As he prepared to climb the scaffolding to be publicly executed by guillotine on 21 January 1793, Louis XVI is reported to have asked: “Is there news of Monsieur de la Pérouse?”

He was referring to Jean-François de Galaup, the count of Pérouse , a naval officer and navigator whom the French king had dispatched in 1785 on a round-the-world scientific mission to rival discoveries made by the English explorer Captain James Cook.

The veiled love letters sent by Jean-André Mongez to Louise Victoire Le Cloustier were discovered in a book of engravings. Photograph: Daguerre Paris

The two ships under La Pérouse’s command, La Boussole and L’Astrolabe, disappeared three years later after leaving Botany Bay in Australia. The wrecks were only formally identified in 2005 and exactly what happened to them remains a mystery to this day.

Now rare letters written by an illustrious scientist who perished in 1788 have resurfaced, giving a new and personal account of the voyage.…