How to track an animated arc?

I've animated an arc by animating the end point so it grows from nothing to about a 270 degree arc.  I'd like to have a graphic of a comet fly into the frame from offscreen toward the start of the arc and then follow it as it extends, so it looks like the comet is drawing the arc.  I can handle the fly-in, but how can I get the comet to track the extending end point?