adminSATELLITES AND ORBITS
A satellite is any space body or object, natural or man-made revolving in space around another bigger body or object with a fix circular or elliptical path. Satellites obey the physical laws of gravity. They move along elliptical trajectories or orbits around the center of a bigger body according to newton’s and Kepler’s law of the physical universe.
There are two types of satellites; natural and artificial satellites.
Natural satellites are celestial bodies that orbit a planet or other body. They include planets orbiting a star, small galaxies orbiting a major galaxy and moons orbiting a planet. The solar system has about 240 moons, including 160 that orbit the eight planets, four that orbit dwarf planets and many more that orbit small bodies.
Artificial satellites are man-made. The world first artificial satellite, sputnik 1, was launched by the ussr in the year 1957. By 2010, about 3000 artificial satellites are known to be in earth’s orbit.
All satellites orbit around centers of mass; galactocentric satellites orbit about the center of a galaxy; like the solar system. Heliocentric satellites orbit around the sun; like the planets. While, geocentric satellites orbit around the earth; like the moon and artificial satellites.
There are two main types of orbits, the inclined and the special orbits. Inclined orbits are either polar, polar synchronous or inclined orbits. While, the special orbits are either prograde, retrograde or graveyard orbits.
Polar orbits pass above or close to the north and south poles on each revolution. Polar orbits typically have an altitude of 700-800km and an orbital period of 90-100 minutes (inclination equal or close to 90degrees).
Polar sun-synchronous orbits are almost polar orbits pass over the equator at the same local time on each revolution (inclination close to 90 degrees).
Inclined orbits are inclined to the equatorial plane, for example GPS satellites orbits are inclined at 55 degrees to the equatorial plane (inclination between 0 and 90 degrees).
Prograde orbits are orbits in which the satellite travels in the same direction as the planets rotation. Satellites are usually launched in prograde orbits to save energy.
Retrograde are orbits in which the satellite travels in opposite direction to a planets rotation. Retrograde orbits require large amounts of energy or additional fuel to overcome the effect of a planets rotation on the rocket launch.
Graveyard orbits are a few hundred kilometers above the geosynchronous orbit (35786km). Satellites can be moved to these orbits at the end of their useful life. Note most earth orbiting satellites are in geostationary or polar orbits.
Also satellites orbits are classified according to the altitude of the orbit which they operate; thus they are classified as low, medium or high altitude orbits. Generally low earth orbit (LEO) satellites have altitude of less than 2000km. medium earth orbit (MEO) satellites have altitude between 2000km and 36,000km. While high earth orbit (HEO) satellites is usually elliptical orbits at altitudes greater than 36,000km.
Satellites in their application have found widespread use in telecommunications industry in both radio and TV broadcasting, as navigational satellites for ships, aircraft and autonomous vehicles, global mobile communication (GSM) and remote sensing. In its remote sensing application, it is used mainly for population and poverty mapping, infrastructure mapping and exposure, crop yield and productivity forecasting, weather forecasting, hazardous events like troop movement, volcanic and earth quake activity, plane crash investigation and general event detection, severity evaluation and many other applications across the human domain.
