In the most basic form, for example just using eg cubes to illustrate how to do this, is all that's needed.
1. Create yourself a basic terrain 1000x1000 (Height unknown)
2. Place a cube into scene
3. Create yourself another basic terrain 1000x1000 (Height unknown)
How would I fly the cube out of the first terrain, into space, so going through some clouds and into space, see stars and other planets (If there are some there, which would also allow me to fly too and enter the atmosphere and land and explore) fly to the other terrain and land.
Bare in mind that the terrains used in this example are planets.
Don;t just bid on this simply because you think you can do this.
I have done my research and have lots of questions. If you think you can do this, please make a very small and yet simple example and provide that example.
Failing to add the example will result in your bid being rejected.
I have probably missed out other important factors here already, so if you've any questions please ask. In your bid along with your demo.
I have customize the budget for this simple reason. I know there may be a few scripts here that are in use here as well. Your profile if you bid on this should have clearly other examples of people whom have used Unity3D, since this is an important feature.