that scale. Gravity dominates all other interactions except gas pressure - and that only for things made mostly of gas.
That being said, I would love to see an accurate, 3D, high resolution simulation of the scenario described in Seveneves.
Absent enough input energy to push one or more of the fragments onto a new geodesic, to zeroth-order the moon would very quickly recollapse into a hot spherical droplet (likely by way of seven increasingly droplet-like components), and it would take an enormous amount of time for surface features -- like seas and so forth -- and lunar mascons[1] to appear.
The undistorted body in the Earth-moon pair under either scenario is distant enough that the squashed Earth or the fragmented moon would still source a close approximation of the Schwarzschild metric[2], with an interface between the two as part of the boundary condition of each. Usually for simplicity we'd just describe the motion of the lighter body in the geodesics of the Schwarzschild solution of the heavier body, and the whole moon, its seven fragments, and the fragments' successor arrangements are very likely amenable to that.
One could always kick part of the mass of the moon such that it escapes "to infinity" from the Earth-moon system. Unfortunately one would have to recalculate the stress-energy tensor around the residual moon (and around the Earth, and around the sun) and then work out the geodesics. This would make for an entertaining project in numerical relativity.
[2] The Schwarzschild solution is after all an exterior solution, and is fine for approximately spherically symmetric bodies with approximately zero angular momentum. Moreover, if we don't care about ~ microsecond accuracies (and we almost certainly don't, because the conditions at lunar fragmentation are not that finely specified), we can do everything with small corrections to Newtonian gravity (cf. sec 6.4 of http://onlinelibrary.wiley.com/doi/10.1002/9783527634569.ch6... for the post-Newtonian Kepler solution and ch 8 on relativistic geodesy).
> Galileo observed that small animals like cats can survive falls much better than large animals like horses even over a couple of meters. The thing is that a things strength increases in proportion to its cross section but its weight increases as its volume
Does the principle have a name? I always felt this in the back of my had but never formulated it, I find it extremely interesting that Galileo wrote about it.
http://hopefullyintersting.blogspot.com/2015/06/seveneves-an...