There's no need to save something like an asteroid as a object file unless it was already loaded from external geometry. Anything that has been created in TG2, such as a planet displaced to look like an asteroid, can just be saved as a "clip file" and loaded separately into other scenes that way. Just select the nodes that create the object or effect you want to save then go to File->Save Nodes As Clip File. Specify a file name and location, then you can load it from there at a later time and incorporate it into another scene.
You can create and export geometry from some scene elements, but it's a bit complicated and should not be used for what it sounds like you want to do. It's the least efficient approach, really. The only time I would think you'd want to use geometry export is if you wanted to load some part of the TG2 scene in another 3d app.
- Oshyan