Game Object floats away

Hello!

I've created a new IPhone project using Unity 3. Right off the drawing board I have a problem with the main game object floating up and away. It's a large inhabited asteroid that's positioned in outer space. My player is to land on it and explore. I've tried making the asteroid a Rigidbody and checking gravity, it drifts upward. Gravity off, it drifts. I've added a mesh collider, or a box collider to the rigidbody and it still drifts. I've removed the rigidbody and left the mesh collider on, and it still floats upward. With no collider and no rigidbody it floats. I've created an empty game object and put the mesh inside and tried different combinations of colliders and rigidbodies and it still drifts. I don't know what else to do. I had this same problem a couple of years ago and left Unity because of it. Unfortunately it's still here. I've spent a couple of days now just trying to stabilize the object with no success. I'm stuck at square one and was hoping someone could tell me how to resolve this problem so I can get my workflow flowing.

Thanks,

Duane