Poul Anderson 
Question and Answer 

For many years the starships of earth searched the heavens for places where men could live. Many planets were found, but always something was wrong: too hot; too cold; atmospheric contaminants; poisonous biochemistry; intelligent natives — something. At last a deep-space survey vessel has reported a planet that seems perfect, an uninhabited paradise where people can roam free. But the first Troas Expedition never returns. And now the crew of the De Gama must find out why or mankind will lose the stars forever.