Fatto generale: 21157 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21157 is an unremarkable number.
Trivia: 21157 is an uninteresting number.
Data storica: December 3rd is the day in 1999 that NASA loses radio contact with the Mars Polar Lander moments before the spacecraft enters the Martian atmosphere.