Two neutron stars are separated by a distance of 1.0 x 1010 m. They each have a mass of 1.0 x 1030 kg and a radius of l.0 x 105 m. They are initially at rest with respect to each other. As measured from that rest frame, how fast are they moving when?

(a) Their separation has decreased to one-half its initial value and

(b) They are about to collide?