The Resonance War - Book 5 of The Celestriad

A time capsule, left by the remnants of Celestra's first civilsation, is found and deciphered. It contains a warning: the asteroid which decimated them was under propulsion, and whoever launched the unprovoked attack is still out there. After a freak communications glitch alerts this ancient enemy, Celestra is invaded by an invisible, synthetic bio-swarm. The Celestrians have but one defence. Their own children.