Mother and son were driving at dusk in SE Wyoming, when her son noticed a strange white light, brighter than the sun, pass behind a cloud, and then descend into it. The object was being pursued by an aircraft, which had a flashing red light on it. The circular object was approximately ten times the apparent size of the red light on the aircraft. The aircraft circled the cloud the object had descended into, and then left. The object was not observed again. Warren AFB at Cheyenne, WY, was contacted, but they reported no radar contact.