I came across this program a couple of days ago (apologies if someone's already mentioned it). It takes heightmaps and texture maps of planets and lets you view it at any detail - from orbiting in space right down to flying through valleys!
I know this is a feature most people want in Celestia - well this is what can be done (and without next-gen gfx cards
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Unfortuanately it doesn't seem to be open source, and there is only a windows download. My guess is it's a research project... maybe source code will become available later on.
(p.s if you download it, use the function keys to vary speed - it took me a while to figure the controls out
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