As of July 2011, there are presently four known moons orbiting Pluto. These are named, in order of increasing distance from the dwarf planet; Charon, Nix, P4 / S/2011 (134340) and Hydra. The following text gives further detailed explanations of their discoveries and what we presently know of these distant moons. S/2011 (134340) was the most recently discovered that lies between the orbits of Nix and Hydra.
On 7th June 1978, Charon, was discovered by J.W. Christy of the U.S. Naval Observatory while doing imaging Pluto with the 1.8 metre telescope at Flagstaff Arizona. Later, previous images were found, showing the unrecorded new moon. Clearly, one of the major problems with Charon is that the observed separation is only minute. It averages just under 1 arcsec, requiring advanced techniques to see it, and is all but invisible to amateur astronomers. Pluto throughout its long elliptical orbit around the Sun, the separation can be anywhere between 0.6 and 1.1 arcsec. Charon was later first clearly resolved by the planetary camera of the Hubble Space Telescope in October 1990. This unexpected faint 17.8v magnitude moon was soon named after the Greek mythological ferryman who transports the dead across the river Acheron (Styx by the Romans) and into the realm of the underworld ruled by the god Hades.
Charon’s diameter was first estimated to be small, whose orbital period is 6.2 days at 19,570 kilometres in distance. When Charon was observed during a rare star occultation on 07th April 1980, this measured diameter proved to be much larger than expected, being some 1,210±10 kilometres across, that is still frequently quoted today (2006). More recently, the JPL estimates finds a slightly smaller 1,186±26 km. across. Charon roughly contains about 12% the mass of Pluto, and similarly has the lower density of about 1.85 g.cm-3. Earth is about 5.5 g.cm-3.
Its orbit is a bit like the Earth’s Moon, being locked in synchronous orbit and always holding the same face towards Pluto. Hence, rotation of Charon equals the 6.2 day long rotational period of Pluto. As mass ratio is about 12%, the gravitational orbital pivot occurs somewhere between Pluto and Charon. Due to this proximity this would make the system more like some ‘double planet’, than the usual planet/moon systems.
Charon’s orbit is significantly inclined to the ecliptic by 94.9°, and like the Uranian axis, rings and moons; can be seen by the observer as edgewise or wide open. Edge-wise orbit crossings, when occultations and eclipses are possible, and this will happen only twice during the 247-odd year orbital period of Pluto. Previous crossings occurred on 26th December 1987 and 18th April 1988, and the next will not be until 5th April 2110 and 30th November 2111 AD. The orbital shape appears more oval between these times, and will be widest in 2048 AD.
NIX and HYDRA
Both these new Plutonian moons were announced on 31st October 2005, being found by the collaboration of astronomers using the HST planetary camera while imaging Pluto. They were known under the provisional International Astronomical Union (IAU) designations, as S/2005 P1 and S/2005 P2, until 23rd June 2006 when after spirited debating, they were named as Nix and Hydra. This announcement was made in the IAU Circular, No. 8625. Nix in Greek mythology was the goddess of night, who bore the brighter moon Charon — the ferryman that transports the dead across the river Styx and reach the Underworld. Hydra was a dreaded nine-headed serpent creature who carefully guards the main entrance to the Underworld from mortal intruders or those wishing to escape.
Once astronomers looked at the earlier HST images taken on 14th June 2002, they found these same moons. This proved helpful in establish the orbital parameters of these new moons, whose respective distances are 48,700 and 64,800 kilometres from Pluto, whose mutual orbital periods are 24.86 and 38.21 days. These two new outer moons are very similar in size, about 90km. across. Like Charon, they orbit in the same orbital plane, whose inclinations are about 96° to the ecliptic.
Pluto IV, P4. S/2011 PI (134340)
Five images taken of Pluto on 28th June, 3rd and 18th July using the Wide-field Camera (WFC3) of the Hubble Space Telescope (HST) looking at; have revealed the fourth Plutonian satellite, now designated (2011) as S/2011 (134340). Shining at a very low 26.1±0.3 magnitude, this new moon appears much fainter than Nix (Pluto II). We know little of the orbital or physical parameters, other than P4 orbits Pluto once every 31±0.3 days in a near circular orbit at the distance of 59000±2000 km from the gravitational centre of Pluto’s equator. P4 is therefore placed between the orbits of Nix and Hydra. Size is estimated as merely 14km to 40km, depending on the adopted albedo.
Credit: NASA, ESA, and M. Showalter (SETI Institute)
Discovery of these four moons may have some implications for Pluto. As no other of the Kuiper belt bodies has more than one moon, this might finally save Pluto its membership among the other planets, which was stripped away by the International Astronomical Union (IAU). No doubt we will learn much about these new moons during the one billion dollar New Horizons mission. Now en-route spacecraft fly-by now scheduled for mid-July 2015.
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Last Update : 9th January 2015
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