Moons of Pluto

Pluto has five moons. They are Charon, Nix, Hydra, Kerberos (P4) and Styx (P5). Charon is the largest moon.

Charon Moon

Charon Moon is the innermost and largest moon of the Pluto system. Charon was discovered in 1978. Its diameter is 1212 km (753 miles) which is more than half as wide in size as Pluto and the Pluto-Charon system is like a double planet. Charon orbits Pluto every 6.4 days and has a synchronous orbit (the pair show the same face to each other all the time). To an observer on the planet, Charon appears to be stationary in the sky like a geostationary satellite orbiting the Earth.

Pluto and Charon have been called a double planet because Charon is larger compared to Pluto.

Moons: Nix and Hydra

Two new moons of Pluto were first spotted in May 2005 Hubble Space Telescope images, then confirmed in archival Hubble pictures taken in June 2002. The moons were provisionally designated S/2005 P 1 (Hydra) and S/2005 P 2 (Nix). 

On June 21, 2006 the International Astronomical Union (IAU) announced the formal names for Pluto’s two discovered small moons: Nix and Hydra. The IAU chose the name to complement Pluto, who was the Greco-Roman god of the underworld. Nix represents the Greek goddess of night and Hydra is the nine-headed monster that guarded Lake Lerna, an entrance to the underworld in Greek mythology. Nix’s off-spring Charon, who ferried souls not the underworld, lent his name to Pluto’s largest moon, which was discovered in 1978.

The correct spelling of Nix is actually Nyx, but as an asteroid had already been given that name, the IAU accepted the alternative spelling.

Picture: The artist’s concept above shows the Pluto system from the surface of one of the moons. The other members of the Pluto system are just above the moon’s surface. Pluto is the large disk at center, right. Charon is the smaller disk to the right of Pluto. The other moon is the bright dot on Pluto’s far left. Credit: NASA, ESA and G. Bacon (STScI)

Hydra

Hydra is the outer-most natural satellite of Pluto. It was named after Hydra, the monster who guarded the waters of Pluto / Hades’s underworld in Greco-Roman Mythology. The name Hydra was announced on June 21, 2006, in IAU Circular 8723, along with the formal designation Pluto III. Hydra was visited along with Pluto by the New Horizons mission in 2015. 

At the time of discovery, Hydra was about 25 percent brighter than its sister moon Nix, which led to the assumption that its diameter was some 10 percent larger. However, in subsequent observations the two moons were about equal in brightness. This is likely due to an oblong shape, although brightness variation over its surface may also be responsible. Hydra appears to be spectrally neutral like Charon and probably Nix, but unlike Pluto, which is reddish.

Although its size has not been directly measured, calculations based on its brightness give it a diameter of between 40 km, if its reflectivity is similar to Charon’s 35 percent and about 130 km, if it has a reflectivity of 4 percent like the darkest Kuiper belt objects (KBOs).

Kerberos (P4)

Kerberos is the fourth moon of Pluto. It was discovered by the Hubble Space Telescope’s Pluto Companion Search Team on 28 June, 2011 using the Wide Field Camera 3. Kerberos is a three-headed dog in Greek mythology.

Styx (P5)

Styx is the fifth moon of Pluto is called P5 or S/2012 (134340) 1. It was visible as a speck of light in nine separate Hubble images taken by Hubble’s Wide Field Camera 3 on 26, 27 and 29 June, and 7 and 9 July 2012.

It is estimated to be irregular in shape and between 10 and 25 km across. It is in a 95 000 km-diameter circular orbit around Pluto that is assumed to lie in the same plane as Pluto’s other known moons.

Styx is the mythological river separating the living world from the dead.

Kerberos and Styx Naming Contest

Astronomers unveiled a contest to name Pluto’s P4 and P5 moons and wanted the public to help vote the names. The discovery team led by Mark Showalter of the SETI Institute in Mountain View, California announced on 11 Monday, February 2013 that they have opened a contest for public voting on the names. 

In July 2013, the IAU (International Astronomical Union) approved ‘Kerberos’ and ‘Styx’ as the new names for two of Pluto’s moons that were previously called P4 and P5 respectively.

The IAU  selected the names based on the results of the 2013 Pluto Rocks Internet poll sponsored by SETI (the Search for Extraterrestrial Intelligence), but Vulcan the top vote-getter ultimately wasn’t chosen as a name for one of the tiny moons. IAU rules states that Pluto’s moons should be named for mythological characters of the underworld in Greek and Roman mythology. Dwarf Planet Pluto is known as the ruler of the underworld in ancient mythos.

Did you know?

In 2006 Planet Pluto was controversially reclassified as a Dwarf Planet.

The International Astronomical Union (IAU) is the organization responsible for naming celestial objects.,

Books

Pluto and Charon : Ice Worlds on the Ragged Edge of the Solar System by Alan Stern, Jacqueline Mitton
From Amazon.com, Amazon.co.uk

Pluto: The Ninth Planet by Michael D. Cole
From Amazon.com, Amazon.co.uk

Beyond Pluto by John Davies (Author)
From Amazon.com, Amazon.co.uk

Moons of Pluto Links:

• NASA – Hubble Spots Possible New Moons Around Pluto: Source of Photo
• NASA – New Moons of Pluto:
• Hubble Discovers New Pluto Moon: date July 2012.
• Names for New Pluto Moons Accepted by the IAU After Public Vote: date 2 July 2012 by IAU.

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