Bright, White, Saucer-Shaped Light in Arizona Desert

My mother was the one who actually saw the craft. She was a little girl at the time. This was in the Arizona desert. It was a large half-dome-shaped or saucer-shaped, bright, white light. She saw it from the back of the family station wagon. Her 2 sisters may have also seen it, but probably don't remeber. My grandfather told her that it was probably military testing.
Time and date are approximate.  PD