Rectangular object with rounded edges approx 20-30,000 ft alt seen above superspeedway

Just minutes prior to the start of the Winston 500 at the speedway, observed, from backstretch grandstands (east side of track) looking west, a reflection way off in the distance to teh west rapidly approaching speedway at astronomical speed. Object abruptly stopped over south end of speedway at approx 20-30,000 ft altitude and hovered for about 1 minute in an oscillating wobbling motion. It was almost as if whoever was in the craft stopped to see what was going on at this big event. Brought this to attention to friends sitting next to me and they saw it and could not explain it. Saw object through binaculars but still could not see any markings or features. Motion of this sort, the abrubt stop after streaking in from the west, would undoubtably kill a human being because of the abrupt force of the stop. After about a one minute hover, object streaked back to the west at just as fast as it came in and to the same direction it came and vanished. It was a clear day with just a few high clouds off to the north and west, so there was no obstruction. Event lasted less than 2 minutes. Object appeared silver as it approached, but when it was overhead, being high noon, it just looked black.