Glowing white spherical large craft (50 to 100 ft dia) travels across mountain range near Green River, Wyo

My father was taking the family from Rock Springs, Wyoming, in the summer of 1951 to Green River, Wyo.  approx half way we saw cars parked on both sides of the two lane highway with everyone (about 20 people) looking north at the peaked hills (small mountain range).   A spherical craft that appeared "glowing" white was slowing moving over the mountain range.  Some said it might be a weather balloon but it was traveling "horizontal" and seemed to be steared rather than floating on it's own.
We got to watch it for about 5 to 7 minutes and my dad said, "Well there it is Freddy, a flying saucer".   It eventually went behind one of the peaks but never came out on the other side.   Also it was very large, I would guess about a good 50 to 100 feet in diameter.