SpaceEngine is not based on Celestia nor any other engine, I wrote the code all by myself (except some common algorithms like quicksort, memory manager, OpenGL tricks, that can be found in books and everywhere else in the Internet). The first public version was 0.74, it was released on the Russian GameDev forum. It worked only on NVidia GPU, has procedural planets, but no procedural terrain (3D terrain was only on Earth and some other Solar System bodies). You can follow SE evolution on the Russian GameDev (Google translate could help):
SpaceEngine thread on GameDev
Links to download all released versions of SE. Stable work is not guaranteed
0.74 (NVidia only)
Until 0.74, SE had a very quick version number change, every small improvement increased the number bo 0.1: 0.5 - 0.6 - 0.7. Then I realized that it is too fast, so I decreased the numerical step and now every big release has a version number increased by 0.0.1, while patches add 0.0.0.1: 0.9.6.0 - 0.9.6.1 - 0.9.6.2
Terrain engine in SE is pretty much the same even now, it wasn't changed since a beginning of SE.
Look at the graphics settings dialog in 0.74