Image of the Plutonian system
|Discovered by||Showalter, M. R. et al.|
|Discovery date||June 28, 2011
(verified July 20, 2011)
|Alternative names||S/2011 P 1, P4|
|Mean orbit radius||(59±2)×103 km|
|Orbital period||32.1±0.3 d|
Kerberos is a small natural satellite of Pluto whose existence was announced on July 20, 2011. Its discovery, following the discoveries of Charon in 1978 and Nix and Hydra in 2005, made it Pluto's fourth known moon.
Kerberos was discovered by the Hubble Space Telescope's Pluto Companion Search Team on June 28, 2011, using the Wide Field Camera 3, during an attempt to find any rings that Pluto might possess.[notes 1] Further observations were made on July 3 and July 18, 2011 and it was verified as a new moon on July 20, 2011. It was later identified in archival Hubble images from February 15, 2006 and June 25, 2010. Kerberos's brightness is only about 10% of that of Nix, and it was found because the discovery team took 8-minute exposures; earlier observations had used shorter exposures.
The provisional designation of the satellite varied based on the source used. The International Astronomical Union announced it as S/2011 (134340) 1, while the New Horizons mission website announced it as S/2011 P 1.
With an estimated diameter of 13–34 km (8–21 mi), Kerberos is the second smallest known moon of Pluto (after Styx, which has an estimated diameter of 10–25 km (6–16 mi)). This diameter range is derived from an assumed possible geometric albedo range of 0.06 to 0.35.
Current[when?] observations suggest a circular, equatorial orbit with a radius of approximately 59,000 km (about 37,000 miles). Kerberos orbits in the region between Nix and Hydra and makes a complete orbit around Pluto roughly every 32.1 days. This period is close to a 1:5 orbital resonance with Charon, with the timing discrepancy being apparently less than 0.6%.[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, 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 believed to have created the Moon.
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. After the initial announcement, William Shatner, the actor who plays 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. The 'Romulus' suggestion was discounted, as there is already an asteroid moon of that name, 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. 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. Cerberus is already the name of an asteroid, 1865 Cerberus, but the Greek form of the name, Kerberos, was acceptable to the IAU.
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