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Medusa is a nuclear delivery payload system and described "crown jewel" of the Colonial Fleet's nuclear initiative just prior to the Fall.
According to a briefing given by Therion's commander, Burkett, the Medusa delivery system is capable of delivering ten high-impact planetary-scale nuclear assault solutions with unprecedented levels of accuracy, adaptability and reliability. Therion and its Ghost Squadron are to provide security for the test on Aquaria.
Testing of this system is aborted just prior to the Fall due to confirmation that the Children of Erebus are operating near the facility, resulting in a ban on civilian air traffic and lockdown of the launch site (Battlestar Galactica: Ghosts 1).