Suitability of FLUKA for Simulation of Backscattered Radiation off an Asteroid This one's pretty simple.  I'm looking at trying to simulate the backscattered radiation coming off of a S-type asteroid shape model due to impinging GCRs and solar particles.  Ideally my simulation would include dynamic factors like the relative motion and attitude of a spacecraft in orbit around the asteroid (like the Rosetta spacecraft for instance), the orientation of the asteroid with respect to solar influx, and internal density non-homogeneities within the asteroid model to look at their effect on surface neutron emissions etc.  
I know this is a decidedly complex simulation, and I'm curious regarding your opinions on whether FLUKA (specifically using Flair GUI) is up to it?  I think GEANT4 or GRAS might be able to do so, but not so sure about FLUKA/Flair.  
Other information: I've got access to a high power computing array in the lab (so I'm not overly worried about crashing the program with an enormous task), and have the latest versions of both FLUKA and GEANT4.
Thanks!
-D. Hodge
 A: A bit meta to answer my own question, but here are some salient points which I've uncovered.  Figured that some one may find this useful so here goes:


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*FLUKA doesn't do multi-stage scattering very easily.  The sim/physics is fine but the extrication of the data is somewhat nightmarish.  You have to build a multi-stage run in FORTRAN, pump primary GCRs into the asteroid model, then use those as the input to a second sim with any detectors you're interested in.  This is, again, to the best of my knowledge (which at this point extends to having successfully run a very basic sim along the above lines). 

*Frankly, the documentation for GEANT4 is vastly superior to FLUKA's, it's not written in FORTRAN (which I hope won't step on any toes, but GEANT4's C++ is much more contemporary to my thinking), and there are fairly lively user forums for GEANT4 with more than FLUKA's apparent ~5-10 people in the entire world who can successfully answer questions and suggest debug options. 


I'd like to emphasize that the second point is subjective, and based upon my personal travails with FLUKA, and my less painful experience with GEANT4.  It is worth noting that the Flair GUI for FLUKA is quite excellent and can do most run of the mill sims quickly and without having to dive into the FORTRAN guts of FLUKA.  Complex and customized sims, however, I'd definitely go with GEANT4. 
-Cheers 
