Star date: 09:21:99

Free Fallin' Space Traveler

Since we are all space travelers aboard the planet Earth, we will look at one aspect of space which nearly everyone believes they already know about. Everyone has seen images taken from space in which objects are floating around the cabin of a spacecraft. Whether it is of astronauts flipping around clipboards in midair, cosmonauts aboard the long weary Mir doing simple "zero gravity" demonstrations for schoolchildren, or Homer Simpson floating around the space shuttle cabin hunting down floating potato chips, almost everyone knows that objects in space are weightless. The trouble is, almost everyone is wrong.

The first part of the explanation of this phenomenon is to understand the difference between mass and weight. You can refer to the weight of an object on the surface of the Earth, and you can assign it a mass, but they are not the same thing.

Mass is simply the measure of the objects inertia. Newton stated, in one of his three well known laws of physics, that an object in motion tends to remain in motion until acted upon by another force, and that an object at rest tends to remain at rest until acted upon by another force. Now, gravity is really just an acceleration toward the center of any object, such as the Earth. If we were to drop an object near the surface of the Earth, it would accelerate at 9.8 meters (or about 32 feet) per second squared. What that means is that a free falling object near the surface of the Earth will fall 9.8 meters (or 32 feet) per second faster than it was falling one second before.

Weight is simply the mass of an object multiplied by the gravitational acceleration wherever it may be. The only unit in the English system of measurement which can easily describe mass is the slug, which is almost never used, so we must use the metric system to express mass. A box of cereal, for instance, might have a weight of 17.2 oz., and a mass of 487 grams. If we were to bring it to the Moon, where the gravity is only 1/6 of what it is on Earth, we would find that it would have a weight of only 17.2 divided by 6, or slightly under 3 ounces. It would, however, still have a mass of the original 487 grams.

Newton also deduced that the gravitational attraction of two objects for each other was proportional to the mass of the two objects multiplied together, and then divided by the square of the distance between them. If we accept the gravity on the surface of the Earth as being equal to one "G", what is the gravity aboard, say, the space shuttle? Since we need to measure the distance from the center of one body to the center of another we can see that if we are on the surface of the Earth, we are about 6,370 km from the center of the Earth. The space shuttle regularly orbits almost 200 kilometers above the Earth, or 6,570 km from the center of the Earth. Since the masses of both objects are remaining the same in both equations, we can compare just the squares of the distances. We find the the gravity aboard the space shuttle is 6,370 squared divided by 6,570 squared, or about 94% what it is on Earth.

Wait a minute! If the gravity aboard the space shuttle is nearly the same as it is on Earth, why do objects seem to float in space? To answer this question, we need to go back to the 17th century, and discuss Galileo. He discovered that all objects (ignoring the effects of air resistance) fall at the same speed. Why is this? Because although heavier objects are attracted more readily to the center of the Earth because of their weight, they also have a greater resistance to change in movement due to their greater mass. These effects perfectly counter act one another, and the objects fall at the same
> speed.

Any orbiting body, including the space shuttle or the International Space Station is in constant free fall. An astronaut who lets go of a tennis ball inside of a cabin will see that ball float in space before him or her, because the spacecraft and the tennis ball are falling towards the Earth at the same speed, therefore the ball can never hit the floor of the spacecraft. The spacecraft has a greater weight than the ball, but also a greater inertia. That is what you are seeing when
> it appears that there is "zero gravity" in space.

You will continue to hear space reporters on news programs speaking of astronauts being in "zero gravity", but now you know better.

Clear skies, and good viewing.

"Understanding is joyous" - Carl Sagan