Exactly 50 years ago, in 1930, the planet Pluto was discovered by a young research student named Clyde Tombaugh. Since that time Pluto has set puzzle after puzzle. It is smaller than expected; it has an unusual path: and it has a moon, Charon, which is one-third the size of itself. Today, at the age of 77, CLYDE TOMBAUGH is one of America's most respected astronomers. For this programme Patrick Moore flew to Arizona to meet Professor Tombaugh and to ask him what he now thought about the curious planet he discovered half a century ago.