Translation:I do not want to fly to another planet by myself.
So... 'sole'. But then, it's nice to have synonyms of course. Makes for better writing.
But they really are not synonyms. "Unuope" does mean "one by one" or "one at a time" and is not the same as "alone". From PIV: "Rim. Oni ne uzu opa en la senco «unuopa, aparta, individua»: tio estas kontraŭsenco."
("sole" and "unuope")
Yes, synonyms are nice to have, and I'm happy to see that even Esperanto has them!