Astronomy Picture of the Day . ... A comet nucleus, formed from the primordial stuff of the solar system, resembles a very dirty iceberg. ... After taking its spectacular pictures of the outer solar system planets, Voyager 1 looked back at six planets from the inner solar system. ... Pluto: The Frozen Planet . ... The Hubble Space Telescope imaged Pluto and its moon Charon in 1994. ... This picture was taken by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Neptune. ...