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The term microgravity is used to describe environments where the force of gravity is present but has a negligible effect. Objects in orbit are not perfectly weightless due to several effects:

• In Low Earth orbit, the force of gravity decreases by one part per million for every three meter increase in altitude. Objects which have a non-zero size will be subjected to a tidal force, or a differential pull, between the high and low ends of the object.
• In a spacecraft in orbit, the centrifugal force is greater on the side of the spacecraft furthest from the Earth. This is also a tidal force.
• Objects within a spacecraft will slowly "fall" toward the densest part of the spacecraft. When they eventually come to rest on the wall of the spacecraft, they will have weight.
• Though very thin, there is some air at the level of the Space Shuttle's orbital altitude of 185 to 1,000 km. This atmosphere causes deceleration due to friction. This has the effect of giving objects a small "weight" oriented in the direction of motion. Above 10,000 km, this effect becomes negligible compared to the effect of the solar wind.
• "Floating" objects in the Space Shuttle are actually in independent orbits around the earth. If two objects are placed side-by-side (relative to their direction of motion) they will be orbiting the earth in different orbital planes. Since all orbital planes pass through the center of the earth, any two orbital planes intersect along a line. Therefore two objects placed side-by-side (at any distance apart) will come together after one quarter of a revolution. If they are placed so they miss each other, they will oscillate past each other twice per orbit. If they are placed one ahead of the other in the same orbital plane, they will maintain their separation. If they are placed one above the other (at different radii from the center of the earth) they will have different potential energies, so the size, eccentricity, and period of their orbits will be different, causing them to move in a complex looping pattern relative to each other.[1]

The symbol for microgravity, µg, was used on the insignia of Space Shuttle flight STS-107, because this flight was devoted to microgravity research. (This flight was also notable as being the final mission of the Space Shuttle Columbia, which broke up during re-entry.)

## ISS MicrogravityEdit

At the station's orbital altitude, the gravity from the Earth is 88% of that at sea level. The state of weightlessness is caused by the constant "free fall" of the ISS, which, because of the equivalence principle, is indiscernible from a state of zero gravity.[2] The environment on the station is, however, often described as microgravity, as the weightlessness is imperfect because of four effects:[3]

• The drag resulting from the residual atmosphere.
• Vibratory acceleration caused by mechanical systems and the crew on board the ISS.
• Orbital corrections by the on-board gyroscopes or thrusters.
• The spatial separation from the real centre of mass of the ISS—any part of the ISS not at the exact centre of mass will tend to follow its own orbit. However, as each point is physically part of the station, this is impossible, and so each component is subject to small accelerations from the forces, which keep them attached to the station as it orbits.[3] This is also called the tidal force.

## ReferencesEdit

1. "Weightlessness and Microgravity", David Chandler, The Physics Teacher, May 1991, pp. 312-13
2. "Microgravity" (aspx). British National Space Council (3 August 2009). Retrieved on 7 September 2009.
3. 3.0 3.1 "European Users Guide to Low Gravity Platforms" (PDF). European Space Agency (6 December 2005). Archived from the original on 24 Jun 2006. Retrieved on 16 May 2006.