The gravitational forces between two masses are inversely proportional to
the square of the distance between the centers of the masses.
So if the distance between the centers of the asteroids is multiplied by 1/2,
then the gravitational forces between them are multiplied by ...
1 / (1/2)² = (2)² = 4 .
The new force is (100N) x (4) = 400N.
