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They are kept in orbit by achieving a balance between their speed and the force of gravity: speed pushes them outward, while gravity pulls them inward. The speed needed to stay in orbit at a given height is called orbital velocity. For orbit at a few hundred kilometres, the orbital velocity is 28,000 kph (17,500 mph).
Gravity is what keeps a satellite in orbit. When launched into a path that is at least fairly parallel to the Earth's surface at the right velocity, it will be in a state of continual free fall around the Earth. For each foot it falls towards the ground, earth moves out from under it by the same amount. Velocity is critical, too slow the satellite falls into the atmosphere and disintegrates, too fast it escapes from the Earth entirely. However, satellites once placed in orbit will not stay there on their own, especially in low orbits. Drag from the upper atmosphere, radiation pressure from the Sun, and the fact the earth is not perfectly spherical ensure that unless a satellite has some sort of propulsion system on-board, it will drift away from it's designated orbit. Eventually, their orbits decay and they crash to Earth as a streak of burning metallic debris, the surviving pieces of which end up reaching the ground or ocean. In high orbit, satellites will stay up there for millennia or even longer, but in low orbit left to themselves they will fall back to earth in a few years or decades unless they're re-boosted by an on-board rocket engine or thrusters. The exact lifetime a satellite has in any given orbit depends on the shape, mass, and the original orbit it was placed in, along with how much fuel it carries. For an rocket engine, a tank of fuel and oxidizer provides propellants which when mixed in the engine ignite on contact, producing thrust. This if often done using nitrogen tetroxide and a form of hydrazine, both of which are violently corrosive, toxic and explosive chemicals. Fueling a satellite is very dangerous because one leak or spill will cause a deadly explosion, and the fumes will kill any unprotected worker exposed to them. For these reasons, highly trained personnel in special protective suits fill the propellant tanks before the satellite is launched. Unlike most liquid rocket fuels, these however can be kept indefinitely in tanks without the insulation liquid hydrogen or oxygen requires. Thus the rockets are very simple and thus reliable. Thrusters often use a mono-propellant that reacts with a catalyst in the thruster, producing thrust when it become hot expanding gases which are expelled from the thrusters.
u have mentioned the low earth orbit satellites,but wat abt the medium and Geo Earth orbit satellites, how they sustain themselves in their orbits as on that much there is no Earth's gravitational force?
Anywhere have earth's gravitational force.
GEO is stationary to the earth.
And medium orbit satellite is moving to the earth observing point. But its orbit have period to scan the earth. Sometimes the satellite corrects its orbit if orbit if drifting away the designed orbit.
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