Starcross — Philip Reeve

There is an old saying: if something is too good to be true, it probably is. Take the holiday that Art Mumby, his irritating younger sister Myrtle, and their mother take to Starcross, the finest sea-bathing resort in the entire Asteroid Belt. The fact that there are no seas anywhere should give you a clue. Sure enough, Art and family quickly find themselves grappling with French spies, Yankee rebels, and man-eating starfish. With stops to the future and prehistoric Mars, it's all Art can do to keep his head about him - which is essential, since everyone else is losing theirs!