Planetary Satellites 379
FIGURE 11 Six small satellites of
Saturn. They are, clockwise from
the top, Atlas, Pandora, Janus,
Calypso, Helene, Telesto,
Epimetheus, and Prometheus.4.3.3 THE INNER SMALL SATELLITES
Four types of unusual inner small satellites have been found
in the Saturnian system: the shepherding satellites, the co-
orbitals, the Lagrangians, and the satellites that orbit in
ring gaps. All of these objects are irregularly shaped (see
Fig. 11) and probably consist primarily of ice. The three
shepherds—Atlas, Pandora, and Prometheus—are mod-
eled as playing a key role in defining the edges of Saturn’s
A- and F-rings. The orbit of Atlas, Saturn’s innermost satel-
lite that is not within the ring system, lies several hundred
kilometers from the outer edge of the A-ring. The other
2 shepherds, which orbit on either side of the F-ring, not
only constrain the width of this narrow ring but may cause
its kinky appearance. TheCassinispacecraft also discov-
ered Pallene and Methone, two small satellites that orbit
between Mimas and Enceladus.
The co-orbital satellites Janus and Epimetheus, which
were discovered in 1966 and 1978, exist in an unusual dy-
namical situation. They move in almost identical orbits at
about 2.5 Saturn radii. Every four years the inner satellite
(which orbits slightly faster than the outer one) overtakes
its companion. Instead of colliding, the satellites exchange
orbits. The 4-year cycle then begins over again. Perhaps
these two satellites were once part of a larger body that
disintegrated after a major collision.
Four other small satellites of Saturn orbit in theLa-
grangian pointsof larger satellites: Two are associated
with Dione (Helene and Polydeuces, which was discovered
byCassiniin October 2004) and two with Tethys (Telesto
and Calypso). The Lagrangian points are locations within
an object’s orbit in which a less massive body can move in an
identical, stable orbit. They lie about 60◦in front of and in
back of the larger body. Although no other known satellites
in the solar system are Lagrangians, the Trojan asteroids
orbit in two of the Lagrangian points of Jupiter, Neptune,
and Mars.
The final class of unusual Saturnian satellite is those that
dwell in ring gaps and sweep and clear particles from the
gaps. Pan, which was discovered in 1990 fromVoyagerim-
ages, sits in the Encke gap. TheCassinispacecraft discov-
ered Daphnis, a small satellite in the Keeler gap, in May
2005.4.3.4 SMALL OUTER SATELLITES
Like the other giant outer planets, Saturn has a large fam-
ily of outer irregular satellites, most of which have been
recently discovered with large telescopes. The 25 known
outer small satellites move in eccentric inclined orbits, and
most of their orbits are retrograde, implying that they are
captured objects. The farthest satellites orbit more than
20 million km from Saturn.4.4 The Satellites of Uranus
4.4.1 THE MEDIUM-SIZED SATELLITES OF URANUS: MIRANDA,
ARIEL, UMBRIEL, TITANIA, AND OBERON
Uranus has a total of 27 known satellites. The main satellites
are medium-sized bodies that orbit the planet from 130,000
to 583,000 km. The orbits of Ariel, Umbriel, Titania, and
Oberon are regular, whereas Miranda’s orbit is slightly in-
clined. Figure 12 is a telescopic image of the satellites typi-
cal of the quality attainable before the advent of spacecraft
missions.
Theoretical models suggest that the satellites are com-
posed of water ice, possibly in the form of methane