Applause erupted on the National Mall in Washington, D.C., Tuesday morning as a space shuttle mounted on the back of a modified 747 airplane flew over the nation's capital. The space shuttle Discovery was en route to the Smithsonian National Air and Space Museum annex near Dulles Airport, its new permanent home. (Watch the video below.) It marked the final flight for the shuttle, which NASA retired in March 2011. Here, a guide to the "extraordinary sight" and the Discovery's legacy:  

Why is Discovery important?
Discovery first flew in August 1984, completed 39 trips into space, and remains NASA's oldest surviving shuttle. Most famously, it delivered the Hubble telescope into space and returned Mercury astronaut John Glenn to orbit at age 77. Over its 27 years in service, Discovery logged nearly 143 million miles and transported more crew members into outer space than any other NASA vehicle.

Why did Discovery fly over D.C.?
The journey from the Kennedy Space Center in Cape Canaveral, Fla., to Dulles Airport in northern Virginia was Discovery's last, as it will "now settle into its retirement home" at the Smithsonian's National Air and Space Museum annex. There, it will replace the Enterprise, a prototype orbiter from the '70s that is being transferred to New York City's Intrepid Sea, Air, and Space Museum later this month. 

Where did it fly exactly?
The shuttle made its journey flying piggyback on a 747, and flew at an especially "low and slow" speed and altitude — about 1,500 feet — for the express purpose of showing off for fans. It made a sweep of the entire capital region, circling over the monuments, the U.S. Capitol, and the Potomac River, before landing at Dulles. 

And people were excited?
Absolutely. The 2,000-car parking lot of the Udvar-Hazy Center next to the airport was full by 9 a.m., while thousands flocked to the National Mall for a sighting.  More than 200 spectators crowded onto the Memorial Bridge, some traveling from as far away as Michigan. Adrienne Watson, an aide to Rep. Loretta Sanchez (D-Calif.), reports that congressional staffers ran outside to see it, cheering as Discovery flew by. "It was a pretty special moment and a great show," she tells the Los Angeles Times.

What else are people saying?
"Breathtaking," said Virginia resident Rupi Stepniczka, who watched the shuttle from Dulles' parking garage deck. "It is the end of an era," said D.C. resident Fred Weiss, as his wife wiped tears from her cheeks. "I think it strikes the psyche of adults more than kids. We remember the Apollo missions." Yes, "it's a very emotional, poignant, bittersweet moment," says former astronaut Mike Mullane, a veteran of three shuttle missions.

What next?
Discovery's two sister ships, Endeavour and Atlantis, have also been retired, and are being sent to exhibits at the California Space Center in Los Angeles and the Kennedy Space Center Visitor Complex in Florida later this year. When the Enterprise arrives in New York City next week, it will have a low-altitude flight parade similar to Discovery's, flying over the Statue of Liberty and other famous sites before being placed on a barge in the Hudson River to be transported to the Intrepid museum.  

Watch Discovery in action:

Sources: CBS News, Gothamist, LA Times, Reuters, TwitterUSA Today, Wash. Post