Expert: Ray Wilkinson - 7/5/2007

Question
what is the effect of gravity on the space rocket while it enter into orbit of earth?


Answer
Hi Navdeep

Nice question (tricky answer). For anything to be in orbit around the Earth (or any other body), the forces acting on it must be in balance. Taking a circular orbit first as it's easier, Newton's laws of motion say that an object in motion will travel in a straight line unless a force acts on it. In this case, the gravitational force will act to make it fall towards the Earth. When this is exactly balanced by the centrifugal force trying to make the object fly off in a straight line then the object will be in orbit and will trace a circular path around it - i.e. it will be in orbit. Another way of looking at it is that the object is constantly falling towards the Earth but because it has a tangential component of its velocity it never falls in the right direction, as though the Earth is constantly dodging out of the way.

Now looking at non-circular orbits, a similar principle applies, but if the speed of the object is not exactly matched to its altitude it will not travel in a circular path. Imagine it is travelling too quickly - the path will be too straight and it will tend to move away from Earth. As it does so it will slow down, due to the Earth's gravitional field. As it slows down, it can't keep up the path it has, and will start to move back towards the Earth. As it does so, it will speed up again, straightening the path, and so on. It will describe an elliptical orbit with the Earth at the centre.

If the orbit, or part of it, is too close to Earth, then it will encounter drag from the atmosphere. This will gradually slow the body down, reducing its ability to maintain orbit. This is called a decaying orbit. Eventually it will pass too far inside the atmosphere, lose too much speed to maintain orbit at all and fall to Earth. It will normally travel very quickly and burn up before reaching the ground. Many satellites are actually in decaying orbits (often because they need to be at low altitudes, such as surveillance satellites), but the rate of decay is very predictable, so the time it will stay in orbit (which may be years) is known.

I hope this answers your question.