Sundiver u-1

No species has ever reached for the stars without the guidance of a patron — except perhaps mankind. Did some mysterious race begin the uplift of humanity aeons ago? Circling the sun, under the caverns of Mercury, Expedition Sundiver prepares for the most momentous voyage in history — a journey into the boiling inferno of the sun.
The book was nominated for Locus Award for Best First Novel in 1981.