Micro-g environment

The International Space Station in orbit around Earth, February 2010. The ISS is in a micro-g environment.

The term micro-g environment (also µg, often referred to by the term microgravity) is more or less synonymous with the terms weightlessness and zero-g, but indicates that g-forces are never exactly zero—just very small (on the ISS, for example, the small g-forces come from tidal effects, gravity from objects other than the Earth (such as astronauts, the spacecraft, and the Sun), and, occasionally, air resistance).[1][2] 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[3] 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 travelling an enormous distance, rendering it highly 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 kilometres 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 kilometres. (On Earth the gravity due to the rest of the Milky Way is already attenuated by a factor greater than one million, so we do not need to move away further from its center[citation needed]). Thus it is not impossible, but it has only been achieved so far by four interstellar probes (Voyager 1 and 2 of the Voyager program, and Pioneer 10 and 11 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.

Location Gravity due to Total
Earth Sun rest of Milky Way
Earth's surface 9.81 m/s2 6 mm/s2 200 pm/s2 = 6 mm/s/yr 9.81 m/s2
Low Earth orbit 9 m/s2 6 mm/s2 200 pm/s2 9 m/s2
200,000 km from Earth 10 mm/s2 6 mm/s2 200 pm/s2 up to 12 mm/s2
6×106 km from Earth 10 μm/s2 6 mm/s2 200 pm/s2 6 mm/s2
3.7×109 km from Earth 29 pm/s2 10 μm/s2 200 pm/s2 10 μm/s2
Voyager 1 (17×109 km from Earth) 1 pm/s2 500 nm/s2 200 pm/s2 500 nm/s2
0.1 light-year from Earth 400 am/s2 200 pm/s2 200 pm/s2 up to 400 pm/s2

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.

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العربية: جاذبية صغرى
беларуская: Мікрагравітацыя
català: Microgravetat
español: Microgravedad
فارسی: ریزگرانش
français: Micropesanteur
עברית: חוסר משקל
português: Microgravidade
slovenčina: Mikrogravitácia
slovenščina: Lebdenje
српски / srpski: Микрогравитација
srpskohrvatski / српскохрватски: Mikrogravitacija
українська: Мікрогравітація