On clear night two objects make large circle in the sky and disappear at very high speed.

I was 15 going on 16 and we had just begun a trip out west from Ohio. We had arrived late to set up the tent in the Badlands of South Dakota.  We had just finished setting up the tent when my dad and I looked up into the sky.  It was a very clear night and very dark so you could see thousands of stars in the sky.  Just about one minute after we started looking at the sky, I saw two objects the size of the stars seem to separate from one location.  They slowly made a huge circle in the sky and came back together again.  I could not believe what I was seeing so without looking away from them (I did not want to lose track of the objects), about half way into the circle I asked my dad if he was seeing what I was seeing.  He said he was.  When they came back together the object was one again. It began to slowly move in one direction and then picked up speed quickly and within a second was gone.  I continued to look for a while, but it was over.  I turned to my dad and asked him what he thought it was.  He said he was not sure.  I said could that have been a UFO and again playing it safe he said he was not sure.  But I know what I saw, and my dad verified the fact that I was not seeing things.  The USA or the rest of the world did not have anything that could have made that circle and disappeared that fast.  I am convinced this was a UFO.
Date is approximate.  PD