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Pluto's smallest moons named Kerberos and Styx

Charles Black, Founder and CEO of Sen
Jul 3, 2013, 7:00 UTC

Sen—Pluto's two smallest known moons have been officially named Kerberos and Styx, the International Astronomical Union (IAU) has announced.

The two moons were discovered in 2011 and 2012 as a result of observations using the Hubble Space Telescope. The names are derived from the Greek mythological underworld where Kerberos was a three headed dog and the river Styx separated the land of the living from the dead.

Five moons have so far been found orbiting Pluto. The largest moon Charon has a diameter of about 1,200 km - about half the size of Pluto. Charon, which was discovered in 1978, orbits Pluto at a distance of less than 20,000 km.

Nix and Hydra, which have diameters between 32 and 113 km, were discovered in 2005 by the Hubble Space Telescope. 

Kerberos, previously designated P4, was discovered in 2011. The tiny moon, with a diameter estimated between 13 and 34 kilometres, is located between the orbits of Nix and Hydra.

The fifth moon, called P5 since its discovery in 2012, has been named Styx. The tiny moon is thought to be an irregular shape and has an estimated diameter of 10 to 25 kilometres. 

The research team who discovered Kerberos and Styx was led by Mark Showalter of the SETI Institute, who invited the public to suggest names for the two moons earlier this year.

Over 450,000 votes were received on the Pluto rocks poll. Vulcan, Mr Spock's homeworld in Star Trek, received the most votes. Cerberus and Styx were the second and third most popular. Showalter said he had been overwhelmed by the public response to the naming campaign.

The names Vulcan and Cerberus were submitted to the IAU, which acts as the international arbiter for naming celestial bodies. However, the IAU vetoed the use of Vulcan because it said the name had already been used for a hypothetical planet between Mercury and the Sun. Although the planet had been found not to exist, the IAU said that asteroids within the orbit of Mercury were still referred to as Vulcanoids. The IAU therefore chose the second and third most popular choices, and changed the spelling of Cerberus to the Greek form Kerberos.

Pluto and its five moons imaged by Hubble

The Hubble Space Telescope's image of Pluto and its five moons taken in July 2012. Credit: NASA

Pluto was discovered in 1930 by Clyde Tombaugh and was considered the ninth planet until 2006 when the IAU reclassified it as a dwarf planet. Pluto has been found to be one of many icy bodies that form the Kuiper Belt. The dwarf planet is one of the largest of the Kuiper Belt Objects with a diameter of about 2,300 km. 

NASA's New Horizons spacecraft will be the first to fly through the Pluto system in 2015. The New Horizons probe is armed with several instruments and should be able to capture images and valuable data about the dwarf planet and its moons. The spacecraft will also search for additional moons which might be too small for Hubble to detect.

Alan Stern, the Principal Investigator for New Horizons, said "The discoveries of Kerberos and Styx add to the mysteries surrounding the formation of the Pluto system."

Pluto’s distance from the Earth varies because it is in an elliptical orbit, but in July 2015 Pluto will be 4.92 billion kilometres away.