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tidal force

The ability of one massive object to cause tides on another object nearby. If the tidal force is large enough it can give rise to tidal heating.

A tidal force comes about because of the differences in gravitational pull on an object due to a large mass around which the object is moving. In the case of a space station in Earth orbit, parts of the station that are further away from the Earth are pulled less strongly so that the centrifugal force of the orbit is not quite balanced by gravity and there is an net upward tidal force. Similarly, for parts closer to the Earth there is a downward tidal force. These opposing forces try to stretch the station along a line that passes through Earth's center. One effect is that tidal forces will make any elongated object tend toward an orbit with its long axis pointing to the Earth's center. Either the space station has to be designed to orbit in this way, or it must have an orientation correction system to counter the orientation drift that the tidal forces will produce. Another effect will be on objects within a space station. Tidal forces are one of the reasons it is impossible to have perfectly zero-gravity conditions in orbit. The fact that microgravity always exists has important consequences for some experiments and manufacturing processes in space.

Dangerous tidal effects would be most evident near highly condensed objects such as black holes. Tidal forces are proportional to d/R3 where d is the density of the gravitating mass and R is the distance from it. Using this formula it is possible to calculate that an astronaut would be torn apart, head from toe, if he approached a 6-solar-mass black hole, feet first, closer than about 5,300 km.

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