366 Encyclopedia of the Solar System
FIGURE 1 The relative sizes of the main planetary
satellites Earth, Mass, and Pluto.More than 100 years would pass before the next satellite
discoveries were made: the Uranian satellites Titania and
Oberon and 2 smaller moons of Saturn. As telescopes ac-
quired more resolving power in the 19th century, the family
of satellites grew (see Table 1). Observations obtained by
the camera on theVoyagerspacecraft led to the discovery
of 3 small satellites of Jupiter, four of Saturn (S27 Pan was
discovered 9 years after the data were obtained), 10 satel-
lites of Uranus, and 6 of Neptune (see Table 1). TheCassini
spacecraft revealed the existence of 4 small moons of Sat-
urn. Many small satellites have been recently discovered by
sensitive charge-coupled device (CCD) cameras attached
to large ground-based telescopes and the Hubble Space
Telescope, including the 2 small satellites of Pluto, Hydra
and Nix, which brings the total of known satellites of Pluto
to three. [SeePluto.]
The natural planetary satellites are generally named af-
ter figures in world mythologies who were associated with
the namesakes of their primaries. They are also designated
by the first letter of their primary and an Arabic numeral
assigned in order of discovery: Io is J1, Europa J2, and so
on. When satellites are first discovered but not yet con-
firmed or officially named, they are known by the year in
which they were discovered, the initial of the primary, and
a number assigned consecutively for all solar system dis-
coveries, for example, 2003 J23. Names for all satellites
are assigned by the International Astronomical Union. See
http://planetarynames.wr.usgs.gov/append7.html for a list
of names and discovery circumstances maintained by the
International Astronomical Union.
After planetary scientists were able to map geologic for-
mations of the satellites from spacecraft images, they named
many of the features after characters or locations from
Western and Eastern mythologies. These names are also
approved by the IAU.1.2 Physical and Dynamical Properties
The motion of a satellite around the center of mass of it-
self and its primary defines an ellipse with the primary at
one of the foci. The orbit is defined by three primary orbital
elements: (1) the semimajor axis, which is the maximum dis-
tance between the ellipse and its center; (2) the eccentricity,
and (3) the angle made by the intersection of the plane of
the orbit and the plane of the primary’s spin equator (the
angle of inclination; for outer satellites the inclination is de-
fined by the plane of the satellite’s orbit to the orbital plane
of the planet). The orbits are said to be regular if they are
in the same sense of direction (the prograde sense) as that
determined by the rotation of the primary, and if their ec-
centricities and inclinations are low. The orbit of a satellite is
irregular if its motion is in the opposite (or retrograde) sense