PLANET OF NO RETURN

Universal paperback, 1978. Cover by Les Edwards (thanks, Staz).

"WANTED: 

terrestroid planets, habitable but uninhabited, clean of major sicknesses, rich enough to support colonists without outside help. 

FOUND: 

in almost a generation, nothing. 

Then a shipful of astronomers chanced on the troas-illium system. The da gama had set out, but never come home. Now, seven years later, the henry hudson is due to leave on the same mission..."