SR-71A Blackbird and D-21 Drone

The centerpiece exhibit at the Museum of Flight.  I've seen several SR-71s in museums before, and am old enough to remember, have seen, and definitely heard them when they were in service.  (In Denver in 1983, a SR-71 cracked Mach 3 at over 60,000 feet, and it shook the city.  I remember thinking that the Russians had finally dropped the big one...we heard later it was "accidental."  How do you accidentally go over Mach 3?)

Whatever the case, the SR-71 is always an impressive sight; it looks fast standing still.  This is the only one in the world that I know of displayed with the D-21 atop it.  What did the D-21 do?  It's still classified.