THE SOLAR SYSTEM
periodically they actually exchange orbits. They do not
collide – otherwise they could not continue to exist as
separate bodies – but they may come within a few kilo-
metres (a mile or two) of each other. Both are very irregu-
lar, and are unquestionably the fragments of a larger object
which met with disaster in the remote past.
Telesto and Calypso move in the same orbit as Tethys,
just as the Trojan asteroids do with respect to Jupiter; they
oscillate around the Lagrangian points 60 degrees ahead
and 60 degrees behind. Dione has two Trojan satellites,
Helene and Polydeuces.
Titan is visible in almost any telescope, and very keen-
sighted people can glimpse it with good binoculars. A
7.5-centimetre (3-inch) refracting telescope will show
Iapetus (when west of Saturn) and Rhea easily, and Tethys
and Dione with more difficulty. The other pre-Voyager
satellites require larger apertures, though all except
Phoebe are within the range of a 30-centimetre (12-inch)
reflector.
In 1904 W. H. Pickering, discoverer of Phoebe, reported
another satellite, moving between the orbits of Titan
and Hyperion. The satellite was even given a name –
Themis – but has never been confirmed, and probably
does not exist. Rhea(Cassini). Rhea is
heavily cratered, but with
few really large formations.
As with Dione, the trailing
hemisphere is darkish, with
wispy features which are not
unlike those in Dione, though
less prominent. Rhea seems
to be made up of a mixture
of rock and ice in almost
equal amounts.Tethys(Voyager 2).
Tethys seems to be made up
of almost pure ice. There is
one huge crater, Odysseus,
with a diameter of 400 km
(250 miles) – larger than
the whole of Mimas! There
is also Ithaca Chasma, a
tremendous trench 2000 km
(1250 miles) long, running
from near the north pole
across the equator and along
to the south pole.Dione(Cassini). It is icy and
cratered; the wispy terrain is
made up by bright ice cliffs
created by tectonic fractures.
The trailing hemisphere is
darker than the leading side.
The largest crater, Aeneas,
is about 231 kilometres (144
miles) in diameter. Iapetus(Cassini). There
are dark areas, high peaks
and a curious equatorial
ridge.Hyperion(Voyager 2).
It has been said that
Hyperion is shaped like
a hamburger! Its rotation
is ‘chaotic’ rather than
synchronous, and the longer
axis does not point towards
Saturn, as it might have beenPhoebe(Cassini). This
image was obtained on
11 June 2004 by the Cassini
spacecraft as it entered the
Saturnian system. Phoebe is
cratered and was probably
formed in the outer part of
the Solar System.expected to do if its rotation
was more settled. There are
many craters, and one long
scarp (Bons-Lassell) running
for 300 km (190 miles). The
surface is in general less
reflective than those of
the other icy satellites.108-151 Atl of Univ Phil'05 7/6/05 12:21 pm Page 117