My father was taking the family from Rock Springs, WY, in the summer of 1951 to Green River, WY. About 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, a 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. It was 50' to 100' in diameter