The forces of gravity between two objects are inversely proportional to the square of the distance between them. So reducing the distance by 1/2 means increasing the gravitational force by 2² = 4 times. The 1 million newtons becomes 4 million newtons.
Note that this does NOT mean the satellite's altitude above the surface. When you're calculating gravitational forces, it's the distance between the centers of the objects. So the question is a meaningful exercise only if we use the distance between the satellite and the planet's center.
