Editing Geocentric orbit (section)

{{Short description|Orbit around Earth}}
{{Refimprove|date=December 2015}}

A '''geocentric orbit''', '''Earth-centered orbit''', or '''Earth orbit''' involves any object [[orbit]]ing [[Earth]], such as the [[Moon]] or [[satellite|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]].<ref name="Goddard-1997">{{cite web |title=Satellite Situation Report, 1997 |publisher=NASA [[Goddard Space Flight Center]] |date=2000-02-01 |url=http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/ssr.html |access-date=2006-09-10 |url-status=dead |archive-url=https://web.archive.org/web/20060823011134/http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/ssr.html <!-- Bot retrieved archive --> |archive-date=2006-08-23}}</ref> More than 16,291 objects previously launched have undergone [[orbital decay]] and [[atmospheric entry|entered]] [[atmosphere of Earth|Earth's atmosphere]].<ref name="Goddard-1997" /> 

A spacecraft enters orbit when its [[centripetal force|centripetal]] [[acceleration]] due to [[gravity]] is less than or equal to the [[centrifugal force|centrifugal]] acceleration due to the horizontal component of its velocity. For a [[low Earth orbit]], this velocity is about {{cvt|7.8|km/s|km/h mph|-2}};<ref name=hill1999>{{citation | first1=James V. H. | last1=Hill | date=April 1999 | title=Getting to Low Earth Orbit | work=Space Future | url=http://www.spacefuture.com/archive/getting_to_low_earth_orbit.shtml | access-date=2012-03-18 | postscript=. | url-status=dead | archive-url=https://web.archive.org/web/20120319163414/http://www.spacefuture.com/archive/getting_to_low_earth_orbit.shtml | archive-date=2012-03-19 }}</ref> by contrast, the fastest crewed airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was {{cvt|2.2|km/s|km/h mph|-2}} in 1967 by the [[North American X-15]].<ref name=shiner20071101>{{citation | url=http://www.airspacemag.com/history-of-flight/x-15_walkaround.html | title=X-15 Walkaround | date=November 1, 2007 | first1=Linda | last1=Shiner | publisher=Air & Space Magazine | access-date=2009-06-19 | postscript=. }}</ref> The energy required to reach Earth orbital velocity at an [[altitude]] of {{cvt|600|km|mi}} is about 36&nbsp;[[Megajoule|MJ]]/kg, which is six times the energy needed merely to climb to the corresponding altitude.<ref name=dimotakis1999>{{citation|display-authors=1 |last1=Dimotakis |first1=P. |last2=Garwin |first2=R. |last3=Katz |first3=J. |last4=Vesecky |first4=J. |title=100 lbs to Low Earth Orbit (LEO): Small-Payload Launch Options |publisher=The Mitre Corporation |date=October 1999 |pages=1–39 |url=http://en.scientificcommons.org/18569633 |archive-url=https://web.archive.org/web/20170829090237/http://en.scientificcommons.org/18569633 |url-status=dead |archive-date=2017-08-29 |access-date=2012-01-21 |postscript=. }}</ref>

Spacecraft with a [[apsis|perigee]] below about {{cvt|2000|km|mi}} are subject to drag from the Earth's atmosphere,<ref>{{citation | last1=Ghosh | first1=S. N. | date=2000 | title=Atmospheric Science and Environment | publisher=Allied Publishers | isbn=978-8177640434 | url=https://books.google.com/books?id=d6Azu3sfPAgC&pg=PA48 | pages=47–48 }}</ref> 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 {{cvt|300|km|mi}}, decay becomes more rapid with lifetimes measured in days. Once a satellite descends to {{cvt|180|km|mi}}, it has only hours before it vaporizes in the atmosphere.<ref name=slsa>{{citation | first1=John | last1=Kennewell | first2=Andrew | last2=McDonald | date=2011 | publisher=Commonwealth of Australia Bureau of Weather, Space Weather Branch | title=Satellite Lifetimes and Solar Activity | url=http://www.ips.gov.au/Educational/1/3/8 | access-date=2011-12-31 | postscript=. | url-status=live | archive-url=https://web.archive.org/web/20111228025141/http://www.ips.gov.au/Educational/1/3/8 | archive-date=2011-12-28 }}</ref> The [[escape velocity]] required to pull free of Earth's gravitational field altogether and move into interplanetary space is about {{cvt|11.2|km/s|km/h mph|-2}}.<ref name=williams2010>{{citation | first1=David R. | last1=Williams | date=November 17, 2010 | title=Earth Fact Sheet | work=Lunar & Planetary Science | publisher=NASA | url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html | access-date=2012-05-10 | postscript=. | url-status=live | archive-url=https://web.archive.org/web/20101030234253/http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html | archive-date=October 30, 2010 }}</ref>