P.K. 19

P.K. 19 is short for Planet Killer 19, and references the number of Mercury Ships in Gudersnipe's fleet which have the capacity to destroy a planet. There have not always been 19 ships on the registry with this power rating, especially early on the Foundation's history, however it was established in A.Y. 112 as the largest number that would ever be placed in service. Note that not all Mercury Ships fall into the P.K. 19, making this a limit on the number of planet-killers and not the number of Mercury-class warships.

P.K. 19 is also sometimes used as slang for any kind of planet-destroying event. Prior to the development of Mercury Ships, the Scorcher system was used to a similar effect.