Jupiter's four largest moons - the Galilean satellites - are among the most interesting members of the Solar System. They were discovered by Galileo in 1610 - hence their nickname - and they have been studied by the Pioneer probes; one of them, lo, affects Jupiter's radio emission. Patrick Moore talks about these planet-sized satellites with Dr Garry Hunt , who is involved in all the space missions and gives the latest news about the Viking probe to Mars.