Geocentric orbit

A geocentric orbit, Earth-centered orbit, or Earth orbit involves any object <a href="/facts/Orbit/nD2TPySG">orbiting</a> <a href="/facts/Earth/HLLmDfj0">Earth</a>, such as the <a href="/facts/Moon/zKHqEMPb">Moon</a> or <a href="/facts/Satellite/PTX8c68t">artificial satellites</a>. In 1997, NASA estimated there were approximately 2,465 artificial satellite payloads orbiting Earth and 6,216 pieces of <a href="/facts/Space_debris/X7YSKlXl">space debris</a> as tracked by the <a href="/facts/Goddard_Space_Flight_Center/WfoEKknM">Goddard Space Flight Center</a>. More than 16,291 objects previously launched have undergone <a href="/facts/Orbital_decay/c9rj3aK6">orbital decay</a> and <a href="/facts/Atmospheric_entry/p7TMk9sa">entered</a> <a href="/facts/Atmosphere_of_Earth/FV97Am25">Earth's atmosphere</a>. 
A spacecraft enters orbit when its <a href="/facts/Centripetal_force/VWBADgMM">centripetal</a> <a href="/facts/Acceleration/UVDmjYvR">acceleration</a> due to <a href="/facts/Gravity/bqAN7DdQ">gravity</a> is less than or equal to the <a href="/facts/Centrifugal_force/CWgvMw6w">centrifugal</a> acceleration due to the horizontal component of its velocity. For a <a href="/facts/Low_Earth_orbit/rq1TwayJ">low Earth orbit</a>, 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 <a href="/facts/North_American_X-15/btDgcYKz">North American X-15</a>. The energy required to reach Earth orbital velocity at an <a href="/facts/Altitude/B5eNVurK">altitude</a> of 600 km (370 mi) is about 36 <a href="/facts/Megajoule/9ndq9jD2">MJ</a>/kg, which is six times the energy needed merely to climb to the corresponding altitude.
Spacecraft with a <a href="/facts/Apsis/VaL6ow9w">perigee</a> 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 <a href="/facts/Escape_velocity/bdtme0tx">escape velocity</a> 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).

Geocentric orbit open-in-new

Geocentric orbit