Collision detection

Hiy guys,

I am working on a game and I want my space craft to have 6 shields 
(front,front right, rear right, rear, etc.)

The problem is doing the collision detect, I have code which is fast for 
checking if the shields are hit at all but finding the angle of the 
collision is non trivial.

I basicly check a line against a circle with this code, this includes 
scaling code but don't worry about that, the circle has radius s

   // screen position of object
    x3=((list[i]->x/vs)*202)+202;
    y3=((list[i]->y/vs)*235)+235;

    x1=202;
    y1=235;                      // start point of line

    x2=202+(500*cos(a1));
    y2=235+(500*sin(a1));        // end point of line

    u =(x3 - x1)*(x2 - x1) + (y3 - y1)*(y2 - y1);
    u/=(x2 - x1)*(x2 - x1) + (y2 - y1)*(y2 - y1);
    if ((u>=0)&&(u<=1))
   {
         xi = x1 + u *(x2 - x1);
         yi = y1 + u *(y2 - y1);

         r=sqrt(((xi-x3)*(xi-x3))+((yi-y3)*(yi-y3)));
         if (r<=s)
         {
                // hit
         }
  }

I know I can get rid of the sqrt but the problem is a line intersects a 
circle at two points and I need to find out the first point impacted.