A geocentric orbit, Earth-centered orbit, or Earth orbit involves any object orbiting Earth, such as the Moon or artificial satellites. In 1997, NASA estimated there were approximately 2,465 artificial satellite payloads orbiting Earth and 6,216 pieces of space debris as tracked by the Goddard Space Flight Center. More than 16,291 objects previously launched have undergone orbital decay and entered Earth's atmosphere. A spacecraft enters orbit when its centripetal acceleration due to gravity is less than or equal to the centrifugal acceleration due to the horizontal component of its velocity. For a low Earth orbit, this velocity is about 7.8 km/s (28,100 km/h; 17,400 mph); by contrast, the fastest crewed airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2.2 km/s (7,900 km/h; 4,900 mph) in 1967 by the North American X-15. The energy required to reach Earth orbital velocity at an altitude of 600 km (370 mi) is about 36 MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude. Spacecraft with a perigee below about 2,000 km (1,200 mi) are subject to drag from the Earth's atmosphere, which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere. Below about 300 km (190 mi), decay becomes more rapid with lifetimes measured in days. Once a satellite descends to 180 km (110 mi), it has only hours before it vaporizes in the atmosphere. The escape velocity required to pull free of Earth's gravitational field altogether and move into interplanetary space is about 11.2 km/s (40,300 km/h; 25,100 mph).
List of terms and concepts
Altitude as used here, the height of an object above the average surface of the Earth's oceans (mean sea level). Analemma a term in astronomy used to describe the plot of the positions of the Sun on the celestial sphere throughout one year. Closely resembles a figure-eight. Apogee is the farthest point that a satellite or celestial body can go from Earth, at which the orbital velocity will be at its minimum. Eccentricity a measure of how much an orbit deviates from a perfect circle. Eccentricity is strictly defined for all circular and elliptical orbits, and parabolic and hyperbolic trajectories. Equatorial plane as used here, an imaginary plane extending from the equator on the Earth to the celestial sphere. Escape velocity as used here, the minimum velocity an object without propulsion needs to have to move away indefinitely from the Earth. An object at this velocity will enter a parabolic trajectory; above this velocity it will enter a hyperbolic trajectory. Impulse the integral of a force over the time during which it acts. Measured in (N·sec or lb * sec). Inclination the angle between a reference plane and another plane or axis. In the sense discussed here the reference plane is the Earth's equatorial plane. Orbital arc an imaginary arc in the sky as seen from any given location on the surface of the Earth. Orbital characteristics the six parameters of the Keplerian elements needed to specify that orbit uniquely. Orbital period as defined here, time it takes a satellite to make one full orbit around the Earth. Perigee is the nearest approach point of a satellite or celestial body from Earth, at which the orbital velocity will be at its maximum. Sidereal day the time it takes for a celestial object to rotate 360°. For the Earth this is: 23 hours, 56 minutes, 4.091 seconds. Solar time as used here, the local time as measured by a sundial. Velocity an object's speed in a particular direction. Since velocity is defined as a vector, both speed and direction are required to define it.
Types The following is a list of different geocentric orbit classifications.
Altitude classifications
Transatmospheric orbit (TAO) Geocentric orbits with altitudes at apogee higher than 100 km (62 mi) and perigee that intersects with the defined atmosphere. Low Earth orbit (LEO) Geocentric orbits ranging in altitude from 160 km (100 mi) to 2,000 km (1,200 mi) above mean sea level. At 160 km, one revolution takes approximately 90 minutes, and the circular orbital speed is 8 km/s (26,000 ft/s). Medium Earth orbit (MEO) Geocentric orbits with altitudes at apogee ranging between 2,000 km (1,200 mi) and that of the geosynchronous orbit at 35,786 km (22,236 mi). Geosynchronous orbit (GSO) Geocentric circular orbit with an altitude of 35,786 km (22,236 mi). The period of the orbit equals one sidereal day, coinciding with the rotation period of the Earth. The speed is approximately 3 km/s (9,800 ft/s). High Earth orbit (HEO) Geocentric orbits with altitudes at apogee higher than that of the geosynchronous orbit. A special case of high Earth orbit is the highly elliptical orbit, where altitude at perigee is less than 2,000 km (1,200 mi).
Inclination classifications Inclined orbit An orbit whose inclination in reference to the equatorial plane is not 0. Polar orbit A satellite that passes above or nearly above both poles of the planet on each revolution. Therefore it has an inclination of (or very close to) 90 degrees. Polar Sun synchronous orbit A nearly polar orbit that passes the equator at the same local time on every pass. Useful for image-taking satellites because shadows will be the same on every pass.
… excerpt ends here. Continue reading the full article.
