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The term micro-g environment (also ''µg'', often referred to by the term microgravity) is more or less a synonym of weightlessness and ''zero-G'', but indicates that g-forces are not quite zero, just very small.〔〕 The symbol for microgravity, ''µg'', was used on the insignias of Space Shuttle flights STS-87 and STS-107, because these flights were devoted to microgravity research in low Earth orbit. ==Absence of gravity== A "stationary" micro-g environment〔Depending on distance, "stationary" is meant relative to Earth or the Sun.〕 would require travelling far enough into deep space so as to reduce the effect of gravity by attenuation to almost zero. This is the simplest in conception, but requires traveling an enormous distance, rendering it most impractical. For example, to reduce the gravity of the Earth by a factor of one million, one needs to be at a distance of 6 million km from the Earth, but to reduce the gravity of the Sun to this amount one has to be at a distance of 3.7 billion km. (The gravity due to the rest of the Milky Way is already smaller than one millionth of the gravity on Earth, so we do not need to move away further from its center). Thus it is not impossible, but it has only been achieved so far by four interstellar probes (Voyager 1 and 2, part of the Voyager program, Pioneer 10 and 11 part of the Pioneer program) and they did not return to Earth. To reduce the gravity to one thousandth of that on Earth's surface, one needs to be at a distance of 200,000 km. From stationarity the gravity from "the rest of the Milky Way" would cause a free fall, covering a distance of 100 pm in one second, 360 nm in one minute, 1.3 mm in one hour, 70 cm in one day, 37 m in one week, 100 km in one year, and 10,000 km in 10 years (at a speed at that last location of 6 cm/s). At a distance relatively close to Earth (less than 3000 km), gravity is only slightly reduced. As an object orbits a body such as the Earth, gravity is still attracting objects towards the Earth and the object is accelerated downward at almost 1g. Because the objects are typically moving laterally with respect to the surface at such immense speeds, the object will not lose altitude because of the curvature of the Earth. When viewed from an orbiting observer, other close objects in space appear to be floating because everything is being pulled towards Earth at the same speed, but also moving forward as the Earth's surface 'falls' away below. All these objects are in free fall, not zero gravity. Compare the gravitational potential at some of these locations. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Micro-g environment」の詳細全文を読む スポンサード リンク
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