Object movement- rounding error?

I made a simple 2d game that makes two planets orbit each other. To each of the planets, I attached the following script:

 #pragma strict
 var velocity = Vector2(0,0);
 var target : Transform;
 var weight = 4000;
 private var acc = Vector2(0,0);
 private var join = Vector2(0,0);
 function Start() {
 }
 
 function FixedUpdate () {
     var pos = transform.position;
     join = target.transform.position - pos;
     acc = 10 / join.sqrMagnitude * join.normalized;
     velocity = velocity + acc;
     pos = transform.position + velocity;
     transform.position = pos;
 }

The two planets are each set to have an opposite initial velocity(0.4 and -0.4), along the vertical axis, and are otherwise identical. This should make a cyclic orbit; however, when I set them to trace their paths, I get the following result:

The planets eventually meet in the middle and fling off into space. Do to the asymmetry, I believe this is an error of some sort, and not simply incorrectly coded orbital mechanics. Does anyone know a method to solve this problem or a workaround?