Satellites

    A satellite is any smaller object traveling around a larger object, but by this definition, the moon is a satellite to the Earth and the Earth is a satelite to the sun. However, in this case, a satellite is a man made object orbiting the Earth.

    Satellites can either be active or passive. Passive satellites contain no radio transmitters or other energy signals, but rather only reflect signals beamed at them from Earth. Active satellites collect data and emit radio signals that transmit the information down to Earth. Satellites are designed to serve one of three general purposes: space science, applcations, or communications. Space science satellites carry instruments to study the Sun, measure magnetic fields or to examine the universe in the different energy wavelengths of the electromagnetic spectrum, including gamma reays, X-rays, ultraviolet, visable light, infared, microwaves and radio waves. Application satellites survey the Earth's resources and supply weather photographs and ther information to forecasters. Communication satellites relay telephone calls and television signals, trasit Scientific information from other satellites down to Earth, and relay voice communications between the astronauts orbiting on the Space Shuttle and mission controllers on Earth.
    The earliest satellites were made for the armed forces of the United States and Soviet Union, which are still in use today. The armed forces use these orbiting satellites to research battlefield information, locate known or missing troops, ensure communication, and take pictures. There are certain satellites that keep watch on signs of a possible potential uclear missile and a nuclear explosion.

    The Defense Support Program, (DSP), satellites have been in Geo orbits since the 1970's. These satellites monitor the Earth's surface using sensors for detection of ballistic middiles. Any ballistic missile will be detected within seconds, which has greatly reduced the chances of a surprise attack by an enemy. The ability to detect the mssiles so quickly provides enough time for a retaliatory strike.

    A ten year program was started by the United States Government in the latter years of 1990 to replace DSP satellites with the Space-Based infrared System,(SBIRS). This fleet of satellites will have sensors capable of detecting missile launches and will provide information.

    The first SBIRS is scheduled to be launched into a gerostationary and a elliptical orbit by the United States Defese Department. These low-orbit satellites will work with the higher orbit satellites to improve the missile warnings. The band frequencies used fall within the infrared and visible radiation areas of the spectrum.

Benefits of Satellites

    There are more then 3, 500 satellites that orbit the Earth today. Data from these satellites help promote an awareness of the environment, the world, and the universe. New technologies developed for these satellites have additional applications that benefit life on Earth. The new technologies created for satellites are tansferable and have additional aplications, called spin offs. Spin offs represent valuable national resource that are applied to new products and processes. For example, when applying the computerized images to enhance photographs, experts are now able to provide pictures of the human body. Unlike X-Rays, this technology allows doctors to see into the bones, making it more possible to find tumors and make them stand out clearly.

    Much of the information gathered by satellies shows what is happening to Earth. For example, Landsat maps can help determine where healthy or diseased vegetation is at. Satellites such as the Hubble Space Telescope have helped broadened our understanding and knowledge of the universe. The hubble has revealed planet-wide atmospheric storms on Saturn, or the birth of stars in near by galaxyss, the evolution of a supernova, and the phenomena that tells us about black holes--a power source for active galaxies.
Orbits

    Each satellie has a set path in space above the Earth's atmosphere called an orbit. If a satellite traveled through the atmosphere, air would push against it and slow it down. To be completely free of the atmopshere, satellites orbit at least 180 miles (300 kiometers) above sea level.

    Satellites are launched into a variety of orbits, depending on the satellite's purpose. Polar or near polar, geosynchronous (Geo), and elliptical; are the orbits that a satellite could be in. Polar or near polar orbits are launched at an inclination of approximately 90 degrees to the equator. These satellites travel in a cirular patern over the North and South Poles so that they can survey all or major portion of the Earth as it runs below them. Some weather satellites use this type of orbit to track the approach or development of a strom. Then there is geosunchronous orbit, which is 22, 300 miles (35, 888 kilometers) above the equator. As this height, a satellite's velocity matches that of a point on the Earth's equator. Weather and communications satellites use geo orbit.

    Other satellites use an ellipitical orbit around the Earth. This orbit is useful for making scientific measurements.

    Today many people use GPS, Global Postioning System is popular for commerical usage. It's orginal function was to be used for the United States Military. The GPS has served, and still does, as a search and rescue satellite for troops in battle. A hand-held device wil receive a signal from each of the four GPS satellites, allowing people to find their surface position to with in a few meters.
Satellite Structure

    Satellites differ majorly in size and weight. Such as, Explorer I was 6.6 feet (2meters) long and weighed 17.6 pounds (8 kilograms). In contrast, NASA's Compton Gamma Ray Observatory is much larger. Compton measures 70 feet (21.3 meters) from the tip of one solar array to the tip of the other solar array and weighs more than 17 tons. Even though satellites varying size and weight, there are elements that all satellites share. All satellites need a power source. Most are powered by solar cells, which gather energy from the Sun and converts the engery into electricity to power the systems.

    All satellites have intruments holding containg of the scientific and engineering sensors that measure changes in a satellite and its surroundings. The intruments detect chanes in the power supply, the temperature, and the pressure of the satellites. Guideance and contorl sensores keep the satellites on its proper course. sensors such as horizon seekers, star trackers, and Sun seekers help determine the satellite's position. A ground station may send commands to make adjustments if the satellite is off course.
Application

    Satellites are used in testing and developng ways to improve global weather forecasting. Weather satellites are important in predicting when and where tropical storms, hurricanes, floods, cyclones, tidal waves and even forest fires may strike. Knowing this it can save many lives and gives us a time to prepare for these events. Weather information that is predicted ahead of time can help famers decide when to plant or harvest their crops or prevent their destruction by hail or snow.
Scientific Satellites

    Scientific satellites are probably the most well- known type of satellite. They get data that gives scientists important information about Earth and our universe. Satellies that look at Earth are part of NASAs Mission to planet Earth series. The information gathered from these satellites clarifies the Earth's history, present condition, and what the future may hold.
Communications Satellites

    In today's world we take communication satellites for granted. At anytime, a phone call can be placed anywhere in the world and be connected. It is also expecte d that the latest breaking news from around the world will be broadcast as it happens. Until recently, this was not possibly. This development in communications has taken place since 1945 when science fiction author Arthur C. Clake proposed an idea for this network communication satellites. Communication satellites were the first commerical satellites. Industrys have incorporated this type of space technology for its own advancement.

    The first communication satellites were aluminum-coated balloons. The first Echo balloon, was launched in 1960, was considered passive because radio signals refelected off of the surface of the balloon instead of being transmitted from the satellite. Since then there have been many advancements and active sattellites soon followed.
Satellites and the Future

    NASA in exploring ways to provide frequent flight opportunities for relatively inexpensice space missions. One of these is the Small Explorer (SMEX) pogram and provides a low-cost, quick-turnaround missions. The SMEX spacecraft weighs about 400 pounds each and even thuough they are small in size they will investigate some of the most important questions raised in space physics.

    Through satellites, NASA hopes to be able to continure learning about the balance of life on our planet. To do this NASA has established the Earth Observing System. Through orbiting platforms , the Earth Obseving System will provide data that will help scientists understand Earth's basic processes and develop and apply models. The first platform launching is still undecied.

    Satellites will continue to improve life on Earth. Around the world, satellites put people and their compters on the informaiton highway. Electronic mail by satellite has already become a reality. It is possible that in the furture, we may recieve information such as newspaers, and magazine over the computer thanks to satellites.


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