National Air and Space Museum | AllCheapFares

The National Air and Space Museum is the most popular museum in the world, with more visiters than even the Louvre or Uffizi. It's also the largest collection of historic air- and spacecraft in the world. Where else can you see planes flown by the old greats like Lindbergh and Earhart, and newer pieces of aviation history like the Voyager plane (first non-stop, non-fueled flight around the world in 1986). And don't forget the space program, although in its case a lot of the craft you see on display are models- they don't always come back to Earth! The Mercury and Gemini programs, Apollo and Viking, up through today's space shuttles and the Hubble Space Telescope. NASM has two theaters; the Samuel P. Langley, which shows IMAX films, and the Albert Einstein Planetarium. Free (except for the theater shows). Metro: L'Enfant Plaza or Smithsonian

The museum also holds the backup mirror for the Hubble which, unlike the one that was launched, was ground to the correct shape. There were once plans for it to be installed to the Hubble itself, but plans to return the satellite to Earth were scrapped after the Space Shuttle Columbia disaster in 2003; the mission was re-considered as too risky. The Smithsonian has also been promised the International Cometary Explorer, which is currently in a solar orbit that occasionally brings it back to Earth, should NASA attempt to recover it.

This is really a museum worth visiting!  The attractions are always changing so you can go over and over.