Kerberos (moon)

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This article is about the fourth moon of Pluto, Kerberos. It is not to be confused with the asteroid 1865 Cerberus.
Moons of Pluto.png
Image of the Plutonian system
Discovered by Showalter, M. R. et al.
Discovery date 28 June 2011
(verified 20 July 2011)
Pronunciation /ˈkɜrbərəs/
Named after
S/2011 P 1, P4
Orbital characteristics[2]
Mean orbit radius
57783±19 km
Eccentricity 0.00328 ± 0.00020
32.16756±0.00014 d
Inclination 0.389°±0.037°
Satellite of Pluto
Physical characteristics
Mean radius
15 km ± ?[3]
Mass ?

Kerberos is a small natural satellite of Pluto, about 30 km (19 mi) in diameter. It was the fourth moon of Pluto to be discovered and its existence was announced on 20 July 2011.[1] It was imaged, along with Pluto and its four other moons, by the New Horizons spacecraft in July 2015.[4] The images have not yet been downlinked, but should have resolutions better than 2 kilometers per pixel.[5]


Hubble Space Telescope discovery images of Kerberos.

Kerberos was discovered by the Pluto Companion Search Team using the Hubble Space Telescope on 28 June 2011, using the Wide Field Camera 3, during an attempt to find any rings that Pluto might possess.[notes 1][8] Further observations were made on 3 and 18 July 2011 and Kerberos was verified as a new moon on 20 July 2011.[1][9] It was later 'precovered' (identified in earlier archival Hubble images) in images from 15 February 2006 and 25 June 2010.[1] Kerberos's brightness is only about 10% of that of Nix, and it was only found because the discovery team took 8-minute exposures; earlier observations had used shorter exposures.[10]

The provisional designation of the satellite varied based on the source used. The International Astronomical Union announced it as S/2011 (134340) 1,[1] whereas the New Horizons mission website announced it as S/2011 P 1.[11]

Physical properties[edit]

When first discovered, Kerberos was given an estimated diameter of 13–34 km (8–21 mi), making Kerberos the second smallest moon of Pluto after Styx, which has an estimated diameter of 10–25 km (6–16 mi); this diameter range was initially derived from an assumed possible geometric albedo range of 0.06 to 0.35.[1][10] More-recent discoveries suggest that Kerberos has an unexpectedly low albedo of 0.04 and is thus towards the higher end of this range.[12]

Orbital properties[edit]

Observations indicate a circular, equatorial orbit with a radius of approximately 58,000 km (36,000 mi).[2] Kerberos orbits between Nix and Hydra and makes a complete orbit around Pluto roughly every 32.1 days.[1][10] This period is close to a 1:5 orbital resonance with Charon, 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 of precession.


Like Pluto's other satellites,[13] it is suspected that Kerberos coalesced from the debris of a massive collision between Pluto and another Kuiper belt object, similar to the giant impact thought to have created the Moon.[8] However, Kerberos's low albedo is a challenge to explain with such a model; if formed in a collision, Kerberos would normally be expected to have a higher albedo, similar to those of Charon, Nix, Hydra, and possibly Styx (whose albedo is yet undetermined).[14] The reason for this difference may be cleared up when more data from New Horizons is received.


Upon discovery, Kerberos received the minor planet designation S/2011 (134340) 1 because it was the first satellite (S) discovered orbiting minor planet (134340) in 2011. It was initially called "P4", meaning the fourth Plutonian moon to be discovered.

The convention for naming Plutonian moons is to use names associated with the god Pluto in classical mythology. To decide on names for P4 and P5, Mark Showalter and the SETI 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.[15] After the initial announcement, William Shatner, the actor who played Captain James T. Kirk in the Star Trek franchise, proposed the names Vulcan and Romulus, ostensibly referring to the fire god Vulcan (a nephew of Pluto), and to Romulus the founder of Rome, but also alluding to the fictional planets of Vulcan and Romulus in the Star Trek universe.[16][17] The 'Romulus' suggestion was discounted, because there is already an asteroid moon of that name,[18] but Vulcan won the poll after Shatner tweeted about it, with Cerberus, (the dog that guards Pluto's underworld) coming second and Styx (the goddess of the river of the underworld) coming third. The winning names were submitted to the International Astronomical Union.[17] However, 'Vulcan' was unacceptable to the IAU because it was not the name of an underworld deity and had already been used for a hypothetical planet inside the orbit of Mercury, as well as having given its name to the vulcanoid asteroids.[16][19] Cerberus is already the name of an asteroid, 1865 Cerberus, but the Greek form of the name, Kerberos, was acceptable to the IAU.[20]

On 2 July 2013, the IAU announced that it formally approved the names Kerberos for P4 and Styx for P5.[21][22]


The New Horizons spacecraft took close-up images of Kerberos during its flyby of the Pluto system on 14 July 2015. The images have not yet been downlinked from the spacecraft, but New Horizons‍' best spatial resolution of Kerberos will be at 3.2 km/pixel.[citation needed]


  1. ^ The search for rings was motivated in part by a desire to avoid damage to New Horizons when it passed through the Pluto system in July 2015.[6][7]
  2. ^ (P_K - 5*P_C)/P_K = 0.0072 where P_K=32.16756\,d is Kerberos's period and P_C = 6.3872304\,d is Charon's period.


  1. ^ a b c d e f g h Showalter, M. R.; Hamilton, D. P. (2011-07-20). "New Satellite of (134340) Pluto: S/2011 (134340) 1". Central Bureau for Astronomical Telegrams. International Astronomical Union. Retrieved 2011-07-20. 
  2. ^ a b Showalter, M. R.; Hamilton, D. P. (3 June 2015). "Resonant interactions and chaotic rotation of Pluto’s small moons". Nature 522 (7554): 45–49. doi:10.1038/nature14469. 
  3. ^ NASA's Hubble finds Pluto's moons tumbling in absolute chaos
  4. ^ Cain, Fraser (2008). "Pluto’s Moon Nix". 
  5. ^ New Horizons flyby timeline
  6. ^ Wall, M. (2011-07-20). "New Pluto Moon Foreshadows More Surprises for NASA Probe En Route". Space.Com web site. TechMediaNetwork. Retrieved 2011-07-21.  External link in |work= (help)
  7. ^ McKee, M. (2006-02-22). "Rings of ice and dust may encircle Pluto". New Scientist web site. New Scientist. Retrieved 2011-07-21.  External link in |work= (help)
  8. ^ a b Boyle, A. (2011-07-20). "Scientists spot Pluto's fourth moon". Cosmic Log on Retrieved 2011-07-20.  External link in |work= (help)
  9. ^ "Pluto Has Another Moon, Hubble Photos Reveal | Dwarf Planet Pluto | Pluto's Moons". Retrieved 2012-07-12. 
  10. ^ a b c Lakdawalla, E. (2011-07-20). "A fourth moon for Pluto". Planetary Society weblog. The Planetary Society. Retrieved 2011-07-20.  External link in |work= (help)
  11. ^ New Horizons news, Johns Hopkins University Applied Physics Laboratory, "Fourth Moon Adds to Pluto's Appeal", 20 July 2011
  12. ^ "Pluto's moons are even weirder than thought". Retrieved 2015-06-20. 
  13. ^ Stern, S. A.; Weaver, H. A.; Steff, A. J.; Mutchler, M. J.; Merline, W. J.; Buie, M. W.; Young, E. F.; Young, L. A.; Spencer, J. R. (2006-02-23). "A giant impact origin for Pluto’s small moons and satellite multiplicity in the Kuiper belt" (PDF). Nature 439 (7079): 946–948. Bibcode:2006Natur.439..946S. doi:10.1038/nature04548. PMID 16495992. Retrieved 2011-07-20. 
  14. ^ "Pluto's moons dance to a random beat". Retrieved 2015-06-20. 
  15. ^ "Ground Rules". Retrieved 12 May 2013. 
  16. ^ a b Marcia Dunn (25 February 2013). "Capt. Kirk's Vulcan entry wins Pluto moons contest". San Francisco Chronicle. Associated Press. Retrieved 25 February 2013. 
  17. ^ a b Alexandra Witze (2013-04-23). "Moon and planet names spark battle : Nature News & Comment". Retrieved 2013-07-06. 
  18. ^ "'Vulcan' tops poll for moon name". 3 News NZ. 26 February 2013. 
  19. ^ Miriam Krame (25 February 2013). "'Vulcan' and 'Cerberus' Win Pluto Moon Naming Poll". Retrieved 25 February 2013. 
  20. ^ Rice, Tony. "Kerberos and Styx named as moons of Pluto". WRAL. 
  21. ^ "Names for New Pluto Moons Accepted by the IAU After Public Vote". IAU. 2 July 2013. Retrieved 2 July 2013. 
  22. ^ "Pluto's Smallest Moons Receive Their Official Names". SETI Institute. 2 July 2013. Retrieved 2 July 2013.