Lets take a brief look at history and see what it can tell us about Satellites:

What is a Satellite?

 There are two categories of satellites: Natural and Artificial.

  Natural Satellites.
 
    A natural satellite is a celestial body that revolves around a planet larger than itself. At least 54 such bodies exist in our SOLAR SYSTEM. Mercury and Venus have no satellites. The Earth and its MOON form virtually a double planetary system, whereas the two Martian satellites, PHOBOS and DEIMOS, are tiny, irregularly shaped bodies--probably large meteorites or small asteroids captured by Mars. JUPITER has at least 16 satellites, SATURN 17, URANUS 14, NEPTUNE 2, and PLUTO 1. In addition, countless small particles revolve around Jupiter, Saturn, and Uranus in the form of rings.

    By mass and size, the Moon; Jupiter's IO, EUROPA, GANYMEDE, and CALLISTO; Saturn's TITAN; and Neptune's Triton predominate. Callisto, Ganymede, Titan, and Triton exceed the size of the planets Mercury and Pluto. The mean densities of these four satellites--between 1.8 and 2.0 g/cu cm (112 and 125 lb/cu ft)--are, however, substantially lower than that of the Moon, indicating that they cannot consist of solid silicate rocks as does the Moon, but that their cores must be encrusted with ices whose composition is similar to that of their central planets. All other satellites are of asteroidal rather than planetary size. Some small satellites orbit their planets in a direction opposite to the planet's rotation--another indication of possible asteroidal origin.

  Artificial Satellites.

    An artificial satellite is an object placed into orbit around the Earth for the purpose of scientific research, Earth applications, or military reconnaissance.  Orbit is achieved when the object is given a horizontal velocity of approximately 28,500 km/h (17,500 mph) at sea level.  At this velocity the Earth's surface curves away from the horizontal as fast as gravity pulls the object downward.  As the altitude of the satellite increases, its velocity decreases, and its period--the time the satellite takes to circle the Earth--increases.  A satellite 275 km (170 mi) above the Earth's surface will have a period of 1.5 hours and a velocity of 27,860 km/h (17,300 mph), but the same object 35,840 km (22,300 mi) above the Earth would have a period of 24 hours (the same as the Earth's rotation, these are also known as Geostationary Satellites) and a velocity of only 11,050 km/h (6,850 mph).  A satellite in the latter orbit is called a synchronous satellite;  if such a satellite orbits in the equatorial plane, it is termed geostationary because it will remain at the same point above the Earth's surface.

History.

  The theoretical possibility of establishing an artificial satellite of Earth had been mentioned in 1687 by the English mathematician Isaac Newton as a consequence of his work on the theory of gravitation.  Only in the early 20th century, however, did the theoretical work of the Russian Konstantin TSIOLKOVSKY and the experimental work of the American Robert GODDARD confirm that a satellite might be launched by means of a rocket.

  During the period from 1943 to 1946 several studies indicated that available rockets would be unable to place a satellite into orbit.  Work on rockets for missiles and upper-atmosphere research was so extensive after World War II, however, that by 1954 the feasibility of launching a satellite was no longer in serious doubt.  In October 1954 the Committee for the International Geophysical Year (IGY) recommended to member countries that they consider launching small satellite vehicles for scientific SPACE EXPLORATION.  In April and July 1955 the USSR and the United States, respectively, announced plans to launch such satellites for the IGY.  Accordingly, the USSR launched SPUTNIK 1 on Oct. 4, 1957, and the United States launched EXPLORER 1 on Jan. 31, 1958.  These two satellites provided an enormous stimulus for further work on artificial satellites, especially with the discovery of the Van Allen radiation belts made possible by Explorer 1.

  On December the 12th 1961, Amateur Radio's first satellite, was launched into orbit, known as OSCAR 1 (Orbiting Satellite Carrying Amateur Radio). It was a small battery - powered box like (running on Mercury batteries) satellite that continually transmitted the Morse Code identifier HI to amateur radio receivers on earth until it's battery ran down. OSCAR 1 lasted 21 days in orbit around the Earth. The cost of this Satellite (which of course it was constructed by Radio Amateurs at their own home's) was only about $26 U.S. The beacon frquency was on 144.983 MHz and it's maximum power output was only 0.1W (100mW). Maximum altitute was 430 km. About 500 contacts have been reported world wide. This was a great achivement in Amateur Radio, ever since the first Amateur Radio satellite was launched, many countries around the world where in action to hear and learn about up comming future satellites.

 View the pictures & info taken on Explorer & Sputnik below. Click on the links below.

 Explorer 1 - Launch Rocket.

 Explorer 1 - Satellite.

 Sputnik 1 (also known as RS-1).

Basic Satellite Elements.

  All artificial satellites have certain features in common. They include radar for altitude measurements, sensors such as optical devices in observation satellites, receivers and transmitters in communications satellites, and stable radio-signal sources in navigation satellites.  Solar cells generate power from the Sun, and storage batteries are used for the periods when the satellite is blocked from the Sun by the Earth.  These batteries in turn are recharged by the solar cells.  In special cases, nuclear power sources are utilized. Attitude-control equipment is needed to keep the satellite in its desired orbit and, in some cases, to point the antennas or sensors properly.  Radio transmitters and receivers are used to send signals to and from the Earth.  TELEMETRY encoders measure various quantities that describe the condition of the onboard equipment and relay this information to Earth.

Scientific Research Satellites.

  Numerous satellites are orbited to observe the space environment of the Earth or to study the Sun, stars, and extragalactic objects over a wide range of wavelengths.  Among those whose tour of active duty has been concluded are the six Orbiting Geophysical Observatories (1964-69)  eight Orbiting Solar Observatories (1962-75)  two Orbiting Astronomical Observatories (1966-72)  three HIGH ENERGY ASTRONOMICAL OBSERVATORIES (1977-79)  the International Ultraviolet Explorer (1978)  the X-RAY ASTRONOMY satellites Uhuru (1970-79) and Exosat (1983) and the Infrared Astronomy Satellite, or IRAS.

Communications Satellites.

  Perhaps the most important technological application of artificial satellites has been to relay radio signals around the Earth for communications purposes.  The experimental communications satellites SCORE, ECHO, TELSTAR, RELAY, and SYNCOM were launched by the United States between 1958 and 1963.  The INTELSAT system now spans the globe, and domestic satellites--such as the USSR's MOLNIYA satellites, Western Union's Westar, and Canada's Anik-- serve individual countries. Using receivers, amplifiers, and transmitters and the electronic technique of multiplexing, these communications satellites can simultaneously relay many telephone and television signals.

Navigation Satellites.

  Navigation satellites provide the means to pinpoint any location on Earth with high accuracy by use of the Doppler effect.  Because the satellite's orbit is already known, an unknown position can be accurately determined by Doppler measurements made, from that position, of the increase or decrease in the radio frequency emitted by the satellite as it orbits the Earth.  The U.S. Transit system has been in worldwide operation continuously since 1964 and is regularly used by more than a thousand stations, most of which are involved in merchant shipping.  NAVSTAR, a more advanced system consisting of 18 satellites positioned at an altitude of 20,000 km (12,500 mi), where operational around the beginning of the 90's.

Reconnaissance Satellites.

  Both the United States and the ex-USSR (and to a much smaller extent, possibly the People's Republic of China) have orbited considerable numbers of reconnaissance satellites for photo surveillance, electronic intelligence, nuclear-explosion detection, and strategic-missile launch detection (for example, COSMOS - SAMOS).

Weather Satellites.

  Since the launch on Apr. 1, 1960, of the first weather satellite, TIROS 1, the extent of cloud-cover pictures taken from satellites has gradually increased until, beginning in 1966, the entire Earth has been photographed at least once daily on a continuous basis.  Satellite data provide information about the ocean, desert, and polar areas of the Earth where conventional weather reports are unavailable or limited.  Satellite photos locate weather features--storm systems, fronts, upper-level troughs and ridges, jet streams, fog, sea ice conditions, snow cover, and, to some extent, upper-level wind direction and speeds--that are characterized by certain cloud formations.  Coastal and island stations can use such data to locate and track major storms.

  Both optical and infrared cameras are used, the latter showing sea-surface temperatures more frequently and over a larger area than is possible by any other means.  These data are useful for shipping and fishing interests and are important for meteorological forecasts.  The United States has used both low-altitude (700 to 1,700 km/435 to 1,050 mi), geostationary, and Sun-synchronous weather satellites.

Land and Sea Observation Satellites.

  By using satellite sensors for microwave, X-ray, and infrared wavelengths, valuable data can be obtained about land and sea resources.  Such sensors can distinguish between land and water, cities and fields, and corn and wheat as well as between distressed corn and vigorous corn.  The U. S. satellites of the LANDSAT series have been used for making estimates of global wheat production, for forest and rangeland inventories, for mineral and oil exploration and geologic mapping, and for environmental monitoring and impact assessments.  The Spot-1 satellite launched in 1986 by France's space agency and equipped with high-resolution cameras is performing comparable surveys of Earth resources for international users. In addition, the Soviet Union has sold survey photographs through an international agent, ContiTrade Service Corporation.

  The instrumentation of SEASAT (1978) included a radar that measured altitude to an accuracy of 10 cm (4 in) and wave heights from 1 to 20 m (3 to 65 ft).  The satellite detected ocean currents, tides, and storm surges.  A scatterometer measured ocean-surface wind speeds from 4 to 26 m/sec (13 to 85 ft/sec) with an accuracy of 2 m/sec (7 ft/sec).  Other equipment measured surface temperatures, water-vapor content in the air, rain rate, and the age, concentration, extent, and motion of sea ice.

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