Of the three known outer satellites, Voyager 1 studied from a distance
only Hyperion and Iapetus. Tiny Phoebe, in its retrograde (clockwise)
orbit, will be studied by Voyager 2 in the summer of 1981. Hyperion and
Iapetus are most likely composed of water ice, although their masses and
densities are uncertain. Iapetus has one bright and one dark hemisphere.
The dark side, which faces forward as Iapetus circles Saturn, reflects
about one-fifth as much light as the trailing, bright side.