1.1. A binary planet system consists of two stars, A and B, as shown in Figure 1.  A has a mass of mass 4.0 × 10³⁰ kg and B has a mass of 8.0 × 10³⁰ kg.  The centre of the stars is separated by a distance of 2 × 10¹¹ m.

Figure 1

Calculate the gravitational potential at the midpoint between the stars.

1.2. The amount of energy required to send a space probe of mass 1800 kg from the surface of star A to the midpoint between stars A and B is 4.2 × 10¹¹ J.

Calculate the gravitational field strength on the surface of star A.

1.3. Star A is drifting away from star B. 

Calculate how far star A will have drifted if its gravitational potential energy decreases by 10% of its initial value.