When we hear the word weightless, it's hard not to think of astronauts floating around in space. Surprisingly, however, an object does not need to be in outer space to experience the sensation of weightlessness. In fact, being in space, by itself, does not make a spacecraft or its occupants weightless.

The weight of an object is a function of its mass -- essentially how much stuff it's made of -- and how far it is from the center of a very large object, like Earth. All else being equal, the more mass an object has, the more it will weigh. Also, the closer an object is to Earth, the more it will weigh.

The space shuttle orbits at an altitude of about 320 kilometers (200 miles) above Earth's surface. At this distance, the spacecraft and the astronauts still weigh about 90 percent as much as they do on the ground. That is, the force of Earth's gravity acting on them is still 90 percent as strong as if they were standing on the ground.

So, the astronauts experience weightlessness not because there is no gravity. Rather, weightlessness occurs because they and their spacecraft are free-falling in gravity. When two objects fall freely, one can float inside the other until they both reach the ground. The astronauts float inside the space shuttle just water might float inside a falling cup -- even if that cup had holes in it.

But what keeps the space shuttle from free-falling to Earth's surface and crashing into it? A spacecraft can maintain its free-fall for a very long period of time by traveling fast enough -- about 7.5 kilometers (4.7 miles) per second -- horizontally, so that even though it is constantly being pulled toward Earth's surface, its free-fall path is parallel to the planet's curvature.