FDMaxey and Cherry are absolutely right. There is a song called "Unforgettable, that's what you are' It illustrates the meaning of your question as well.

unforget = remember?

unforget + mars.. *thinking..* they don't add up

Hello Paul, I heard about this expression " on my memory like Mars at perihelion" . Mars is at perihelion when it is at its closest point on its orbit to the sun,which is a remarkable phenomena .So the saying compares anything that is remarkable and difficult to forget with the occurrence of Mars at perihelion. * peri =a prefix meaning near , helion = from greek helios =sun*

I'm not sure I understand. There is no real word "unforget". It is more likely you are thinking of "unforgettable", which means making a big impression - something you would never forget. "The Chinese fireworks display during the new year's festival was unforgettable." It is actually possible that somebody used "unforget" in science fiction, perhaps, as a play on words. It would mean to remember something which was forgotten. In any sort of actual writing or speaking though, "unforget" does not exist as a real word and you would never hear it.