Of the 100 probes that have been sent out into space, only one probe - Voyager 2 in 1989 - has ever sent back any information about the solar system's outer planets. From afar, they seem featureless and devoid of any significant geological activity.Since then, long-range telescopes have shown that there is much more to Neptune and Uranus than previously thought and that they both warrant closer inspection. Time, however, is of the essence. Deep-space missions are dependent on Jupiter. The slingshot effect it provides to move probes along the way is only available to us every 25 years.