Celestia does zoom in; the amount of zoom is dependent upon how large the resolution is of your map.
I've been itching to play with celestia a bit anyway; I could probably help you out if you can wait a couple weekends. Did you have a planet already in progress? Celestia textures are basically 3 maps -- an equirectangular map (think Ascension's atlas tut and much much darker than is intuitive), a bump map that corresponds, and a cloud layer. So if you have an equirectangular (or any projection I can flexify into it) sketch, I can go from there.