SATELLITES AND ORBITS

« direction parallel with the ground.

It will then travel round and round the Earth at the same height as a satellite.

But we must make sure that the body is launched high enough so that it is above the Earth's atmosphere.

Then there is no air resistance to slow it down.

So it remains lapping the Earth at the same speed. 2.Satellite orbits We call the path a satellite takes through space, its orbit.

And the speed it must have to remain in orbit is called the orbital velocity.

The orbital velocity varies according to the height of the satellite's orbit.

It gets lower as the orbit gets higher. This is because gravity gets weaker as distance from the Earth increases; in other words, the satellite does not have to travel so fast to overcome it. The orbital velocity quoted earlier, 28,000 km an hour, is for an orbit about 150 km high.

For a satellite in an orbit 1,500 km high, the orbital velocity is less than 26,000 km an hour. In an orbit 150 km high, a satellite takes less than 90 minutes to circle the Earth once.

This length of time is called the orbital period.

Naturally, the orbital period of a satellite varies according to the height of its orbit.

It increases with increasing height.

This is because the orbit is longer and the satellite is travelling more slowly in it. So at a height of 1,500 km, the orbital period has increased to nearly two hours.

At a height of 15,000 km, the orbital period is nine hours.

And at a height of 35,900 km, it is no less than twenty-four hours. At a height of 385,000 km, the orbital period of a satellite would be twenty-seven-and-a-third days.

In fact there is a satellite of the Earth at that height, and with this orbital. »