Just curious but: Suppose that in the future we begin mining asteroids and other rocky objects in the asteroid belt for resources. #1 how much material would we be able to bring back to earth before we start noticing increased gravitational effects (especially considering that recovered material would almost exclusively return to a land mass therefore making the distribution uneven)? #2 how much additional material would be needed to affect the orbit of the moon? IIRC the moon is moving something like 1 cm a year away from us. At some point the additional mass will cause additional gravitational pull on the moon stopping or even reversing this movement. What would be this critical mass? #3 What other problems could we potentially face due to space mining?