Photo by jsmjr

After yesterday’s Space Shuttle Discovery fly-over of D.C., plenty of people wondered how exactly a 747 could fly with a shuttle strapped to its back.

Pretty easily, it seems. CNN reports today that the shuttle—which is stripped of just about everything inside, making it an empty shell—is substantially lighter than what a 747 would usually carry in passengers and luggage. Space Shuttle Discovery weighed some 488,000 pounds, while a fully-loaded 747 can carry up to 800,000 pounds worth of people, luggage and equipment:

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.

That being said, flying a 747 with a shuttle on top certainly isn’t the same as flying one with nothing more than passengers in it. According to NASA, the shuttle-shuttling 747s handle more sluggishly at slow speeds, burn about twice the amount of fuel and have to be more careful about avoiding inclement weather. (A normal 747 can go over 5,000 nautical miles; the retrofitted NASA 747s barely exceed 1,100 nautical miles in one go.) In 2009, a NASA pilot wrote of the experience of shuttling Space Shuttle Atlantis on a 747:

We took off from Columbus AFB on their 12,000 foot runway, of which I used 11,999 1/2 feet to get the wheels off the ground. We were at 3,500 feet left to go of the runway, throttles full power, nose wheels still hugging the ground, copilot calling out decision speeds, the weight of Atlantis now screaming through my fingers clinched tightly on the controls, tires heating up to their near maximum temperature from the speed and the weight, and not yet at rotation speed, the speed at which I would be pulling on the controls to get the nose to rise.

Yikes. Makes yesterday’s fly-over even more impressive, doesn’t it?