Early Tuesday morning, the space shuttle Discovery landed at California's Edwards Air Force Base. The shuttle had been scheduled to land at the Kennedy Space Center in Cape Canaveral, Fla., but was diverted due to bad weather. Since Kennedy Space Center is the shuttle's only launch site, NASA now has to bring Discovery from California to Florida. How do you transport a space shuttle across the country?
Hitch it to a tricked-out jumbo jet. NASA owns a pair of Boeing 747s that stay at Edwards Air Force Base for most of the year awaiting wayward space shuttles. The two jets, dubbed Shuttle Carrier Aircraft, are almost identical in appearance and functionality. The SCAs, known as NASA 905 and NASA 911, are both erstwhile commercial airplanes with a few modifications. All of the interior seats and furnishings have been removed. And, most notably, each carrier aircraft has an extra pair of vertical stabilizers that act like rudders and help the plane compensate for its unwieldy cargo.
Before attaching Discovery to the SCA, NASA personnel will lift the shuttle off the ground using a crane-like device. They'll then roll the 747 underneath before lowering the shuttle onto to the top of the airplane. The same devices that attach the shuttle to its orange, external fuel tank during blast-off are used to bind the shuttle to its host plane. A crew of approximately 170 will prep the shuttle for its ride home. The process typically takes about a week.
The shuttle is roughly three-fourths the length of the carrier plane. Binding the two together creates an unwieldy, double-decker contraption. (For photos of the SCA in action, click here.) Due to the piggybacked plane's lousy aerodynamics and sizeable weight, the SCA's fuel efficiency is significantly less than that of a commercial airliner. As a result, an SCA can't fly more than 1000 nautical miles without refueling. The trip back will take up to 12 hours of flight time spread over two or three days.
Throughout the voyage, a plane known as a "pathfinder" flies ahead and acts as a kind of weather scout. The shuttle's external tiles are highly susceptible to the elements. To make sure they don't get damaged, the pathfinder looks out for storm clouds and warns the SCA pilots of upcoming high winds or turbulence.
According to NASA, the price tag for a cross-country shuttle trip is around $230,000. Before this year, NASA last ferried a shuttle back to Florida in 2002. Since the early 1980s, NASA has carried out 51 shuttle-transport missions, including a trip across the Atlantic to take the Enterprise to the Paris Air Show.
[Update, August 10: Many readers have been wondering how the Enterprise made the long journey across the Atlantic Ocean to the Paris Air Show. The shuttle traveled over the water by making the (relatively) short trip from Goose Bay, in northeast Canada, to Keflavik, on the southwestern tip of Iceland. The Enterprise then continued on to England and West Germany before reaching Paris.]
Special thanks to Jim Rostohar at NASA and several Slate readers for asking the question.