View source for Cytherocentric orbit

{{Short description|Orbit around planet Venus}}
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[[File:Animation of Akatsuki trajectory around Venus.gif |thumb |right |Animation of [[JAXA]] probe [[Akatsuki (spacecraft)|Akatsuki]]'s trajectory around [[Venus]] from 1 December 2015<br>{{legend2|magenta| ''Akatsuki''}}{{·}}{{legend2| Lime |[[Venus]]}}]]

A '''Cytherocentric orbit''' is an orbit around the [[planet]] [[Venus]]. Venus has no moon, but several man-made objects orbit the planet.

The name is analogous to the term [[geocentric orbit]] for an orbit around Earth and [[heliocentric orbit]] for an orbit around the Sun. The [[apsides]] of an Cytherocentric orbit are pericytherion, the [[pericenter]] (analogous to [[perigee]]), and the [[apocenter]] is named apocytherion (analogous to [[apogee]]).

==Etymology==
The ''Cythero'' prefix is derived from [[Kythira]] or "Cythera." In [[Greek mythology|Greek]] mythology, Cythera was an island associated with the goddess [[Aphrodite]], who is the equivalent of the Roman goddess [[Venus (mythology)|Venus]]. Therefore, naming an orbit around Venus "cytherocentric" is a way of referencing Venus' association with this goddess.{{Citation needed|date=January 2024}}

==Satellites in Cytherocentric orbit==

[[Venera 9]] was the first satellite to achieve Venus orbit in 20 October 1975. [[Akatsuki (spacecraft)|Akatsuki]] was the latest probe to achieve Venus orbit in 2015.<ref name="Exploration: Venus">{{cite web |title=Exploration: Venus|work=The National Aeronautics and Space Administration|date= 2010|url=https://science.nasa.gov/venus/exploration/ }}</ref>

Eight probes have achieved Venus orbit: 
* 4 [[Soviet Union]] probes, [[Venera 9]], [[Venera 10]], [[Venera 15]], [[Venera 16]]
* Two [[NASA]] probe, [[Magellan (spacecraft)|''Magellan'']], [[Pioneer Venus 1]]
* One [[JAXA]] probe, [[Akatsuki (spacecraft)|Akatsuki]]
* One [[ESA]] probe, [[Venus Express]].

In order to enter Venus orbit, a  satellite has to perform an engine burn to reduce the speed. Otherwise, the probe moves too fast to achieve orbit and will be a [[Flyby (spaceflight)|flyby]]. A noteworthy case is that of Japanese probe [[Akatsuki (spacecraft)|Akatsuki]], which failed to enter orbit around Venus on 6 December 2010.<ref>JAXA's press briefing, 22:00, 7 December 2010 JST</ref> JAXA stated on 8 December that the probe's orbital insertion maneuver had failed,<ref name="ABC">{{cite web | title=Japan's Venus Probe Fails to Enter Orbit | url=https://abcnews.go.com/Technology/wireStory?id=12339589 | work=[[ABC News (United States)|ABC News]] | access-date=8 December 2010}}</ref> because of a defect in the orbital insertion burn. After the craft [[heliocentric orbit|orbited the Sun]] for five years, engineers successfully placed it into an alternative Venusian [[elliptic orbit]] on 7 December 2015 by firing its [[Spacecraft attitude control|attitude control]] thrusters for 20 minutes.

[[Magellan (spacecraft)|''Magellan'']] was the first interplanetary probe to use [[aerobraking]] to reduce the apocytherion.<ref name="lyons">{{cite journal |last1=Lyons |first1=Daniel T. |last2=Saunders |first2=R. Stephen |last3=Griffith |first3=Douglas G. |title=The Magellan Venus mapping mission: Aerobraking operations |journal=Acta Astronautica |date=1 May 1995 |volume=35 |issue=9 |pages=669–676 |doi=10.1016/0094-5765(95)00032-U |bibcode=1995AcAau..35..669L |url=https://www.sciencedirect.com/science/article/abs/pii/009457659500032U |language=en |issn=0094-5765}}</ref> By passing through the dense atmosphere, a probe can reduce its speed and attain the necessary [[delta-v]]. [[Atmosphere of Venus|Venus's thick atmosphere]] supports [[aerobraking]]. This reduces fuel needs.

==Stationary and synchronous orbits==

A [[satellite]] with revolutionary period that matches the planet's rotational period appears fixed at a position in the sky relative to an observer on the planet. Such an orbit on Earth is a [[Geostationary orbit]].<ref name="ST">{{cite web |title=Space Today Online - Answers To Your Questions|work= Spacetoday.org|date= 2010|url=  http://www.spacetoday.org/Questions/PolarSats.html }}</ref>

The height of a stationary or synchronous orbit can be calculated as follows:

: <math>R_{syn} = \sqrt[3]{{G(m_2)T^2\over 4 \pi^2}}</math><ref>{{Cite news|url=https://www.askwillonline.com/2012/12/calculating-radius-of-geostationary.html|title=Calculating the Radius of a Geostationary Orbit - Ask Will Online|date=2012-12-27|work=Ask Will Online|access-date=2017-11-21|language=en-GB}}</ref>
where G is the [[gravitational constant]], m<sub>2</sub> is the mass of the celestial body, and T is the [[Sidereal time|sidereal rotational period]] of the body.

By this formula one can find the [[geostationary]]-analogous cytherostationary orbit. Around Venus, such an [[orbit]] would be 1,536,600&nbsp;km or about 253 Venus radii from the planet's surface. This is because Venus has the slowest rotation rate of any planet. The slower the rotation, the farther away a satellite must be in order to be stationary. The [[hill sphere]] of a celestial body describes the region in which the gravity of that body is dominant. The [[hill sphere]] radius of Venus is about 1 million kilometers; and as the cytherostationary orbital distance lies outside of it, no stable cytherostationary satellite can exist.

==See also==
* [[Orbit of Venus]]
* [[Areocentric orbit]]

==References==
{{reflist}}

[[Category:Orbits]]
[[Category:Venus]]