The Ragged Astronauts

Land and Overland — twin worlds a few thousand miles apart. On Land, humanity faces a threat to its very survival — an airborne species, the ptertha, has declared war on humankind, and is actively hunting for victims. The only hope lies in migration. Through space to Overland. By balloon.  — first volume in an epic adventure filled with memorable characters, intense action, engaging notions, exotic locales. Won BSFA Award for Best Novel in 1987.