Twenty years ago today — November 15, 1990 — Space Shuttle Atlantis launched from the Kennedy Space Center on a classified Department of Defense mission.
(STS-38 crewmembers in the crew compartment trainer. NASA image.)
STS-38 astronauts Richard O. Covey, Frank L. Culbertson, Jr.,* Charles “Sam” Gemar, Robert C. Springer and Carl J. Meade deployed their classified payload — which was probably classified higher than SECRET — and then returned to Earth on November 20th.
This Wikipedia article includes speculation about what that payload might have been. Don’t ask me: I don’t know, and if I did I still wouldn’t say.
*I met CAPT (Ret.) Culbertson last year, at the NASA Industry/Education Summit. In the “small world” department, our high schools used to be in the same conference, back when my high school still existed.