The Smithsonian National Air and Space Museum on the National Mall, the fourth-most-visited museum in the world, has a slightly less well-known sibling in Chantilly, Virginia, near Dulles International Airport. The Steven F. Udvar-Hazy Center, opened in 2003, occupies two giant hangars and an observation tower overlooking Dulles’s busy runways. Besides the Enola Gay and a Concorde, its treasures include biplanes, a World War II German Arado 234, the world’s first jet bomber, and the space shuttle Discovery, all of which one passes on the way to the museum’s gallery and its current exhibition: “Clouds in a Bag,” a fascinating glimpse of the early history of ballooning.