# Why Does A Satellite Stay In Orbit?

Short Answer: It's a combination of inertia and gravity.

In space, where there is (almost) no air resistance because there is (almost) no air, a moving object would tend to just drift along in a straight line at a constant speed, unless a force is applied that changes the direction of the object's movement (eg: Collision with another object, firing a rocket, etc). This tendency to keep "moving right along" is called inertia.

In the case of an orbiting satellite, the force being applied is the Earth's gravitational pull, which tries to pull the satellite back towards the Earth's surface. The further away from Earth you get, the less effect the force of gravity will have.

So, to put it all together, NASA (or whoever) release a satellite into space at just the right speed so it is not going quite fast enough to drift away and escape Earth's gravity, and at just the right distance from Earth so gravity can't quite pull it back down.

The combination of the satellite's inertia and the Earth's gravity results in the satellite travelling in a circular path around the Earth, ie: A stable orbit. It is exactly the same combination of forces that cause the moon to orbit the Earth, and the Earth and other planets to orbit the Sun.

Of course, there are some problems with this. Space isn't a perfect vacuum, so there's a tiny amount of "air resistance", causing ever-so-slight changes in the satellite's movement. Secondly, even NASA can't get the speed and distance accurate enough to keep the satellite in a stable orbit for much longer than a few years.

Without slight corrections to the satellite's course, one of two things happens: Either the satellite ends up travelling too fast and escapes Earth's gravity, floating away into space, or gravity eventually wins this little game of celestial tug-of-war, and the satellite comes back down too close to the Earth, entering the atmosphere and burning up.

To extend a satellite's lifetime, small rockets can be built into the satellite, and used to correct its orbit, or a satellite can be captured by a spacecraft (eg: Space Shuttle) and moved back into a more stable orbit. In most cases, though, a satellite will outlive its usefulness well before this needs to happen. SkyLab, for example, was going to be moved into a higher orbit by the first Space Shuttle in 1979, but when the first Space Shuttle's completion was delayed for two years, Skylab was left to re-enter the atmosphere and burn to pieces. The Hubble Telescope might be an exception to this - it will probably be useful for many years to come.

-B
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Once a satellite gets into an orbit after that even with a little energy it can revolve around the planet with the help of gravity.
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A satellite in orbit around a planet is actually falling while moving forward. But it is moving forward just fast enough that its rate of fall matches the curvature of the planet. A satellite is fired into orbit and is pushed hard by the rocket fuel to reach a very high speed, so it starts with a lot of energy. As there is very little atmosphere at the sort of height that satellites orbit at, there is very little friction to slow it up. So it keeps moving at the same speed.

Depending on the strength of gravity of the planet, the satellite will be pulled downwards at a particular rate. So on different planets there will be different speeds required to keep a satellite in orbit around it.

But while the amount of atmosphere at the great heights at which satellites orbit is very low, there will often be a tiny bit of air, and a tiny bit of friction will occur. Over a long period of time this can be enough to slow it down a bit and then it slowly starts to spiral closer, meets more air and slows up more. Eventually it burns up due to the friction.
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A satellite stays in the orbit due to the balance of two factors:

1- Velocity or the speed at which it would travel in a straight line.
2- The gravitational pull between the earth and the satellite.

To illustrate this principle, attach a small weight or a ball to a string and swing it around in a circle. If the string were to break, the ball would fly off in a straight line but because it is tethered (like gravity tethers a satellite) it orbits you.

We can explain it with the general example:

Imagine that you could climber an imaginary mountain who's summit spokes above the earth's atmosphere (It would be about 10 times higher then Mount Everest), if you threw a baseball from the mountain top, it would fall to the ground in a curving path. Two motions act upon it: trying to go in a straight line and falling toward earth. The faster you throw the ball, the farther it will go before it hits the ground. If you could throw the ball at a speed of 17,000 mph the ball would not reach the ground. It would circle the earth in a curved path; it would be in orbit. It would be travelling at 5mps and take about 10 minutes to cross the United States.

This is the speed needed to put satellites into orbit, which is why the space shuttle and other satellites require such powerful boosters.
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A satellite "stays up" because the centripedal force from its orbiting offsets the force of gravity pulling it back down to earth. If you think about a merry-go-round spinning around, you have to hold on to it to keep from getting thrown off. For a satellite in orbit, the force of gravity is being offset by that same type of force that you feel on a merry-go-round.
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