The amount of force needed to sustain motion of a rock in outer space is

0 N

Explanation:

According to Newton's first law, object subjected to no force or 0 net force will stay in the same motion. In outer space, there's no gravity or air resistance. So if the net force is 0, a rock in motion would be still in motion forever without a need of an extra force to sustain it.

asthenosphere and convection

grav force prop 1/r squared 2r all squared is 4rsquared 1/4rsquared quartered