Pluto

"Originally considered a full planet by many people, Pluto was ultimately classified as a dwarf planet in 2006 when the International Astronomical Union set down the official conditions for planethood, because it had not 'cleared the neighborhood' around its orbit.Since the early days of interplanetary exploration Pluto has been a goal for many people, but its frigid conditions and distant orbit made it a challenge. It is up to you to rise to meet that challenge."