Fatto generale: 21609 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21609 is an unremarkable number.
Trivia: 21609 is a boring number.
Data storica: February 28th is the day in 1959 that Discoverer 1, an American spy satellite that is the first object to achieve a polar orbit, is launched.