The Earth's pull, or the force of gravity, is very powerful. So how can overcome it and send bodies into space? In our normal experience, what goes up, must come down. If you throw a ball into the air, it travels up a short way and then arcs over and falls back to Earth, pulled back by gravity.
But the harder you throw the ball, the faster and farther it travels before gravity pulls it back to Earth. This gives a clue as to beat gravity and send a body into space - by speed.
Imagine you are on a very high mountain and can throw a ball at any speed parallel with the ground. If you throw the ball at, say, 2,000 km an hour (about the speed of a bullet), it will travel quite a way before falling back to the ground. If you throw it at 10,000 km an hour, it will travel way over the horizon before returning to the ground. The faster you throw it, the farther it travels.
But when you throw the ball at a speed of 28,000 km an hour, a strange thing happens. The ball curves around the Earth and comes back to you, staying the same height above the ground! It has become a satellite of the Earth.
The ball is still gripped by gravity and is still falling towards the Earth. But it is travelling so fast that the amount it falls towards the Earth is the same as the amount the Earth's surface curves away beneath it. And so it in effect stays at the same height above the ground. We say it is in a state of free fall.
So to launch a body into space, then, we must boost it to a speed of some 28,000 km an hour in a 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.
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 period. It is the Earth's only natural satellite, the Moon. The Moon obeys just the same rules as man-made satellites!
The height at which a satellite circles the Earth also determines how long it will remain in orbit. Satellites orbiting at heights of a few hundred kilometres, will fall back to Earth in a matter of years. This is because there are still minute traces of air at such heights. And eventually these traces slow down the satellites. They drop lower and lower until their speed drops below orbital velocity.
Then they succumb to gravity and plummet back to Earth. As they re-enter the fringes of the atmosphere, travelling at nearly 28,000 km an hour, friction with the air generates large amounts of heat. This causes them to burn up like meteors.
As satellites are launched into higher orbits, so their life expectancy increases to decades and centuries. It is estimated that the small, dense satellite Lageos 1, launched in 1976 into a 6,000-km high orbit, will remain in orbit for ten million years!
By then the map of the world will look quite different because of the drift of the continents. To inform any Earth people who may find Lageos in ten million years time, it carries a pictorial message telling them when and where it was launched. The time
scale is indicated in binary code. The three maps show: One, the Earth 270 million years ago. Two, the continents today. And three, the continents as they will look in ten million years.
Space scientists choose the orbit for a particular satellite according to what work it will perform in space. Many weather satellites, for example, are launched into an orbit that takes them over the North and South Poles. In this polar orbit, a satellite travels around the Earth, while the Earth spins beneath it. It thus views a different part of the surface each orbit, and scans the whole Earth every 12 hours.
Some satellites circle in an orbit that takes them over the equator, in an equatorial orbit. International communications satellites are launched into an equatorial orbit 35,900 km high. If you remember, in an orbit at this height, a satellite takes 24 hours to make one revolution.
Think what happens when a satellite is launched into such an orbit so that it travels over the equator and in the same direction that the Earth is travelling. It circles once round the Earth in 24 hours. But the Earth itself rotates on its axis in 24 hours. So, relative to the Earth, the satellite appears to be stationary in the sky. For this reason, this 35,900-km high, equatorial orbit is called a geostationary orbit.
The geostationary communications satellites of the international Intelsat network operate from three locations above the Equator over the Atlantic, Pacific and the Indian Oceans. From these vantage points, spaced roughly 120 degrees apart around the globe, they can relay communications between most countries.
Geostationary satellites travel in the plane of the Earth's equator. Most other satellites travel in an orbit at an angle to the plane of the equator. This angle is called the orbital inclination. An orbital inclination of a few degrees indicates that a satellite is travelling close to the equator; an orbital inclination of nearly 90 degrees indicates that it is travelling close to the Poles.
Geostationary satellites also travel in a circular orbit, remaining the same height above the ground. Most satellites, however, travel in an orbit in the shape of an ellipse, which takes them closer to Earth at some times than at others. The point in an orbit nearest to Earth is called the perigee; the point farthest away, the apogee.
The greater the difference between perigee and apogee the more elongated, or eccentric, is the orbit. The term orbital eccentricity describes the amount of elongation. It is the difference between perigee and apogee, divided by the distance between them (the major axis). A circular orbit (apogee equals perigee) has an orbital eccentricity of zero.
Let's look at the orbital characteristics of a practical satellite, such as the Russian Molniya communications satellite. The orbital inclination is about 63 degrees. The perigee is about 450 km in the southern hemisphere. The apogee is about 40,000 km in the northern hemisphere. The orbital eccentricity is about 0.75.
This orbit is chosen so that the satellite spends most of its orbit in northern skies, where it is above the horizon for ground stations in Russia's satellite communications networks.
