Heavy Turrets are turrets that are more heavily armored than their more common counterparts. This makes them more capable of defending positions. During Balak's raid to Asteroid X57 for a slave grab, he instead saw potential and ordered the asteroid's three fusion torches lit and accelerate the asteroid to collide with the Alliance colony of Terra Nova. He had turrets deployed around the torch stations to defend them against anyone that may interfere with his plans, and then had additional turrets deployed at the entrance to the main facility. Shepard must get past these turrets to stop the asteroid and save the colony.