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Orcus (dwarf planet)
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=== Size and magnitude === [[File:Orcus, Earth & Moon size comparison.png|left|thumb|upright|Orcus compared to [[Earth]] and the [[Moon]]]] [[File:Orcus-apmag19-2009Dec26-11UT.jpg|left|thumb|upright|Long-exposure photograph of Orcus at visual magnitude 19.2]] The absolute magnitude of Orcus is approximately 2.3.<ref name=TNOsCool8 /> The detection of Orcus by the [[Spitzer Space Telescope]] in the [[far infrared]]<ref name="Stansberry 2007" /> and by [[Herschel Space Telescope]] in [[submillimeter]] estimates its diameter at {{convert|958.4|km|mi|abbr=on}}, with an uncertainty of {{convert|22.9|km|mi|abbr=on}}.<ref name=TNOsCool8 /> Orcus appears to have an [[Albedo#Astronomical albedo|albedo]] of about 21β25%,<ref name=TNOsCool8 /> which may be typical of trans-Neptunian objects approaching the {{convert|1000|km|mi|abbr=on}} diameter range.<ref name="tnoalbedo" /> The magnitude and size estimates were made under the assumption that Orcus is a singular object. The presence of a relatively large satellite, Vanth, may change them considerably. The absolute magnitude of Vanth is estimated at 4.88, which means that it is about 1/11 as bright as Orcus itself.<ref name="Brown2010" /> The [[Atacama Large Millimeter Array|ALMA]] submillimeter measurements taken in 2016 showed that Vanth has a relatively large size of {{convert|475|km|mi|abbr=on}} with an albedo of about 8 percent while Orcus's has a slightly smaller size of {{convert|910|km|mi|abbr=on}}.<ref name="brown2018"/> Using a [[stellar occultation]] by Vanth in 2017, Vanth's diameter has been determined to be {{convert|442.5|km|mi|abbr=on}}, with an uncertainty of {{convert|10.2|km|mi|abbr=on}}.<ref name="Sickafoose2018"/>
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