It was a strange sight indeed -- one large aircraft carrying another large aircraft strapped on top. But that's exactly what skyward-looking folks in Washington saw Tuesday as Space Shuttle Discovery, atop a Boeing 747, made an inspiring pass over the capital in its final flight.

(CNN) -- It was a strange sight indeed -- one large aircraft carrying another large aircraft strapped on top. But that's exactly what skyward-looking folks in Washington saw Tuesday as Space Shuttle Discovery, atop a Boeing 747, made an inspiring pass over the capital in its final flight.

It makes you wonder: How can the 747 transport something that big and heavy? Well, the shuttle-airliner combo is not as heavy as you might first think. In fact, it's a relatively easy load to hoist.

When the shuttle missions started in the 1980s, NASA designated several places as landing strips for the orbiter, including Edwards AFB in California.

But all the launches were from Florida, so NASA needed a way to transport to Kennedy Space Center from Edwards. A shuttle can't fly itself after it leaves orbit, as it is basically a big glider. So NASA created the designed the Shuttle Carrier Aircraft, which is a Boeing 747 designed to fly with a shuttle on top.

All that's well-known, at least to space buffs. But what may surprise many people is that a Boeing 747 with a space shuttle strapped on its roof weighs less -- that's right, less -- than a 747 airliner full of passengers.

At first that might seem counterintuitive, but Lisa Malone, a spokeswoman for NASA, says the 747 they use is stripped clean of anything in the main cabin. There's a cockpit and a big empty shell.

But the 747s used by airlines have hundreds of seats, galleys, lavatories -- even a bar in some cases. Add 300-plus passengers, their luggage, food, water and assorted other cargo, and the weight adds up.

The 747 that carried Discovery weighed about 488,000 pounds. Jim Proulx, a spokesman for Boeing, says a commercial 747 airliner can weigh more than 800,000 when carrying passengers.

There will be another chance to see the Shuttle Carrier Aircraft in action later this week. That's when it will carry Space Shuttle Enterprise, which was used for tests but never flew in space, will be flown from Dulles International Airport in Virginia to its new home on board the USS Intrepid, a retired aircraft carrier turned floating museum in New York City.