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There are many reasons for space flight. Space flight carries scientific instruments, and sometimes humans, high above the ground, permitting us to see Earth as a planet and to study the complex interactions of atmosphere, oceans, land, energy, and living things. Space flight lofts scientific instruments above the filtering effects of the atmosphere, making the entire electromagnetic spectrum available and allowing us to see more clearly the distant planets, stars, and galaxies. Space flight permits us to travel directly to other worlds to see them close up and sample their compositions. Finally, space flight allows scientists to investigate the fundamental states of matter - solids, liquids, and gases - and the forces that affect them in a microgravity environment. The study of the states of matter and their interactions in microgravity is an exciting opportunity to expand the frontiers of science. Investigations include materials science, combustion, fluids, and biotechnology. Microgravity is the subject of this teacher's guide.

What Is Microgravity?

The presence of Earth creates a gravitational field that acts to attract objects with a force inversely proportional to the square of the distance between the center of the object and the center of Earth. When measured on the surface of Earth, the acceleration of an object acted upon only by Earth's gravity is commonly referred to as one g or one Earth gravity. This acceleration is approximately 9.8 meters/second squared (m/s 2 ).

The term microgravity (mg) can be interpreted in a number of ways depending upon context. The prefix micro - (m) is derived from the original Greek mikros, meaning "small." By this definition, a microgravity environment is one that will impart to an object a net acceleration small compared with that produced by Earth at its surface. In practice, such accelerations will range from about one percent of Earth's gravitational acceleration (aboard aircraft in parabolic flight) to better than one part in a million (for example, aboard Earth-orbiting free flyers).

Another common usage of micro- is found in quantitative systems of measurement, such as the metric system, where micro- means one part in a million. By this second definition, the acceleration imparted to an object in microgravity will be one-millionth (10 -6 ) of that measured at Earth's surface.

The use of the term microgravity in this guide will correspond to the first definition: small gravity levels or low gravity. As we describe how low-acceleration environments can be produced, you will find that the fidelity (quality) of the microgravity environment will depend on the mechanism used to create it. For illustrative purposes only, we will provide a few simple quantitative examples using the second definition. The examples attempt to provide insight into what might be expected if the local acceleration environment would be reduced by six orders of magnitude from 1g to 10 -6 g.

If you stepped off a roof that was five meters high, it would take you just one second to reach the ground. In a microgravity environment equal to one percent of Earth's gravitational pull, the same drop would take 10 seconds. In a microgravity environment equal to one-millionth of Earth's gravitational pull, the same drop would take 1,000 seconds or about 17 minutes!

Microgravity can be created in two ways. Because gravitational pull diminishes with distance, one way to create a microgravity environment is to travel away from Earth. To reach a point where Earth's gravitational pull is reduced to one-millionth of that at the surface, you would have to travel into space a distance of 6.37 million kilometers from Earth (almost 17 times farther away than the Moon). This approach is impractical, except for automated spacecraft, since humans have yet to travel farther away from Earth than the distance to the Moon. However, a more practical microgravity environment can be created through the act of free fall.

We will use a simple example to illustrate how free fall can achieve microgravity. Imagine riding in an elevator to the top floor of a very tall building. At the top, the cables supporting the car break, causing the car and you to fall to the ground. (In this example, we discount the effects of air friction on the falling car.) Since you and the elevator car are falling together, you will float inside the car. In other words, you and the elevator car are accelerating downward at the same rate. If a scale were present, your weight would not register because the scale would be falling too (Figure 1).

drawing of 4 elevators demonstrating relationship between acceleration and weight