http://bit.ly/faslZi - video of Robin Canup's lecture about satelite formation at this year's Lunar and Planetary Science conference (fast forward to about 13 minutes to get to the start, before that is largely admin stuff and student awards). There's an awesome 3D animated simulation of the impact believed to have formed the moon at around the 17 minute mark.  
 
Which makes me wonder... can we do this in GravSim? We can make individual particles, we can make discs/belts of them too... but doesn't that mean that there's the capacity to make spherical arrangements of particles too? If we could define a 'core' and 'mantle' of a planet as differently coloured solid shells of particles and then have them crash into eachother then we could do the same kind of thing as that simulation couldn't we?  
 
(it'd probably be slow as heck, but it'd be interesting to try!)

Possible setups are of course countless .  
The animation here shows the outcome of above simulation

I ran into problems when adding a polar ring . The system got me an overflow . Maybe the number of bodies in GravSim is limited to 1000 ?

I guess that doesn't bode well for having spherical groupings of objects...  
 
Also, is it me or are the smaller particles not colliding with the central object at its surface radius? Shouldn't they be disappearing when they hit that, not going through it?

Some help needed :  
 
I created the following sim :  
* started with 2 bodies , Earth (1 Earth mass ) and Moon ( 0.1 Earth mass )  
* tuned the orbital parameters so that the come very close  
* ran the sim backwards in order to have some safe distance between the two bodies .  
* then changed their masses to 0.5 M and 0.05 M respectively , being the cores  
* then added 300 bodies to each , with diameter 50 km and Sma 3500,6000 for Earth and Sma 1250,2000 for the moon , and having masses 0,00165 and 0,000165 Me , so that each envellope has a mass of 0.5 Me and 0.05 Me for the moon  
* the result should be 2 bodies surrounded by a dense cloud , being their envellopes .  
 
To my surprise , when running the sim al of the 300 + 300 bodies got a mass 0 instead of the inputted masses  
( visible through the "Edit" menu ) .  
Also the diameters ( set at 50 km +/- 0 ) seem to vary , ie not being equal to 50 km  
 
I'm wondering if this odd behaviour is due to the "." , "," issue ?  
Can somebody repeat some part of the setup ?