Ben Bova – Voyagers

Keith Stoner, ex-astronaut turned physicist, knows the signal that his research station is receiving from space is not random. Whatever it is, it’s real.
And it’s headed straight for Earth.
He’ll do anything to be the first man to go out to confrint this enigma. Even lose the only woman he’s ever really loved.
And maybe start a world war.

Jo Camerata, the ambitious young student who fell in love with Stoner, is now head of Vanguard Industries. Jo’s dogged determination has forced the recovery of the alien ship, and now her company is in control of the vast new technology—and in control of Keith Stoner. What Camerata doesn’t know, however, is that when Stoner wakes, someone else awakens, too. The alien presence in Stoner’s mind that has kept him alive all these years is now free, and intends to explore the world.
And it will let nothing stand in its way.

Now Keith Stoner is something more than just a man. While asleep he was inoculated with virus-sized machines designed to repair his body, expand his brain, and let him live forever. Stoner also carries the knowledge and memories of the being whose body was enshrined in the alien craft. He now knows of many different intelligent races who destroyed themselves, battering themselves to death on the stony problems of war, famine, runaway population, and ill-planned technological breakthroughs.

Now, after more than a century of exploring the stars, Keith Stoner returns to find that the world he has come back to does not match the one he left. The planet is suffering the consequences of disastrous greenhouse flooding. Most nations have been taken over by ultraconservative religion-based governments, such as the New Morality in the United States. With population ballooning and resources running out, Earth is heading for nuclear war. Stoner, the star voyager, wants to save Earth’s people. But first he must save himself from the frightened and ambitious zealots who want to destroy this stranger—and the terrifying message he brings from the stars.