Satellites

Satellite is a word that simply refers to one body in orbit around another. There are natural satellites, such as our Moon, which orbits the Earth and artificial, man-made satellites that can serve a number of different purposes. They may be part of a television or telephone network or they can carry instruments to investigate the Earth and its atmosphere. Others monitor the Sun, or travel for many years carrying probes or landers to investigate the mysteries of the distant planets.

Until 4 October 1957, the Earth's only satellite was the Moon. Then the Soviet Union launched the first artificial satellite, a tiny metal sphere called Sputnik, and the world stood transfixed as it circled the Earth once every 90 minutes. Civilisation would never be the same again. Many thousands of satellites have been sent aloft since Sputnik launched the Space Age. Most have orbited the Earth, but several hundred have travelled to the Moon and beyond.

The Soviet Luna 1, launched on 2 January 1959, became the first man-made object to leave the Earth's gravitational influence and fly past the Moon. Its rocket was the first to reach the 25 000mph needed to escape the Earth's gravitational field.

By the early 1960s, the United States and the Soviet Union had begun to look further afield. NASA's Mariner 2 spacecraft skimmed past Venus in December 1962 and the first successful mission to Mars followed two years later.

During the 1970s, more advanced orbiters and landers were sent to Venus and Mars. By using planets to give spacecraft a gravitational 'kick' as they flew past, scientists discovered a faster, cheaper way to deliver satellites to their targets. This technique enabled Mariner 10 to investigate the innermost planet, Mercury, and opened the way for a 'grand tour' of the outer solar system by two Voyager spacecraft.

The Voyager spacecraft which was sent to explore the gas giants in the outer Solar System
Image credit: NASA

By modern standards, the satellites launched at the dawn of the Space Age were small and unsophisticated, although the size of the spacecraft largely depended on the performance of the available rockets. In this respect, the Soviet Union was well ahead of its American competitors.

Both superpowers and Europe continued to launch new missions so that, by the early 1960s, space was populated by satellites performing a variety of tasks devoted to weather, navigation, communications and military reconnaissance.

Satellites to find out about our weather

The weather experienced at any point on the Earth's surface is controlled by the state of the atmosphere. For centuries meteorologists have been measuring and recording atmospheric elements, such as pressure and temperature. However, it was not until the 1960s that it was possible to study the atmosphere from space. The first meteorological satellite, called TIROS I, was launched by the USA in 1960.

A meteorological satellite image of Hurricane Hugo
Image credit: ESA

Today many countries operate meteorological satellites and there is worldwide co-operation so that all countries can benefit from the data collected. There is increasing concern about the possible effects of global climatic change caused by human action, such as the depletion of ozone and the Greenhouse Effect. Weather satellites are playing a major role in monitoring the changing climate. Successful weather forecasting for one location depends on accurate observations of the atmosphere over a large area.

Satellites that tell you where you are

Global Navigation Satellite Systems (GNSS) were first developed for military applications. The best-known system is the Global Positioning System (GPS) developed by the United States. It consists of 24 satellites at an altitude of about 20 000 km providing virtually continuous global coverage. Each satellite has a very accurate clock on board and sends out radio signals. We know that radio waves travel at a certain speed (the speed of light), therefore the time taken for the radio signals to travel from at least 3 satellites can be used to calculate the position of a receiver on the Earth's surface. Positions on the Earth's surface can be located to millimetre accuracy.

Galileo will be Europe's own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It is planned to be compatible with GPS, so that a user will be able to take a position with the same receiver from any of the satellites in both systems.

Europe's planned Galileo GNSS (Credit: ESA)

Satellites for communication

Satellite Communication Systems allow the high-speed transmission of telephone calls, television pictures and news around the globe. This use of satellites was established at the same time as the development of weather satellites - from the 1960s onwards.

The shape of the Earth, and features such as mountains, can block signals transmitted by land-based systems. Satellites provide a clear line of sight and can act as relay stations, linking a number of ground-based antennas. Satellites make it possible to telephone anywhere in the world and over 30% of all transatlantic phone calls now go via satellite.

In many countries of the world people still do not have access to telephones. Land-based telephone networks are expensive to build and maintain. Satellites enable a simpler system, without cables, to provide a vital link for people everywhere.

Satellites for military purposes

Over the years, the methods Man has used to defend his land and to attack others have become increasingly sophisticated. Governments and defence agencies like the North Atlantic Treaty Organisation (NATO) are prepared to spend large sums of money protecting their civilians.

The need to locate enemy positions and map features of the land or the ocean floors has led to the development of many remote-sensing techniques. These techniques are often useful for a whole range of non-military applications, providing further benefits. Radar, for example, was invented during the Second World War as a means of detecting the presence of enemy ships or aircraft.

Satellites can detect tanks hidden under trees and distinguish decoys (plastic or cardboard "pretend" tanks) from the real thing.

Global Navigation Satellite Systems (GNSS) were first developed for military applications. They enabled military targets to be located to millimetre accuracy ensuring civilian casualties could be minimised. Rapid and reliable communication systems are essential to the military and satellites are a key tool for defence agencies.