Celestia v1.6 (and v1.5.1) assumes that dxt3 and dxt5 textures are transparent even if they are opaque. This causes depth sorting problems.
Celestia should examine dxt5 and dxt3 files to see if they should be treated as opaque or transparent.
An Addon demonstrating this problem is available at
http://www.lepp.cornell.edu/~seb/celestia/opaque_dxt5_bug.zip (1MB -- too big to attach)