Silver disk high in blue sky over Iowa 1975

I was 9 yrs old, playing in my front yard in Iowa City Iowa in 1975.  It was mid-afternoon or so.  I noticed a silver disk directly over head where the Sun would be at noon time.  It was very high up.  If you saw a dime about 15 Feet from you, that would be the size it looked in the air. It may have actually been about the size of a small plane.  The Afternoon sun shined on it and it was clearly a silver disk, or baloon shaped object. It didn't move for as long as we (my family and I) looked at it. My mother dismissed it as a weather balloon. But this was a warm summer day, no clouds at all. Completely blue skies. Why send up a weather balloon? Also, balloons drift with the air currents. This object didn't drift at all. In fact, it stayed exactly in one spot. We looked at it for about 10 minutes, and then went back into the house. We didn't see it leave. Witnesses were my mother, a nursing proffessor at the University of Iowa, myself, and my brother who was about 13 at the time. It was too high to be a teathered balloon. Plus it was directly over the town, can't have a teather over a town with an airport. Not a blimp either. Too high and stationary. Not a plane, it didn't move at all. Not a weather aberation. It was a warm cloudless day. Blue skies only. It was definately there. silver object, shining in the bright sun against blue skies. No movement. Totally silent. Not as high as the commercial jets, but higher than small craft. Again, definately not a blimp, too high and too perfectly round. Stationary for over  10 minutes. Wish I knew what it was.
Witness indicates that the date of the incident is approximate.  PD