The Hexagon-Sized Hole in the Smithsonian

That's not a misprint for "hexagonal" or "hexagon-shaped" either. Though it's the latter, not the former.

OK, I better explain.

From The Space Review :

Last week, the Smithsonian National Air and Space Museum officially opened its James S. McDonnell Space Hangar at the Steven F. Udvar-Hazy Center, near Dulles Airport. The hangar is filled with numerous space objects, the most notable being the Space Shuttle Enterprise. There is one object that was supposed to be there but is not: a schoolbus-sized KH-9 HEXAGON spy satellite, developed by the National Reconnaissance Office (NRO), whose headquarters is only a few miles down the street from the museum annex.

Many years ago, when the Smithsonian was still trying to raise money for what was then called the Dulles Annex, they produced a plastic model of what the facility would eventually look like. Small clear plastic markers were cut in the shape of aircraft, spacecraft, and other artifacts and placed inside the model. Off in one corner of the Space Hangar was a little blue object shaped like the Hubble Space Telescope and labeled “KH-9.” That model was occasionally displayed to the public at special events and was usually on display on the downtown museum’s third floor, where the offices are. A map of the Dulles Annex exhibits was also published in Air & Space Magazine, and it too had the KH-9 off in the corner. So now that the space hangar is finally open, some people may wonder why there is no KH-9 to be seen anywhere in the facility.

As to why? Well, you'd better read the article. Another snippet :

Whatever the real story, the results are clear—the NRO will not acknowledge any role in developing the KH-9, and will not declassify its satellite. Instead of proudly standing alongside the space shuttle Enterprise where millions of Americans can see it, the KH-9, the fabled Big Bird, gathers dust in a classified warehouse (probably only a few rows down from the Lost Ark of the Covenant). When it will eventually see the light of day is anybody’s guess.

Now for an exclusive: Lots of things that should be classified accidentally end up on the Internet. Sometimes pictures from Labs and Clean Rooms, where satellites are being worked on by white-coated technicians. It doesn't take a Rocket Scientist to recognise that the white-coated technician in this picture is checking out a Big Bird.