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 the earth. All else being equal, the more mass an object has, the more it will weigh. Also, the closer an object is to the earth, the more it will weigh.

The space shuttle orbits at an altitude of about 320 kilometers (200 miles) above the 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 the earth's gravity acting on them is still 90 percent as strong as if they were on the earth's surface.

So, the astronauts experience weightlessness not because there is no gravity. Rather, weightlessness occurs because they and their spacecraft are free falling in gravity. Just as the ZOOM cast demonstrates in this activity, 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 as the water floats and stays inside the perforated cup.

But what keeps the space shuttle from free falling to the earth like the cup of water? 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 being pulled toward the center of the earth, its free-fall path is parallel to the earth's curvature. In other words, the spacecraft continually falls all the way around the earth.