• satellite of Saturn some 1500km in diameter, in a 3.56 million km, 79 day orbit. Iapetus was discovered by Cassini in 1671 and is one of the oddest worlds in the solar system. Its magnitude as seen from the Earth varies drastically because one hemisphere, that facing backwards in Iapetus’s orbit, is icy and of high albedo while the other side is very much darker, possibly because of material thrown out from another of Saturn’s satellites by impact.