Whilst reading up on one of the Challenge Questions, I came across a cool program called "Universe Sandbox" ;
http://universesandbox.com/
It was suggested I post it up ( Ta, Roger !). Here's the brief summary I posted in the Challenge Question, though following the above link will tell you plenty.
***************
"For anyone who's interested, the link gives pretty good info on what it is; a sort of cosmology design tool/toy. It set's you up with an amazing animated universe, and then you can change parameters/create new objects. It all move before your eyes, with ability to zoom/pause/change view angle etc. The thing is set to allow you 60 minutes play, then the author just asks you to pay what you feel is a reasonable amount. I haven't used more than 10 minutes yet, but it's amazingly good. I'm pretty sure I'll give the author some money for it (just for the sceptics : I have absolutely no connection to the author) - it's one of the best simulators I've seen.
I'm not qualified to vouch for the accuracy of the program, but I doubt the guy/gal made many errors on fact with that much effort. It's both a fun program (destroying planets at whim !!), and a great learning tool.
Usual disclaimers apply."
*********************
The author is Dan Dixon. He asks for feedback when you use the programme, so I'll invite him to visit us here.
Enjoy having a play - I'd be interested to know what folks here think ; Does it appear accurate (I think so), have you created any noteworthy havoc with it (Oh, the poor Moon 
), did your kids enjoy it just as much ( he ain't getting a look in yet !), have you managed to create a gravity flyby with it (not after 10 mins play) etc etc.
4
Universe Sandbox 
Re: Universe Sandbox
Good Answers: