SPACE SURFERS by Anne E Johnson
Teenage siblings Lerris and Graika of the planet Soranen want to go into Outer Space, an act forbidden by their people, the Sor. The Sor, it is believed, are the descendants of the Ellarisor and the Humans, now both extinct on Soranen. When Lerris surfs above the clouds in his minijet, he spots a vessel owned by Lorfallin, a chemical magnate. The ship is spraying a green mist that, it turns out, causes a disease which affects those with Human ancestry, including the siblings’ father.