Moons of Pluto
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
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
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
The four minor moons  orbiting Pluto are much smaller without the mass/self-gravity
necessary to assume a spherical shape:
named after the Greek goddess of darkness and night, and mother of
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.
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
measures approximately 55 by 40 kilometers (34 by 24 miles) and appears largely made of water ice. It has no atmosphere.
S/2011 (134340) 1 or P4) was discovered on June 28, 2011, by the Hubble
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
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.
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.
 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."
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.
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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