Robert Silverberg 
Planet Killers 

It was in wide-eyed horror that Roy Gardner heard the news from the Chief of Security. In just sixty-seven years the Earth would be totally destroyed by the planet Lurion.That data had been compiled by the invincible computer. With unwavering faith in the machine, humans had only one thing to do – destroy Lurion first.And the man to do the job was Gardner. If he did it successfully the blood of billions would be on his hands; if he fouled up he would be the worst traitor in Terrestrial history. And not even he knew which course he would pursue when he finally learned that even the all-wise machine had not known all the right answers.First published in 1959.