The Galilean Moons

Jupiter is now prominently visible in the evening sky. Any telescope will show it together with its four large satellites, known as the Galileans because they were studied in 1610 by Galileo with his first telescope. They are not alike; two are icy and cratered, one smooth as a billiard ball, and one brilliant red, with violently active volcanoes. Patrick Moore talks about the Galileans, and what we have learned about them from space missions.