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In the 30 years of the space age, manned missions and probes to the planets have made the headlines. But satellites are where the huge investment will pay off

MANY countries are now committed to developing the technology to take advantage of space. Commercial companies, too, are interested in making profits from building and launching satellites and working in space. The risks and costs are great, however. As yet, it is governments that are the main investors in this new and growing area of science and technology.

Since 1957, when the Soviet Union launched Sputnik I, the world has put more than 3000 satellites into space. Today, satellites relay telephone calls, monitor the weather, survey the Earth鈥檚 surface for minerals, and gather sensitive military information.

Satellites fill the space round the Earth like a swarm of bees, kept there by the gravitational pull of our planet. Their orbits can vary from a few hundred to about 200 000 kilometres above the surface of the Earth. As long as a satellite moves at the correct speed to counteract the pull of gravity, engineers can choose the height for the job intended.

For communications, engineers may select a geostationary orbit 鈥� hovering over the same spot on Earth. For surveying, a low orbit will give very fine detail, or a higher orbit will cover a larger area. Perhaps the satellite needs to pass over the same area every day, or gradually move across to scan the whole globe.

The shape of orbits can vary too. A satellite that travels with just enough speed to stay up will be in an almost circular orbit. There are also elongated or eccentric orbits where a satellite travels in an ellipse around the Earth. As a satellite travels against the pull of gravity, it loses speed. At the farthest point or apogee, gravity pulls it back down and it accelerates towards the closest point of its orbit, the perigee. Here, it is travelling fast enough to start moving away from Earth again.

Communications 鈥� Voices from the sky

The best developed application of space is that of communications satellites 鈥� for relaying both telephone conversations and television pictures, for example. The latest satellites can simultaneously handle several thousand telephone calls, plus several TV pictures.

Microwaves carry information to and from the satellites: These are better than longer wave-length signals, because they are not reflected by the Earth鈥檚 atmosphere. On board the satellite are antennas for receiving and transmitting communications, and for receiving instructions that control its position in space.

A transponder amplifies signals, and beams them back down to Earth using a range of frequencies for the different channels, just as a radio station uses different frequencies. The signal the satellite beams down to the Earth spreads out, like a torch beam, covering a large area, its footprint.

Crucial for satellite operations are ground stations. These have antennas, plus equipment to convert outgoing signals into microwaves and vice versa. The ground station must be within the footprint.

Most satellites for communications use a geostationary or geosynchronous orbit that is 35 784 kilometres high. At this altitude, a satellite takes 24 hours to orbit round the equator 鈥� the same time as the Earth takes to spin round once on its axis. Consequently, the satellite always remains in the sky above the same point on the Earth鈥檚 surface.

One satellite in geosynchronous orbit can continuously relay signals between, say, the US and Europe. Three satellites, spaced at 120-degree intervals, can cover most of the world. For instance, a telephone call from London can reach Sydney via a satellite above the Atlantic and another above the Pacific.

The geosynchronous orbit is becoming crowded. At the moment, international regulations limit the number of satellites that can occupy the orbit to 180. If there were more, the satellites would be less than 2掳 apart, and their signals might interfere with each other.

Although satellites in geosynchronous orbit can transmit signals to most of the world, they cannot reach the high latitudes. This has caused headaches for the Soviet Union, which has had to adopt a highly elliptical orbit for its Molniya communications satellites. These have a perigee of about 500 kilometres and an apogee of 40 000 kilometres.

At the apogee, over the northern hemisphere, the satellite is travelling slowly, and it stays visible to the ground stations for several hours at a time. Three satellites, therefore, can provide 24-hour coverage. Not as elegant as satellites in geostationary orbit, but good enough.

Remote sensing 鈥� Observing the Earth

Another important application for satellites is remote sensing 鈥� gathering information about the Earth鈥檚 surface. The satellites carry a variety of instruments that measure different parts of the electromagnetic spectrum 鈥� visible light or infrared, for example.

Every material on Earth reflects, transmits and absorbs electromagnetic radiation in a characteristic way, and so has its own 鈥渟ignature鈥�. For example, wheat has a different signature to barley, healthy crops to unhealthy ones, an outcrop of coal to one of iron, a snow storm to a rain storm.

The satellites record this information as electrical signals, and send them back to Earth as radio messages. Computers then process the mass of data to produce images. With an eye in the sky, we can monitor crops and the weather, survey for minerals, spot forest fires and detect oil slicks.

Most remote-sensing satellites are in polar or near-polar orbits. In a true polar orbit, a satellite circles over both poles several times a day. As the Earth rotates below, it passes above a different area each time. In this way, it covers the whole surface of the world every few days.

A refinement of this is the Sun-synchronous orbit. The orbit is within a few degrees of a polar orbit, but the satellite stays in line with the Sun shining on the Earth, regularly passing over the same spot at the same time of day. The Sun-synchronous is a low, almost circular orbit, usually about 1000 kilometres high.

Remote-sensing satellites can now provide extremely detailed pictures. For example, a new French civilian satellite, Spot, can pick out individual London buses on the road. Military ones, which tend to be in lower orbits, can pick out individual tiles on a roof!

Microgravity 鈥� Industry in space

An orbit that industrialists and scientists are very interested in exploiting is the low-Earth orbit, which is between 500 and 1500 kilometres high. Low gravity there could allow us to make in space some materials that are difficult, or even impossible, to make on Earth. This space application, known as microgravity, is still in its infancy.

Typical products might include growing crystals and making alloys, medicines and lenses 鈥� all of which are affected by gravity on Earth. In addition, space is relatively free of impurities, and there is an abundant supply of cheap solar energy. Both the Soviet Union and the US have experimented to see which products are best manufactured in space.

Navigation satellites act as artificial stars, sending down as radio signals their position in space and the correct time. Ships and aeroplanes use signals from three satellites to calculate their position on Earth.

Many small sailing boats, equipped with small receivers, also use satellite navigation. Electronics companies are devising navigation systems for cars. The military, however, are the largest users 鈥� for guiding armies or missiles to their target. On their sophisticated systems, accuracy is to within 16 metres, and speed calculated to within 0.1 metres a second.

Most navigation satellites occupy a high-Earth orbit 鈥� at an altitude of between 10 000 and 20 000 kilometres. This orbit, though, is not popular. It coincides with one of the Van Allen belts 鈥� a region of energetic electrons, trapped by the Earth鈥檚 magnetic field. The electrons can upset the delicate electronics on board satellites.

Finally, there are scientific satellites in orbit. Most carry remote-sensing instruments, either to observe the Earth and its atmosphere or to look into space.

Many stars and bodies in space, for example, emit X-rays which the Earth鈥檚 atmosphere absorbs. Astronomers interested in this source of electromagnetic radiation need telescopes in orbit.

Satellites with instruments pointed down at our planet can measure ozone levels in the atmosphere, detect geological faults, observe the magnetic field, plot ocean currents and even track large colonies of animals.

Satellites need launchers to take them into space and to release them at the right speed and angle to enter into orbit. At present, space organisations use either expendable rockets that work for one launch only, or vehicles where most of the hardware is recovered after each flight.

A single rocket with a payload, however, cannot derive enough energy from the combustion of fuel to reach Earth orbit in one leap. The solution is to use multistage rockets.

In a three-stage rocket, such as Europe鈥檚 Ariane, one rocket plus its payload, which might be a satellite, is a payload of a larger rocket, which in turn is the payload of a still larger rocket. The empty tanks and engine of the largest, and then the second largest are discarded at relatively low altitudes after each completes its firing.

The final stage, which carries the satellite, drops away when it has reached the precise height and speed for the satellite鈥檚 orbit. During the flight, the rocket tilts so that it is travelling parallel to the Earth鈥檚 surface when it finally releases the satellite.

Only the two superpowers are autonomous in space, able to put both equipment and people in space and to explore the planets of the Solar System. The Soviet Union, which has so far developed several different types of expendable vehicles, is now trying to tempt the West to launch satellites on its rockets.

The US has concentrated on a partially recoverable vehicle 鈥� the space shuttle. It can ferry satellites as well as people into low-Earth orbit. The Orbiter, which looks like an aircraft, is mounted on a huge expendable fuel tank with two solid-fuel rocket boosters.

The rocket boosters and three main engines fire simultaneously to launch the shuttle. About two minutes into the flight, when the shuttle is at a height of 45 kilometres, it jettisons the boosters which fall by parachute into the sea to await recovery. Just before the craft reaches orbit, about eight minutes into the flight, it discards the large fuel tank. The crew then manoeuvres the Orbiter using the on-board engines.

The Orbiter can carry up to 29.5 tonnes in its cargo bay. Apart from launching satellites, it can bring them back in for repair or return them to Earth for refurbishment. At the end of the mission, the engines put the Orbiter into a re-entry path. It enters the atmosphere in a shallow glide to make an unpowered landing.

The European Space Agency, which now represents 13 countries in Europe, has its own satellite launcher in Ariane, designed to deliver satellites into geosynchronous orbit. There are three rockets in the family. Ariane 1 can lift 1.7 tonnes. Ariane 2 is more powerful, and will lift up to 2 tonnes. Ariane 3, with two solid-fuel boosters strapped onto the first stage, can carry 2.5 tonnes.

Next year, Europe will launch the yet more powerful Ariane 4. It will have a new first stage and up to four solid-fuel boosters, enabling it to put 4.3 tonnes into geostationary orbit.

Of the other space nations, Japan is developing a new range of rockets, which it hopes will compete with Ariane and the US space shuttle for international payloads. China is also seeking to carry out satellite launches for other countries. Sweden and the US have signed contracts to launch satellites on China鈥檚 Long March Rockets, which can carry up to 1.4 tonnes into orbit. India and Brazil are developing their own launchers and satellites for communications and remote sensing.

In future, perhaps early next century, fully reusable spacecraft will be launching satellites. This will help reduce the cost, because expensive hardware will not be wasted after every flight. Engineers believe that with modern light-weight materials and the possibility of burning oxygen scooped from the atmosphere, it will be feasible to reach orbit in one stage. Hotol, Britain鈥檚 proposed design for a spaceplane, is an example of a single-stage- to-orbit vehicle.

The space nations have spent many billions of dollars over the past 20 years in developing satellites and their associated technology. Already, millions of people around the world benefit. Communications, entertainments, national security, agriculture, air transport, weather forecasting and mineral prospecting are just some of the areas where satellites have made an impact. Now comes the time for profit for the satellite business.

How a rocket works

ALL ROCKETS work on the principle that an action in one direction causes an equal reaction in the opposite direction 鈥� Newton鈥檚 third law of motion. In a rocket, burning fuel gives off hot gases which are expelled through a nozzle to produce the force, or thrust, that lifts the vehicle off the ground.

A rocket works as easily in empty space as in the atmosphere. This is because it is powered not by hot gases pushing against the atmosphere, but by reaction to action. It is rather like a man stranded at the centre of a very slippery ice rink.

No matter how hard he flails his arms and legs, he will not move. But if, by some stroke of luck, he has a stack of briefcases with him, he can use these to move. If he throws them in one direction, one after another, he will begin to travel slowly in the opposite direction.

Early rockets, such as those developed by the ancient Chinese, used a solid fuel 鈥� gunpowder. Rocket development this century has focused on liquid fuels. Not only do they liberate more energy, weight for weight, than solid fuels, but they allow more control.

In a liquid-fuel rocket, the fuel will not burn unless mixed with an oxidiser. Unlike an aeroplane jet, it cannot take in oxygen from the atmosphere so it has to carry its own supply. The fuel and oxidiser are stored separately, and pumped into the combustion chamber where they burn explosively, producing gases that rush out through a nozzle. The amount of thrust is controlled by increasing or decreasing the rate at which fuel and oxidiser are pumped.

The fuel for early rockets, such as Germany鈥檚 V2 missile in the Second World War, was kerosene and oxygen. Now, rockets burn either hydrazine or cryogenic fuels 鈥� liquid hydrogen and liquid oxygen.

Hydrazine is an hypergolic fuel 鈥� in other words, it combusts spontaneously in the presence of an oxidiser such as dinitrogen tetroxide. Hydrazine is about 15-20 per cent less efficient than cryogenic fuels, but is simpler and more reliable.

Cryogenic fuels, for example, need to be refrigerated to remain liquid. The engine, therefore, needs a complicated system of pipes to carry the refrigerated fuel. The fuels also need an ignitor.

It is during the launch that cryogenic fuels come into their own, when as much efficiency as possible is needed to lift the rocket and its cargo off the ground. For repeated firings in orbit, the simpler hydrazine engine is a better choice. Satellites in geostationary orbit, for example, have small hydrazine engines that fire occasionally to keep them in the correct position.

Despite the complexity, Japan has developed a cryogenic engine for its H1 rocket. The Japanese have tested the H1 and, next year, it makes the first of seven planned launches, carrying a cargo of 0.55 tonnes. At the same time, they are developing the H2, which will be able to carry 2 tonnes into geosynchronous orbit.

Why a satellite doesn鈥檛 fall down

THE LAUNCHERS that take satellites into space have to travel upwards at a very high speed to counteract gravity. To deliver a satellite into orbit, a rocket has to accelerate the satellite to at least 8 kilometres a second 鈥� more than 30 times the speed of a jet aircraft.

To understand why this is so, and why a satellite stays up once in orbit, imagine firing a cannon ball at a shallow angle to the ground. After a short flight, gravity will pull the cannon ball back down. With more power, the cannon ball may go beyond the horizon but, eventually, it will fall back to Earth.

Now imagine a launcher that can shoot a cannon ball faster than any cannon ever built. As the cannon ball travels, gravity will pull it back to the ground. But because the Earth is round, the surface will seem to drop away below it.

If the cannon ball is travelling at 8 kilometres a second, the Earth will be falling away from it at exactly the rate it falls. The cannon ball will never get any closer; it will have achieved orbit. And once in space, there is nothing to slow the cannon ball, so it falls forever in a circle around the planet.

In the real world, a satellite can drop out of orbit. In a very low orbit, for example, it will meet resistance from the tenuous upper reaches of the Earth鈥檚 atmosphere. After a few months or even years, this resistance will slow the satellite so that it falls back to Earth.

In higher orbits, the irregular gravitational field of Earth and the gravity from the Sun and Moon all act to push satellites out of their correct orbit. For this reason, satellites have their own propulsion system. Computers continually monitor the satellite鈥檚 position, and can command the small rockets to fire so as to maintain the exact position.

The farther a satellite鈥檚 orbit is from Earth, the weaker the pull of gravity on it, and the slower the speed needed to stay in orbit. In a geosynchronous orbit, for instance, a satellite travels at just over 3 kilometres a second. If a craft travels faster than 11 kilometres a second, it will wrench free entirely of the Earth鈥檚 gravity and head off into the Solar System.

Further reading

The Space Business by Peter Marsh (Pelican) provides facts and figures about the uses of space that technology has made, and will make, possible.

The lllustrated Encyclopedia of Space Technology by Kenneth Gatland (Salamander) is a popular exposition, with excellent diagrams.

Jane鈥檚 Spaceflight Directory is the definitive source for details of past and future space missions. Its price (拢70.50 inc p&p) and the jargon may deter the casual reader.