I believe it *used* to be the case that the apostrophe should be used, and it has fallen out of favour in the last few decades. I have a very hazy memory of being taught to use it in the 1970s.
If you watch Gerry Anderson's "UFO", the teleprinter at the start does contain something like "SEARCHING FOR UFO'S IN EARTH'S ATMOSPHERE." I'm pretty sure that wouldn't be an error, so it was considered reasonable usage in 1972.
It certainly isn't *now* though. It gives greengrocer's an excuse ;-)
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If you watch Gerry Anderson's "UFO", the teleprinter at the start does contain something like "SEARCHING FOR UFO'S IN EARTH'S ATMOSPHERE." I'm pretty sure that wouldn't be an error, so it was considered reasonable usage in 1972.
It certainly isn't *now* though. It gives greengrocer's an excuse ;-)