Kerberos, formal designation(134340) Pluto IV, is a smallnatural satellite of Pluto, about 19 km (12 mi) in its longest dimension. Kerberos is also the second-smallest moon ofPluto, afterStyx. It was the fourth moon of Pluto to be discovered and its existence was announced on 20 July 2011.[1] It was imaged, along withPluto and its four other moons, by theNew Horizons spacecraft in July 2015.[8] The first image of Kerberos from the flyby was released to the public on 22 October 2015.[9]
Kerberos was discovered by researchers of the Pluto Companion Search Team using theHubble Space Telescope on 28 June 2011, using theWide Field Camera 3, during an attempt to find anyrings that Pluto might possess.[10][11] The search for rings was motivated in part by a desire to avoid damage toNew Horizons when it passed through the Pluto system in July 2015.[12] Further observations were made on 3 and 18 July 2011 and Kerberos was verified as a new moon on 20 July 2011.[1][13] It was laterprecovered or identified in earlier archival Hubble images from 15 February 2006 and 25 June 2010, though Kerberos was obscured bydiffraction spikes in those images.[14] Kerberos's brightness is only about 10% of that ofNix, and it was only found because the discovery team took eight-minute exposures; earlier observations had used shorter exposures.[15] It was officially designated S/2011 (134340) 1,[1] and was informally referred to as P4.[15]
Kerberos has a double-lobed shape and is approximately 19 km (12 mi) across its longest dimension and 9 km (5.6 mi) across its shortest dimension. It is the second-smallest of Pluto's moons, after Styx. The larger lobe of Kerberos is approximately 8 km (5 mi) across while the smaller lobe is measured to be approximately 5 km (3.1 mi) across. The double-lobed shape of Kerberos is likely formed by two smaller objects merged, suggesting that Kerberos along with Pluto's other moons may have been formed by the coalescence of debris around Pluto.[17]
Kerberos has a highalbedo or reflectivity similar to Pluto's other small moons, likely due to the presence of water ice on its surface.[18] Before theNew Horizons flyby, Kerberos was initially thought to have a larger size and a darker surface.[19]
Like the other small moons of Pluto, Kerberos is nottidally locked and its rotation ischaotic, varying quickly over geological timescales. The varying gravitational influences of Pluto andCharon as they orbit theirbarycenter causes the chaotictumbling of Pluto's small moons, including Kerberos.[20] At the time of theNew Horizons flyby, the rotational period of Kerberos was about 5.33 days and its rotational axis was tilted about 96 degrees to its orbit.[4][7] The high axial tilt of Kerberos meant that it was rotating sideways relative to its orbit around the Pluto-Charon barycenter.[7]
Observations indicate a circular, equatorial orbit around the Pluto-Charon barycenter at a distance of 57,783 km (35,905 mi).[3] All of Pluto's moons including Kerberos have very circular orbits with very loworbital inclinations to Pluto's equator. Kerberos orbits between Nix andHydra and makes a complete orbit around Pluto roughly every 32.167 days.[1][15]
Its orbital period is close to a 1:5orbital resonance withCharon,[20] with the timing discrepancy being about 0.7%.[notes 2] As with the near resonances between Nix or Hydra and Charon (1:4 and 1:6, respectively), determining how close this relationship is to a true resonance will require more-accurate knowledge of Kerberos's orbit, in particular its rate ofprecession.
TheNew Horizons spacecraft took images of Kerberos during its flyby of the Pluto system on 14 July 2015. Three months later, on 22 October, the first image of the moon was published. It is the last moon of Pluto to have its image released, revealing that Kerberos was small and had a bright surface contrary to the initial idea that the moon was covered in dark material.[9]
Upon discovery, Kerberos received theminor planet designation S/2011 (134340) 1 because it was the first satellite (S) discovered orbiting theminor planet 134340 Pluto in 2011. It was initially called "P4", indicating that it was the fourth Plutonian moon to be discovered.[12]The convention for naming Plutonian moons is to use names associated with the godPluto inclassical mythology.[21] To decide on names for P4 and P5,Mark Showalter and theSETI Institute, on behalf of the discovery team, conducted a non-binding Internet poll in 2013, in which the general public was invited to vote for their favorite names. The public could choose from a selection of Greek mythological names related to the god Pluto, or could propose their own names.[22] After the initial announcement,William Shatner, the actor who played CaptainJames T. Kirk in theStar Trek franchise, proposed the names Vulcan and Romulus, ostensibly referring to the fire godVulcan (a nephew of Pluto), and toRomulus the founder ofRome, but also alluding to the fictional planets ofVulcan andRomulus in theStar Trek universe.[23][24] The "Romulus" suggestion was discounted, because there is alreadyan asteroid moon of that name, but Vulcan won the poll after Shatner tweeted about it, withCerberus (the dog that guards Pluto's underworld) coming second andStyx (the goddess of the river of the underworld) coming third. The winning names were submitted to theInternational Astronomical Union.[24]However, "Vulcan" was unacceptable to the IAU because it was not the name of an underworld deity and had already been used fora hypothetical planet inside the orbit of Mercury, as well as having given its name to the hypotheticalvulcanoids.[23][25] "Cerberus" is already the name of an asteroid,1865 Cerberus, but the Greek form of the name, "Kerberos", was acceptable to the IAU.[26]
On 2 July 2013, the IAU announced that it formally approved the names Kerberos for P4 andStyx for P5.[27][28]
The names of features on the bodies in the Pluto system are related to mythology and the literature and history of exploration. In particular, the names of features on Kerberos must be related to dogs from literature, mythology, and history.[21]
^abStern, Alan; Grinspoon, David (May 1, 2018). "Chapter 12: Into Unknown Danger".Chasing New Horizons: Inside the Epic First Mission to Pluto. Picador.ISBN978-1-250-09896-2.
^"Kerberos Revealed".pluto.jhuapl.edu. NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. October 22, 2015. Archived fromthe original on October 29, 2018. RetrievedOctober 22, 2015.
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