If the separation distance between two masses is halved, the force of gravity between them would increase by a factor of four.

Here's why:

Newton's Law of Universal Gravitation states:

* F = G * (m1 * m2) / r²

Where:

* F is the force of gravity

* G is the gravitational constant

* m1 and m2 are the masses of the two objects

* r is the distance between their centers

As you can see, the force of gravity is inversely proportional to the square of the distance between the objects. This means:

* If you halve the distance (r/2), you square the force (F * 4).

So, halving the distance between the masses would quadruple the force of gravity between them.