Planetary Satellites 381
FIGURE 14 Image of Miranda obtained by theVoyager 2
spacecraft at 30,000–40,000 km from the Moon. Resolution is
560–740 m. Older, cratered terrain is transected by ridges and
valleys, indicating more recent geologic activity.
4.4.2 THE SMALL SATELLITES OF URANUS
Voyager 2discovered 10 new small satellites of Uranus, in-
cluding two that act as shepherding satellites for the outer
(epsilon) ring of Uranus (see Table 1). All these satellites lie
inside the orbit of Miranda. Images of two satellites, Puck
and Cordelia, provided sufficient resolution to directly de-
termine their radii (see Table 1). The sizes of the other
bodies were derived by making the assumption that their
surface brightnesses are equal to those of the other inner
satellites and estimating the projected area required to yield
their observed integral brightnesses. Puck appears to be
only slightly nonspherical in shape. It is likely that the other
small satellites are irregularly shaped. The satellites’ visual
geometric albedos range from 0.04 to 0.09, which is slightly
higher than that of Uranus’s dark ring system. No reliable
color information was obtained byVoyager 2for any of the
small satellites, although their low albedo suggests that they
are C-type objects. Ground-based observers have discov-
ered another 12 outer irregular satellites to bring the total
of known Uranian satellites to 27. Most of the small outer
satellites are moving in retrograde orbits, implying they are
probably captured bodies.
4.5 The Satellites of Neptune
4.5.1 INTRODUCTION
Neptune has 13 known satellites: one is the large moon
Triton and the remaining 12 are small, irregularly shaped
bodies (see Table 1). The small satellites can be divided into
two categories: the 6 inner bodies, which move in highly reg-
ular, circular orbits close to Neptune (<5 planetary radii),
and the irregular outer satellites. At the time of theVoyager
2 encounter, Nereid, which moves in an eccentric prograde
orbit bringing it from 57 to 385 planetary radii from Nep-
tune, was the only known satellite in the latter category.
Five more moons were discovered in 2003, including one
with a period of 26.3 years, which corresponds to a distance
of 47 million km from Neptune. Triton has an appreciable
atmosphere, seasons, and currently active geologic process-
ing. [SeeTriton]
Only Triton and the outer satellite Nereid were known
before the reconnaissance of Neptune by theVoyager 2
spacecraft in 1989. Nereid was discovered in 1949 by Ger-
ard P. Kuiper at McDonald Observatory in Texas. In keep-
ing with the theme of water and oceans for the Neptunian
system, the satellite was named after the sea nymphs known
in Greek mythology as Nereids. Reliable ground-based ob-
servations of Nereid were limited to estimates of its visual
magnitude.4.5.2 ORBITAL AND BULK PROPERTIES
The six inner satellites were all discovered within a few
days during theVoyagerencounter with Neptune in August- They were given names of mythical nautical figures
by the International Astronomical Union. For four of these
satellites (Proteus, Larissa, Galatea, and Despina), as well
as Nereid,Voyagerimages provided sufficient resolution
to determine their dimensions (see Table 1). All five bod-
ies are irregularly shaped. The sizes of Thalassa and Naiad
were derived by making the assumption that their albedos
are equal to those of the other inner satellites. The size
of the satellites increases with the distance from Neptune.
Proteus is the largest known irregularly shaped satellite in
the solar system (see Table 1). The satellite has probably not
been subjected to viscous relaxation; rather its mechanical
properties have been determined by the physics of water
ice, with an internal temperature below 110 K.
Spacecraft tracking of the six inner satellites, and ground-
based observations of Nereid, provided accurate orbit de-
terminations, which are listed in Table 1. All the small
inner satellites except Proteus orbit inside the so-called syn-
chronous distance, which is the distance from Neptune at
which the planet’s rotational spin period equals the satellite’s
orbital period. The rotational periods of these satellites are
unknown, but they are most likely in synchronous rotation.
Voyagerobservations and more recent telescopic mea-
surements suggest that Nereid is in nonsynchronous rota-
tion, with a period of 11.5 hours.
The masses of the satellites were not measured directly
byVoyager.Limits may be obtained by assuming reasonable
values for their bulk densities. These values range from
0.7 g/cm^3 , corresponding to water ice with a bulk porosity
of about 30%, to 2 g/cm^3 , corresponding to water ice with
a significant fraction of rocky material. If the satellites were