Dragonflight by Anne McCaffrey

The planet Pern has been colonized for centuries by humans. When humans first settled on this world, they did not take notice of its sister planet, which had an indigenous life form that attempted to land on Pern when it came within reach. These silver "threads" fell in a destructive wave on the temperate lands of Pern once every 200 years, destroying all life they encountered. To combat this menace, the inhabitants of Pern developed a species of dragon that could burn these threads out of the sky before they touched down. Now, centuries have passed between threadfalls, and the danger of thread is considered a myth.