If you are using an older version (like I am) that may happen if you visit too many planets in one session. This is becasue with the older versions, once you leave a planet, it does not unload it from the RAM, and once your RAM is full, orbiter will crash. Try quicksaving, closing orbiter, closing the launchpad, and the reopen the launchpad. Also, how much RAM do you have? If you only have 512 MB or something the higher level earth, moon, and mars might not load at all. Also, make sure you are not using level 9 heightmaps (they have been removed from download but were availaibe at one time). Hope this helps.