Editing Highly elliptical orbit (section)

{{Short description|Elliptic orbit with high eccentricity}}
[[File:NASA molniya oblique.png|thumb|upright|[[Molniya orbit]] for the Northern hemisphere]]
{{about|a variation of elliptic orbit|geocentric orbit|High Earth orbit}}
A '''highly elliptical orbit''' ('''HEO''') is an [[elliptic orbit]] with high [[orbital eccentricity|eccentricity]], usually referring to [[geocentric orbit|one around Earth]]. Examples of inclined HEO orbits include [[Molniya orbit]]s, named after the [[Molniya (satellite)|Molniya]] Soviet communication satellites which used them, and [[Tundra orbit]]s. Many US satellites also have used these orbits, satellites such as the [[Trumpet (satellite)|Trumpet]] electronics intelligence satellites. 

The acronym HEO often is expanded to Highly Eccentric Orbit by orbital analysts since all orbits around planets, etc are ellipses - the term "highly elliptical" is not very clear as to what is exaggerated. It would be more proper to call these orbits "elongated" than "highly elliptical". 

Highly eccentric orbits have two main uses - as transfer orbits and as good orbits for communication with or surveillance of the Polar regions. The transfer orbit was proposed by the [[Walter Hohmann|German scientist Walter Hohmann]] in 1925, it connects two circular orbits, a lower one and a higher one, with an eccentric orbit.<ref>{{cite book |last1=Bate |first1=Roger |last2=Mueller |first2=Donald |last3=White |first3=Jerry |last4=Saylor |first4=William |title=Fundamentals of Astrodynamics |date=2020 |publisher=Dover Books |isbn= 978-0-486-60061-1 |page=131 |edition=Second}}</ref> There are many HEO orbits where a launch vehicle initially injects a spacecraft into a LEO orbit and then an upper stage injects the payload, intended to go to geosynchronous orbit, into a HEO transfer orbit and then a final stage or engine circularizes the payload in the intended geosynchronous orbit. 

When chosen for communications or surveillance, these extremely elongated orbits also can provide long dwell times at a point in the sky during the approach to, and descent from, [[apsis|apogee]]. Bodies moving through the long apogee dwell appear to move slowly, and remain at high [[altitude (astronomy)|altitude]] over Polar latitude ground sites for long periods of time. This makes these elliptical orbits useful for [[communications satellite]]s. The Northern Hemisphere has far more landmass, and it belongs to historically competitive countries, so most HEO satellites are in orbits which maximize time over the Northern Polar regions. Most of the Southern Hemisphere is covered by water and has been less interesting to these adversarial countries. 

[[Geostationary orbit]]s cannot serve polar latitudes because their elevation above the horizon from these ground sites is too low.<ref name="fortescue" />  The latitude limit for a GEO sat is 81 degrees with a practical limit of just above 75 degrees.<ref>{{Cite web |title=Coverage of a geostationary satellite at Earth |url=https://www.planetary.org/space-images/coverage-of-a-geostationary |access-date=2024-07-06 |website=The Planetary Society}}</ref>  Many GEO comm sats have custom "foot prints" and focus their signals at their primary service areas, so above 60 degrees a larger ground antenna and clear line of sight is needed.

Note: some incorrectly expand the acronym HEO as "High Earth Orbit" which does not make a lot of sense. They mean an orbit that has a high altitude above the Earth, to refer to a likely circular orbit that is much higher than a Low Earth Orbit (LEO) and lower than a Geosynchronous Orbit (GEO).

[[File:molniya.jpg|thumb|Groundtrack of a Molniya orbit]]
[[File:Qzss-45-0.09.jpg|thumb|upright|The groundtrack of a [[QZSS]] orbit]]
[[Sirius Satellite Radio]] used inclined HEO orbits, specifically the Tundra orbits, to keep two satellites positioned above [[North America]] while another satellite quickly sweeps through the southern part of its 24-hour orbit. The [[longitude]] above which the satellites dwell at apogee in the small loop remains relatively constant as [[Earth's rotation|Earth rotates]]. The three separate orbits are spaced equally around the Earth, but share a common [[ground track]].<ref name="castor-tundra"/>