 a geocentric orbit involves any object orbiting the earth, such as the moon or artificial satellites. in 1997 nasa estimated there were approximately 2,465 artificial satellite payloads orbiting the earth and 6,216 pieces of space debris as tracked by the goddard space flight center. over 16,291 previously launched objects have decayed into the earth's atmosphere. == list of terms and concepts ==  ;altitude: as used here, the height of an object above the average surface of the earth's oceans. ;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 (nÂ·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 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
Earth_orbits