if you describe that something is unforget do you might say :on my memory like Mars at perihelion?MY collages and I were appointed to read a book--,author of Elbert Hubbard,in this book,there has a common plot story.but at first chapter ,this sentence "memory like Mars at perihelion" let me think that whether this is a old expression in USA.

Question

if you describe that something is unforget
do you might say :on my memory like Mars at perihelion?MY collages and I were appointed to read a book--<>,author of Elbert Hubbard,in this book,there has a common plot story.but at first chapter ,this sentence "memory like Mars at perihelion" let me think that whether this is a old expression in USA.

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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.

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unforget = remember?

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

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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*

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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.