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Microgravity What is Microgravity? Gravity is a force that governs motion throughout the universe. It holds us to the ground, keeps the Moon in orbit around the Earth, and the Earth in orbit around the Sun. Many people mistakenly think that there is no gravity above the Earth's atmosphere, i.e., in "space," and this is why there appears to be no gravity aboard orbiting spacecraft. Typical orbital altitudes for human spaceflight vary between 192 and 576 km above the surface of the Earth. The gravitational field is still quite strong in these regions, since this is only about 1.8% the distance to the Moon. The Earth's gravitational field at about 400 km above the surface maintains 88.8% of its strength at the surface. Therefore, orbiting spacecraft, like the Space Shuttle or Space Station, are kept in orbit around the Earth by gravity. The nature of gravity was first described by Sir Isaac Newton, more than 300 years ago. Gravity is the attraction between any two masses, most apparent when one mass is very large (like the Earth).
The acceleration of an object toward the ground caused by gravity alone, near the surface of the Earth, is called normal gravity, or 1g. This acceleration is equal to 9.8 m/sec2). If you drop an apple on Earth, it falls at 1g. If an astronaut on the Space Station drops an apple, it falls too; it just doesn't look like it's falling. That's because they're all falling together; the apple, the astronaut, and the Station. But they're not falling towards the earth, they're falling around it. But since they're all falling at the same rate, objects inside of the Station appear to float in a state we call zero gravity (0g), or more accurately microgravity (1x10-6g.)
Creating Microgravity The condition of microgravity comes about whenever an object is in "free fall": that is, it falls faster and faster, accelerating with exactly the acceleration due to gravity (1g). As soon as you drop something (like an apple) it is in a state of "free fall". The same is true if you throw something: it immediately starts falling towards the Earth. But how does something fall around the Earth? Newton developed a "thought experiment" to demonstrate this concept. Imagine placing a cannon at the top of a very tall mountain.
Once fired, a cannonball falls to the Earth. The greater the speed, the farther it will travel before landing. If fired with the proper speed, the cannonball would achieve a state of continuous free-fall which we call orbit. The same principle applies to the Space Shuttle or Space Station. While objects inside them appear to be floating and motionless, they are actually traveling at the same orbital speed as their spacecraft: 28 000 km per hour! Objects in a state of free-fall or orbit are said to be "weightless." The object's mass is the same, but it would register "0" on a scale. Weight varies depending on if you are on the Earth, the Moon, or in orbit. But your mass stays the same, unless you go on a diet!
Experiment Methods Microgravity, which can be fully utilized in space, can also be utilized on Earth for a limited time by several types of facilities. Methods of generating microgravity are shown in the table below.
Why research in microgravity? A microgravity environment provides the basis for a unique laboratory in which scientists can investigate the three fundamental states of matter: solid, liquid, and gas. Microgravity conditions allow scientists to observe and explore phenomena and processes that are normally masked by the effects of Earth's gravity. Since there is almost no buoyancy in the space environment, light materials and heavy materials can be mixed uniformly. Defect-free crystal structures can be formed because there is almost no weight or depth pressure. These features will enable researchers to perform research which will bring about scientific and engineering evolutions, such as discovering and exploiting phenomena peculiar to the space environment or creating new material or medicines. Also, research to clarify the effects of gravity on biological structures and functions, such as development, differentiation, and growth of living things on the Earth will be able to be performed. The results of experiments in microgravity are used to challenge and validate contemporary scientific theories, identify and describe new physical phenomena that can be only explored in a microgravity environment, and engender the development of new theories as a result of unexpected or unexplained discoveries -- often the most exciting part of research. The different research domains within the field of microgravity are elaborated in relation to the question "Why research in microgravity?" on the following pages: Text Sources
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