In another MST3K group, a friend brought up the question of what the “cool thing” Joel and the bots saw really was, and it struck me that we might actually have an answer now.
Tom saw the opening of a portal in space, and called the others over. It got even better when they could see through it to the surface of the Moon. Then, even more amazing (which is why they couldn’t tell us), the SOL from the end of season 13 came through.