The Moons of Pluto


Charon
, in Greek mythology the ferryman of the dead, discovered on June 22, 1978, by astronomer James Christy, has a diameter of 1,208 kilometers (751 miles) is spherical and the largest of Pluto's five known moons. It has an unusual relationship with its primary in that both keep the same face turned toward the other during rotation (tidally locked). New images
(thanks to New Horizons) shows a canyon/fracture system 1,600 km long (four times as long as the Grand Canyon) stretching horizon to horizon. The surface is largely dominated by water ice, the mean surface temperature a frosty -220 degrees Celsius. It appears to have no significant atmosphere and its physical makeup is still a mystery, possibly a mixture of rock and ice and even a frozen or partially frozen ocean. Upon its discovery Charon replaced Luna, the moon of Earth, as the largest moon relative to its primary in the Solar System.

The four minor moons [1] orbiting Pluto are much smaller without the mass/self-gravity necessary to assume a spherical shape:

Nix, [2] named after the Greek goddess of darkness and night, and mother of Charon, was discovered in June 2005 by the Hubble Space Telescope Pluto Companion Search Team (from images taken in May). It was imaged by the New Horizons spacecraft in July 2015, its size estimated to be about 35 kilometers (roughly 20 miles) across. Physically it's elongated, cratered, has a high geometric albedo and appears largely made of water ice. It has no atmosphere.

Hydra, in Greek mythology a nine headed water monster and guardian of the underworld (the nine heads a nod to Pluto's tenure as the ninth planet), was also discovered in June 2005 by the Hubble Space Telescope Pluto Companion Search Team (from images taken in May). It's irregular in shape, measures approximately 55 by 40 kilometers (34 by 24 miles) and appears largely made of water ice. It has no atmosphere.

Kerberos (formerly S/2011 (134340) 1 or P4) was discovered on June 28, 2011, by the Hubble Space Telescope Pluto Companion Search Team while searching for a possible ring system. It was verified as a new moon on July 20, 2011. Based on its strange double-lobed shape there is speculation that it was formed by the merging of two smaller objects—the greater lobe is 11.9 kilometers (7.4 miles) across, the lesser 4.5 kilometers (2.8 miles) across. It appears largely made of water ice. It has no atmosphere.

Styx (formerly S/2012 (134340) 1 or P5) was discovered on June 26, 2012, by astronomers using the Hubble Space Telescope while further surveying the system in preparation for the arrival of the New Horizons spacecraft. It was verified as a new moon on July 7, 2012. Its shape is irregular, measuring 5 by 7 kilometers (3.1 by 4.3 miles).
It appears largely made of water ice and like its companion moons it has no atmosphere.


[1] The presence of minor moons suggests the possibility of a variable ring system, either tightly confined to a width of less than 1,000 km or tenuous like those of Jupiter—fortunately, a little nail biting by the hazard analysis team aside, the New Horizons flyby passed without incident (at 30,800 mph [49,600 kph] a particle as small as a grain of rice could be lethal). "The suspense
—at least most of it—is behind us," said hazard team leader and mission co-investigator John Spencer of SwRI, his relief evident. "As a scientist I'm a bit disappointed that we didn't spot additional moons to study, but as a New Horizons team member I am much more relieved that we didn't find something that could harm the spacecraft."

[2] Actually the classical spelling is Nyx, but to avoid confusion with asteroid 3908 Nyx, the spelling was changed to Nix the Spanish rendering of the Greek name.

*Further to the above, additional analysis of the data (Hubble) has revealed some strange anomalies: Nix, Hydra, and possibly Styx and Kerberos are tumbling end over end as they move through their orbits, the chaotic dance apparently caused by a shifting gravitational field generated by the dumbbell configuration of their larger companions and further exaggerated by the football-like, rather than spherical shape of the satellites themselves. Also unusual are variances in appearance, Kerberos appearing as black as asphalt while the other minor moons are the color of sand. NASA hopes New Horizons which flew through the Pluto-Charon system in July of 2015 will provide some answers.




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