Smart missiles were projectile weapons that had some degree of sentience, to the point where they were living bombs. Because of this, the Doctor considered them "suicidal robots" because he thought it deplorable to create a creature just to purposely destroy it. He compared smart missiles to the dolphins the American CIA would strap with bombs and train to swim near German submarines.
They were able to fly, hover and take orders. Ace had acquired smart missiles, possible from her time in Spacefleet, but the majority of them deserted her on Peladon to form a union with the mining machinery. Because of their sentience, they possessed living minds that were susceptible to strong hypnosis. Ace's last smart missile, being used to threaten Azathoth, was instead converted by it into a follower. (PROSE: All-Consuming Fire)
Earlier versions of smart missiles were used in the early 21st century by the United States of America during its war with Mexico. Lewis Christian worked as a troubleshooter for the missiles' neural computers. (PROSE: Cat's Cradle: Warhead)