Ok, so chalk one up for the terrestrial hypothesis.

https://www.news.com.au/technology/innovation/military/secret-behind-best-ever-ufo-sighting-revealed/news-story/105a08c1a7caf70f6f41c477a563fd3a

This researcher, David Clarke, apparently has discovered, quite by accident, a link between the Calvine UFO photo and the huge Belgian wave flap from the early 1990s. This was a gigantic event in UFO lore, so this possible connection is quite tantalizing.

However, it would be VERY nice if we had the actual identity of the source.