A thermal bomb was a large sleek explosive that was fired from Imperial compounds on Despayre whenever there was a prisoner uprising.
As Nova Stihl described it, if a prisoner took a guard hostage, a thermal bomb would strike that location, killing the prisoner, the guard, and anyone within a hundred meters, leaving a smoking crater. Additionally, the bomb was set to home in on an placed inside the guard, with a location that was different for every guard on the planet. Finding the implant would mean skinning the guard to find the implant and even if the guard was killed, the implant would continue to work reporting its wearer dead. By that time, it would be too late to escape the range of the bomb.