The Hammer of God

By Arthur C. Clarke

Set in the 2100s, the book finds humanity facing doom in the form of a big wayward space rock on a collision course with the planet, with catastrophic consequences.
Captain Singh has the responsibility of hauling a powerful engine out to the location of the rock and placing the engine on the rock and firing it up, in the hopes that it will nudge the rock into a different orbit, one that will not send it crashing into Earth or the moon or Mars, all of which now have human populations. It doesn't have to move it much, just a little nudge would be enough to send it into a safer orbit.
But, not unexpectedly, there are religious zealots who are eager for the end, for God's judgment of wicked humanity. The zealots have managed to throw a monkey wrench into the proceedings: the engine spills its fuel instead of doing what was intended. And the space rock remains on its killer trajectory. Now it is up to Captain Singh and the bright minds of the 22nd Century to try and save Earth and its people, including the nutters who want to see it all go up in smoke.

A pretty interesting tale, but filled with a lot of technical jabber that I just skipped over.