Page 10 - Curriculum Visions Dynamic Book

Neutralizing gravity
The grAvitAtionAl force of the Earth decreases with the square of the distance from the Earth’s surface. So, to begin with, the reduction of grAvity with height is small. The spacecraft has to travel several million kilometres before the gravitational effect of the Earth becomes insignificant for it. That is why a spacecraft cannot just switch off its engines when it gets out of the Earth’s atmosphere. If it did that,
it would fall back to the Earth.
Similarly, astronauts cannot remain stationary and
float in space. If they did that, they would quickly fall to the Earth.
The only way to stay in space and be close to the Earth is to orbit the Earth. That is why all near-Earth spacecraft spin around the Earth. Even those that stay over the same part of the Earth all the time are in orbit—they are just traveling at the same speed as the Earth is revolving.
There is no single optimum height for a successful orbit. But in lower orbits objects will tend to move faster than the Earth’s rotation, while in higher orbits they will match it (geosyncHronous orbit). In very high orbits (the Moon, for example) they will be slower than Earth’s turning.
Launches and flight paths
To get into orbit around the Earth, the spacecraft begins by rising vertically as described above. But that will not get it into a position where it can orbit the Earth. It must incline its path (called a trAjectory) so that at the desired height the craft points along a tAngent to the Earth. When it is achieved, the engine can be cut off.
At this point the centrifugAl force on the spacecraft, due to its velocity, is exactly balanced by the force of gravity pulling it back to the Earth.
The orbital altitude must be chosen so that the object
is above most of the Earth’s atmosphere. In this position friction with the air is very small, and the object will stay in orbit for some considerable time before it falls back to the Earth, or until motors have to be fired.
 A trajectory for going into orbit.
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Begin turn toward orbit
Launch