Marcus Chown & John Gribbin 
Double Planet 

The Dragon, an enormous comet, is on a trajectory that will bring it perilously close to an Earth that is still suffering from the scars of a nuclear incident, and from the problems of the Greenhouse Effect. For the optimists — those that remain — it is a sign of change for the better; for others, the comet foreshadows humanity’s final doom.But to Francis Reese and the hard-pressed astronauts of the depleted space programme, the Dragon presents a third outrageous, yet irresistible possibility — the transformation of a barren world into a new home for the beleaguered peoples of Earth.