USING SPACE

Anything in orbit that around another object can be called as satellite. For an example, the Moon is a natural satellite of Earth. Since 1957, hundreds of artificial satellites have been launched into orbit around the Earth. They come in many shapes and sizes and occupy different types of orbit, depending on what they are designed to do. Many communications satellites occupy geostationary orbit, for example, many weather satellites are in polar orbit. Which orbit they follow, satellites must remain stable so that their instruments always point in the right direction.

If a satellite is not stable, it swings about in an unpredictable way. For an example, the dish of communications satellites must always point towards its receiving station or towards the right country if it is transmitting television signals. Two techniques commonly used to maintain stability are spin and three-axis stabilization.

Telephones calls, television broadcasts and the Internet can all be relayed by communications satellites. All these satellites connect distant places and make communication possible with remote antennas. Many of it is in geostationary orbit but so great the demand for communications that this orbit has become crowded.

Communications links such as antennas on the ground and on satellites send or receive radio waves that carry telephones calls, television signals or data. Telephones calls for an example, Europe to USA might pass through the public telephones networks to a nearby Earth station, that transmit the radio waves to a satellites in Geostationary orbit. The satellites would amplify and retransmit the radio waves to an antenna in the USA, the signals is routed over the telephone network to its destination.

To steer an accurate course between two places, navigation satellites need to know the exact position. Astronauts calculated their positions using the Moon, stars and Sun. when clouds obscure the sky, it is easy to go far off course. Navigation satellites system has solved this problem. Satellites transmit radio waves that can be detected on Earth even when it is cloudy. A navigation satellite is now possible in any weather. The Global Positioning System (GPS) developed in the USA had become the most reliable and accurate navigation system ever.

GPS consists of 24 satellites as well as equipment on the ground. The satellites broadcast their positions and the time. They are spaced in orbits so that a receiver signals from at least four satellites. The GPS receiver knows precisely when the signal was sent, it arrived and it can calculate the distance between itself and each of the satellites. It also calculates the speed and direction of travel.

Meteorology satellites can see the way weather system develope and more around the globe. It record the images that appear nightly on our television screens will show cloud cover and monitor hurricanes growing and moving across the oceans. Meteorology satellites also carry instruments that take reading, which are converted to the temperatures, pressures and humidity needed for weather forecasting. Together with the information from sources such as weather buoys, balloons and ships, help forecasters to improve their predictions.

Before weather satellites existed, hurricanes develop unseen over oceans and strike land with very little warning. A notorious hurricane could kill 6,000 people in Texas, 1906. Hurricanes are extreme tropical storms with wind speeds persistently in excess of 120 km/h. in tropical storms, winds circle a calm eye of low air pressure. Now, weather satellites constantly view of oceans where such storms gather strength.

Satellites help scientists to study Earth's surface are called Earth resources satellites. It can show whether crops are failing or ice caps are melting and pinpoint resources such as metal ores or coal. This is possible because satellites instrument analyze light and other radiation reflected and emitted from surface features. Satellites pass regularly over the whole globe, allow scientists to produce maps that trace how a particular area change over time.

Military satellites are widely used nowadays. From the safety of orbit, satellites can gather information about battlefields, it take pictures so detailed they can show where a person is standing, locate missing troops and provide secure communications. Some of the satellites monitor the globe, watching for signs of the launch of a nuclear missiles or a nuclear explosion.

USA has been launching a Defense Support Program (DSP) satellite into geostationary orbit since 1970s. Each satellite can monitor large sections of the Earth's surface. They carry sensors to detect the launch of a missile and can send a warning to Earth within seconds of a missile igniting. These satellites launched by Russia have reduced the advantage of a surprise attack.

Anything in the orbit that has no use is called space debris. This also includes discarded rockets and obsolete satellites that could stay in orbit for millions of years, as well as fragments from satellites that exploded or were destroyed. After half a century of the first satellites, more than 90% of the objects orbiting Earth are space junk. Each break up or

adds to the rubbish and increase the risk of an orbiting spacecraft being hit by a piece of debris. A collision with a fleck of paint could put a spacecraft out of action. Space nations have begun to examine how they could reduce the junk left in space.

The rubbish is created at many stages of a space operation, such as during separation when the nose cone is discarded once a satellite is released to orbit. Space junk accumulates most quickly in those orbits that are used most often. Satellites and debris could collide at speeds up to 40,000km/h, causing serious damage.