I'm continuing to work on optimal control for rockets. Practical Methods for Optimal Control and Estimation Using Nonlinear Programming looks like a very good book on the subject. I just got a copy, but I found a PDF of it online somewhere that convinced me it was worthwhile.

In the meantime here are a couple of color studies for 2D planets: Which of the following two options would be better?
A foreground cap could add to the exploration aspect of the game by hiding underground tunnels. On the other hand I'd have to draw a spherical planet on it or something, which would be tricky to do well.
[null, {"document_url": "http://playtechs.blogspot.com/2012/06/", "unformatted_src": "http://1.bp.blogspot.com/-7EmInpmU3v4/T98zsJAbGUI/AAAAAAAAAyw/ZkTjeBIqTFo/s400/planet.jpg", "src": "http://1.bp.blogspot.com/-7EmInpmU3v4/T98zsJAbGUI/AAAAAAAAAyw/ZkTjeBIqTFo/s400/planet.jpg", "formatted_filename": "planet", "rendered_width": 400, "rendered_height": 400}, {"document_url": "http://playtechs.blogspot.com/2012/06/", "unformatted_src": "http://1.bp.blogspot.com/-jz8yfv5O0as/T98zsVCCLKI/AAAAAAAAAy8/pPCApfqpIlk/s400/planet_w_tunnels.jpg", "src": "http://1.bp.blogspot.com/-jz8yfv5O0as/T98zsVCCLKI/AAAAAAAAAy8/pPCApfqpIlk/s400/planet_w_tunnels.jpg", "formatted_filename": "planet w tunnels", "rendered_width": 400, "rendered_height": 400}, null, null, null, null, null, null]