The Knights of Mars (Castle in the Stars, Bk. 3)

What if man journeyed into space in 1869, not 1969?

When Seraphin, Hans, and Sophie returned from their voyage to the moon, they didn't come home empty-handed - they brought with them aetherite, a miraculous substance that defies gravity! To keep their secret safe from the Prussians, the trio lives in hiding on a remote island in Brittany. Meanwhile, Seraphin's father has founded the International Society of Aether, a group of scientists dedicated to the peaceful pursuit of space exploration. But on the eve of their inaugural meeting, he vanishes without a trace!

The Knights of Aether's last hope lies in a daring rescue of King Ludwig and Professor Dulac. They will find them on the farthest outskirts of the Prussian empire - the planet Mars!