Cloud-like object at extreme range changes shape.

This account is taken from a personal report made shortly after the sighting:  "I was driving westbound on Interstate 80 between Cheyenne and Laramie, Wyoming when I spied what I at first thought to be a tiny cloud in an otherwise cloudless sky.  Very soon after sighting it, we passed from Laramie County (containing Cheyenne) into Albany County (containing Laramie).  The heavens were still bright (sunset had been at 19:39).  The only other object in the sky was the first-quarter Moon.  The "cloud" had an almost vertical orientation at first, and apparently hung over the Vedauwoo area (a formation of exposed granite mountains east of Laramie).  After a minute or so, the "cloud" took on more of a spherical shape but later did resume its near-vertical attitude for a short while.  As I drove toward the very bright object at 65 mph, it only seemed to get further away, more starlike, and closer to the horizon, until I lost it at about 20:05, some nine minutes after first noticing it."  The most likely explanation for this sighting is a high-altitude westbound aircraft reflecting the rays of the Sun and emitting a short contrail.  But it might have been something less conventional.  It certainly impressed me enough at the time to record the details, even though I am a licensed pilot.